100+ Java Interview Questions and Answers In 2019

100+ Java Interview Questions and Answers In 2019

In this article, I am going to list some of the most important Java Interview Questions and Answers in 2019 which will set you apart in the interview process

Java Interview Questions

Java is used by approx 10 Million developers worldwide to develop applications for 15 Billion devices supporting Java. It is also used to create applications for trending technologies like Big Data to household devices like Mobiles and DTH boxes. And hence today, **Java is used everywhere! **

We have compiled a list of top Java interview questions which are classified into 7 sections, namely:
Basic Interview QuestionsOOPs** Interview QuestionsJDBC Interview Questions**Spring** Interview Questions**Hibernate** Interview QuestionsJSP Interview Questions****Exception and thread Interview Questions**
As a Java professional, it is essential to know the right buzzwords, learn the right technologies and prepare the right answers to commonly asked Java Interview Questions. Here’s a definitive list of top Java Interview Questions that will guarantee a breeze-through to the next level.

So let’s get started with the first set of basic Java Interview Questions.

Basic Java Interview Questions Q1. Explain JDK, JRE and JVM?

Q2. Explain public static void main(String args[]) in Java.

main() in Java is the entry point for any Java program. It is always written as public static void main(String[] args).
public: Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class.static: It is a keyword in java which identifies it is class-based. main() is made static in Java so that it can be accessed without creating the instance of a Class. In case, main is not made static then the compiler will throw an error as main() is called by the JVM before any objects are made and only static methods can be directly invoked via the class. void: It is the return type of the method. Void defines the method which will not return any value.main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.String args[]: It is the parameter passed to the main method.## Q3. Why Java is platform independent?

Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.

Q4. Why Java is not 100% Object-oriented?

Java is not 100% Object-oriented because it makes use of eight primitive data types such as boolean, byte, char, int, float, double, long, short which are not objects.

Q5. What are wrapper classes in Java?

Wrapper classes convert the Java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class. Refer to the below image which displays different primitive type, wrapper class and constructor argument.

Q6. What are constructors in Java?

In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

There are two types of constructors:
Default Constructor: In Java, a default constructor is the one which does not take any inputs. In other words, default constructors are the no argument constructors which will be created by default in case you no other constructor is defined by the user. Its main purpose is to initialize the instance variables with the default values. Also, it is majorly used for object creation. Parameterized Constructor: The parameterized constructor in Java, is the constructor which is capable of initializing the instance variables with the provided values. In other words, the constructors which take the arguments are called parameterized constructors.## Q7. What is singleton class in Java and how can we make a class singleton?

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Q8. What is the difference between Array list and vector in Java?

Q9. What is the difference between equals() and == in Java?

Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.

“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

Q10. What are the differences between Heap and Stack Memory in Java?

The major difference between Heap and Stack memory are:

Q11. What is a package in Java? List down various advantages of packages.

Packages in Java, are the collection of related classes and interfaces which are bundled together. By using packages, developers can easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused. Below I have listed down a few of its advantages:
Packages help in avoiding name clashesThey provide easier access control on the codePackages can also contain hidden classes which are not visible to the outer classes and only used within the packageCreates a proper hierarchical structure which makes it easier to locate the related classes## Q12. Why pointers are not used in Java?

Java doesn’t use pointers because they are unsafe and increases the complexity of the program. Since, Java is known for its simplicity of code, adding the concept of pointers will be contradicting. Moreover, since JVM is responsible for implicit memory allocation, thus in order to avoid direct access to memory by the user, pointers are discouraged in Java.

Q13. What is JIT compiler in Java?

JIT stands for Just-In-Time compiler in Java. It is a program that helps in converting the Java bytecode into instructions that are sent directly to the processor. By default, the JIT compiler is enabled in Java and is activated whenever a Java method is invoked. The JIT compiler then compiles the bytecode of the invoked method into native machine code, compiling it “just in time” to execute. Once the method has been compiled, the JVM summons the compiled code of that method directly rather than interpreting it. This is why it is often responsible for the performance optimization of Java applications at the run time.

Q14. What are access modifiers in Java?

In Java, access modifiers are special keywords which are used to restrict the access of a class, constructor, data member and method in another class. Java supports four types of access modifiers:
DefaultPrivateProtected**Public

Q15. Define a Java Class.

A class in Java is a blueprint which includes all your data. A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class.

class Abc {
member variables // class body
methods}

Q16. What is an object in Java and how is it created?

An object is a real-world entity that has a state and behavior. An object has three characteristics:
StateBehaviorIdentity
An object is created using the ‘new’ keyword. For example:

ClassName obj = new ClassName();

Q17. What is Object Oriented Programming?

Object-oriented programming or popularly known as OOPs is a programming model or approach where the programs are organized around objects rather than logic and functions. In other words, OOP mainly focuses on the objects that are required to be manipulated instead of logic. This approach is ideal for the programs large and complex codes and needs to be actively updated or maintained.

Q18. What are the main concepts of OOPs in Java?

Object-Oriented Programming or OOPs is a programming style that is associated with concepts like:
*Inheritance: *Inheritance is a process where one class acquires the properties of another.*Encapsulation: Encapsulation in Java is a mechanism of wrapping up the data and code together as a single unit. Abstraction: *Abstraction is the methodology of hiding the implementation details from the user and only providing the functionality to the users. *Polymorphism: *Polymorphism is the ability of a variable, function or object to take multiple forms.## Q19. What is the difference between a local variable and an instance variable?

In Java, a local variable is typically used inside a method, constructor, or a block and has only local scope. Thus, this variable can be used only within the scope of a block. The best benefit of having a local variable is that other methods in the class won’t be even aware of that variable.

Example

if(x > 100)
{
String test = "Edureka";
}

Whereas, an instance variable in Java, is a variable which is bounded to its object itself. These variables are declared within a class, but outside a method. Every object of that class will create it’s own copy of the variable while using it. Thus, any changes made to the variable won’t reflect in any other instances of that class and will be bound to that particular instance only.

class Test{
public String EmpName;
public int empAge;
}

Q20. Differentiate between the constructors and methods in Java?

Q21. What is final keyword in Java?

**final **is a special keyword in Java that is used as a non-access modifier. A final variable can be used in different contexts such as:
final variable
When the final keyword is used with a variable then its value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.
final method
When a method is declared final then it can’t be overridden by the inheriting class.
final class
When a class is declared as final in Java, it can’t be extended by any subclass class but it can extend other class.

Q22. What is the difference between break and continue statements?

Example break:

for (int i = 0; i < 5; i++)
{
if (i == 3)
{
break;
}
System.out.println(i);
}

Example continue:

for (int i = 0; i < 5; i++)
{
if(i == 2)
{
continue;
}
System.out.println(i);
}

Q23.What is an infinite loop in Java? Explain with an example.

An infinite loop is an instruction sequence in Java that loops endlessly when a functional exit isn’t met. This type of loop can be the result of a programming error or may also be a deliberate action based on the application behavior. An infinite loop will terminate automatically once the application exits.

For example:

public class InfiniteForLoopDemo
{
public static void main(String[] arg) {
for(;;)
System.out.println("Welcome to Edureka!");
// To terminate this program press ctrl + c in the console.
}
}

Q24. What is the difference between this() and super() in Java?

In Java, super() and this(), both are special keywords that are used to call the constructor.

Q25. What is Java String Pool?

Java String pool refers to a collection of Strings which are stored in heap memory. In this, whenever a new object is created, String pool first checks whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable else new object will be created in the String pool and the respective reference will be returned.

Q26. Differentiate between static and non-static methods in Java.

Q27. What is constructor chaining in Java?

In Java, constructor chaining is the process of calling one constructor from another with respect to the current object. Constructor chaining is possible only through legacy where a subclass constructor is responsible for invoking the superclass’ constructor first. There could be any number of classes in the constructor chain. Constructor chaining can be achieved in two ways:
Within the same class using this()From base class using super()## Q28. Difference between String, String Builder, and String Buffer.

Q29. What is a classloader in Java?

The Java ClassLoader is a subset of JVM (Java Virtual Machine) that is responsible for loading the class files. Whenever a Java program is executed it is first loaded by the classloader. Java provides three built-in classloaders:

  1. Bootstrap ClassLoader
  2. Extension ClassLoader
  3. System/Application ClassLoader
Q30. Why Java Strings are immutable in nature?

In Java, string objects are immutable in nature which simply means once the String object is created its state cannot be modified. Whenever you try to update the value of that object instead of updating the values of that particular object, Java creates a new string object. Java String objects are immutable as String objects are generally cached in the String pool. Since String literals are usually shared between multiple clients, action from one client might affect the rest. It enhances security, caching, synchronization, and performance of the application.

Q31. What is the difference between an array and an array list?

Q32. What is a Map in Java?

In Java, Map is an interface of Util package which maps unique keys to values. The Map interface is not a subset of the main Collection interface and thus it behaves little different from the other collection types. Below are a few of the characteristics of Map interface:
Map doesn’t contain duplicate keys.Each key can map at max one value.## Q33. What is collection class in Java? List down its methods and interfaces.

In Java, the collection is a framework that acts as an architecture for storing and manipulating a group of objects. Using Collections you can perform various tasks like searching, sorting, insertion, manipulation, deletion, etc. Java collection framework includes the following:
InterfacesClassesMethods
The below image shows the complete hierarchy of the Java Collection.

In case you are facing any challenges with these java interview questions, please comment on your problems in the section below.

OOPS Java Interview Questions Q1. What is Polymorphism?

Polymorphism is briefly described as “one interface, many implementations”. Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form. There are two types of polymorphism:


Compile time polymorphismRun time polymorphism
Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface.

Q2. What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. Let’s take a look at the example below to understand it better.

class Car {
void run()
{
System.out.println(&ldquo;car is running&rdquo;);
}
}
class Audi extends Car {
void run()
{
System.out.prinltn(&ldquo;Audi is running safely with 100km&rdquo;);
}
public static void main(String args[])
{
Car b= new Audi();    //upcasting
b.run();
}
}

Q3. What is abstraction in Java?

Abstraction refers to the quality of dealing with ideas rather than events. It basically deals with hiding the details and showing the essential things to the user. Thus you can say that abstraction in Java is the process of hiding the implementation details from the user and revealing only the functionality to them. Abstraction can be achieved in two ways:

  1. Abstract Classes (0-100% of abstraction can be achieved)
  2. Interfaces (100% of abstraction can be achieved)
Q4. What do you mean by an interface in Java?

An interface in Java is a blueprint of a class or you can say it is a collection of abstract methods and static constants. In an interface, each method is public and abstract but it does not contain any constructor. Thus, interface basically is a group of related methods with empty bodies. Example:

public interface Animal {

public void eat();

public void sleep();

public void run();

}

Q5. What is the difference between abstract classes and interfaces?

Q6. What is inheritance in Java?

Inheritance in Java is the concept where the properties of one class can be inherited by the other. It helps to reuse the code and establish a relationship between different classes. Inheritance is performed between two types of classes:
Parent class (Super or Base class)Child class (Subclass or Derived class)
A class which inherits the properties is known as Child Class whereas a class whose properties are inherited is known as Parent class.

Q7. What are the different types of inheritance in Java?

Java supports four types of inheritance which are:
Single Inheritance: In single inheritance, one class inherits the properties of another i.e there will be only one parent as well as one child class.**Multilevel Inheritance: **When a class is derived from a class which is also derived from another class, i.e. a class having more than one parent class but at different levels, such type of inheritance is called Multilevel Inheritance.**Hierarchical Inheritance: **When a class has more than one child classes (subclasses) or in other words, more than one child classes have the same parent class, then such kind of inheritance is known as hierarchical.**Hybrid Inheritance: Hybrid inheritance is a combination of two or more types of inheritance.## Q8. What is method overloading and method overriding?

Method Overloading :

In Method Overloading, Methods of the same class shares the same name but each method must have a different number of parameters or parameters having different types and order.Method Overloading is to “add” or “extend” more to the method’s behavior.It is a compile-time polymorphism.The methods must have a different signature.It may or may not need inheritance in Method Overloading.
Let’s take a look at the example below to understand it better.

class Adder {
Static int add(int a, int b)
{
return a+b;
}
Static double add( double a, double b)
{
return a+b;
}
public static void main(String args[])
{
System.out.println(Adder.add(11,11));
System.out.println(Adder.add(12.3,12.6));
}}

**Method Overriding: **

In Method Overriding, the subclass has the same method with the same name and exactly the same number and type of parameters and same return type as a superclass.Method Overriding is to “Change” existing behavior of the method.It is a run time polymorphism.The methods must have the same signature.It always requires inheritance in Method Overriding.
Let’s take a look at the example below to understand it better.

class Car {
void run(){
System.out.println(&ldquo;car is running&rdquo;);
}
Class Audi extends Car{
void run()
{
System.out.prinltn("Audi is running safely with 100km");
}
public static void main( String args[])
{
Car b=new Audi();
b.run();
}
}

Q9. Can you override a private or static method in Java?

You cannot override a private or static method in Java. If you create a similar method with the same return type and same method arguments in child class then it will hide the superclass method; this is known as method hiding. Similarly, you cannot override a private method in subclass because it’s not accessible there. What you can do is create another private method with the same name in the child class. Let’s take a look at the example below to understand it better.

class Base {
private static void display() {
System.out.println("Static or class method from Base");
}
public void print() {
System.out.println("Non-static or instance method from Base");
}
class Derived extends Base {
private static void display() {
System.out.println("Static or class method from Derived");
}
public void print() {
System.out.println("Non-static or instance method from Derived");
}
public class test {
public static void main(String args[])
{
Base obj= new Derived();
obj1.display();
obj1.print();
}
}

Q10. What is multiple inheritance? Is it supported by Java?

If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes.

The problem with multiple inheritance is that if multiple parent classes have the same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class.

Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred to as Diamond Problem.

Q11. What is encapsulation in Java?

Encapsulation is a mechanism where you bind your data(variables) and code(methods) together as a single unit. Here, the data is hidden from the outer world and can be accessed only via current class methods. This helps in protecting the data from any unnecessary modification. We can achieve encapsulation in Java by:
Declaring the variables of a class as private.Providing public setter and getter methods to modify and view the values of the variables.## Q12. What is an association?

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationships can be one to one, one to many, many to one and many to many.

Q13. What do you mean by aggregation?

An aggregation is a specialized form of Association where all object has their own lifecycle but there is ownership and child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belong to multiple departments, but if we delete the department teacher object will not destroy.

Q14. What is composition in Java?

Composition is again a specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object does not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of a relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different houses if we delete the house room will automatically delete.

Q15. What is a marker interface?

A Marker interface can be defined as the interface having no data member and member functions. In simpler terms, an empty interface is called the Marker interface. The most common examples of Marker interface in Java are Serializable, Cloneable etc. The marker interface can be declared as follows.

public interface Serializable{
}

Q16. What is object cloning in Java?

Object cloning in Java is the process of creating an exact copy of an object. It basically means the ability to create an object with a similar state as the original object. To achieve this, Java provides a method clone() to make use of this functionality. This method creates a new instance of the class of the current object and then initializes all its fields with the exact same contents of corresponding fields. To object clone(), the marker interface **java.lang.Cloneable **must be implemented to avoid any runtime exceptions. One thing you must note is Object clone() is a protected method, thus you need to override it.

Q17. What is a copy constructor in Java?

Copy constructor is a member function that is used to initialize an object using another object of the same class. Though there is no need for copy constructor in Java since all objects are passed by reference. Moreover, Java does not even support automatic pass-by-value.

Q18. What is a constructor overloading in Java?

In Java, constructor overloading is a technique of adding any number of constructors to a class each having a different parameter list. The compiler uses the number of parameters and their types in the list to differentiate the overloaded constructors.

class Demo
{
int i;
public Demo(int a)
{
i=k;
}
public Demo(int a, int b)
{
//body
}
}

In case you are facing any challenges with these java interview questions, please comment on your problems in the section below. Apart from this Java Interview Questions Blog, if you want to get trained from professionals on this technology, you can opt for a structured training from edureka!

Servlets Interview Questions Q1. What is a servlet?
  • Java Servlet is server-side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.
  • The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets.
  • All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services.
  • Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image.

Q2. What are the differences between Get and Post methods?

Q3. What is Request Dispatcher?

RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response.

There are two methods defined in this interface:

1.void forward()

2.void include()

Q4. What are the differences between forward() method and sendRedirect() methods?

Q5. What is the life-cycle of a servlet?

There are 5 stages in the lifecycle of a servlet:


Servlet is loadedServlet is instantiatedServlet is initializedService the requestServlet is destroyed## Q6. How does cookies work in Servlets?
Cookies are text data sent by server to the client and it gets saved at the client local machine.Servlet API provides cookies support through javax.servlet.http.Cookie class that implements Serializable and Cloneable interfaces.HttpServletRequest getCookies() method is provided to get the array of Cookies from request, since there is no point of adding Cookie to request, there are no methods to set or add cookie to request.Similarly HttpServletResponse addCookie(Cookie c) method is provided to attach cookie in response header, there are no getter methods for cookie.## Q7. What are the differences between ServletContext vs ServletConfig?

The difference between ServletContext and ServletConfig in Servlets JSP is in below tabular format.

Q8. What are the different methods of session management in servlets?

Session is a conversational state between client and server and it can consists of multiple request and response between client and server. Since HTTP and Web Server both are stateless, the only way to maintain a session is when some unique information about the session (session id) is passed between server and client in every request and response.

Some of the common ways of session management in servlets are:

  1. User Authentication
  2. HTML Hidden Field
  3. Cookies
  4. URL Rewriting
  5. Session Management API

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JDBC Interview Questions 1. What is JDBC Driver?

JDBC Driver is a software component that enables java application to interact with the database. There are 4 types of JDBC drivers:

  1. JDBC-ODBC bridge driver
  2. Native-API driver (partially java driver)
  3. Network Protocol driver (fully java driver)
  4. Thin driver (fully java driver)

2. What are the steps to connect to a database in java?
  • Registering the driver class
  • Creating connection
  • Creating statement
  • Executing queries
  • Closing connection
3. What are the JDBC API components?

The java.sql package contains interfaces and classes for JDBC API.

Interfaces:

ConnectionStatementPreparedStatementResultSetResultSetMetaDataDatabaseMetaDataCallableStatement etc.#### Classes:

  • DriverManager
  • Blob
  • Clob
  • Types
  • SQLException etc.
4. What is the role of JDBC DriverManager class?

The DriverManager* class* manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection.

5. What is JDBC Connection interface?

The Connection interface maintains a session with the database. It can be used for transaction management. It provides factory methods that returns the instance of Statement, PreparedStatement, CallableStatement and DatabaseMetaData.

6. What is the purpose of JDBC ResultSet interface?

The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database.

7. What is JDBC ResultSetMetaData interface?

The ResultSetMetaData interface returns the information of table such as total number of columns, column name, column type etc.

8. What is JDBC DatabaseMetaData interface?

The DatabaseMetaData interface returns the information of the database such as username, driver name, driver version, number of tables, number of views etc.

9. What do you mean by batch processing in JDBC?

Batch processing helps you to group related SQL statements into a batch and execute them instead of executing a single query. By using batch processing technique in JDBC, you can execute multiple queries which makes the performance faster.

10. What is the difference between execute, executeQuery, executeUpdate?

Statement execute(String query) is used to execute any SQL query and it returns TRUE if the result is an ResultSet such as running Select queries. The output is FALSE when there is no ResultSet object such as running Insert or Update queries. We can use getResultSet() to get the ResultSet and getUpdateCount() method to retrieve the update count.

Statement executeQuery(String query) is used to execute Select queries and returns the ResultSet. ResultSet returned is never null even if there are no records matching the query. When executing select queries we should use executeQuery method so that if someone tries to execute insert/update statement it will throw java.sql.SQLException with message “executeQuery method can not be used for update”.

Statement executeUpdate(String query) is used to execute Insert/Update/Delete (DML) statements or DDL statements that returns nothing. The output is int and equals to the row count for SQL Data Manipulation Language (DML) statements. For DDL statements, the output is 0.

You should use execute() method only when you are not sure about the type of statement else use executeQuery or executeUpdate method.

Q11. What do you understand by JDBC Statements?

JDBC statements are basically the statements which are used to send SQL commands to the database and retrieve data back from the database. Various methods like execute(), executeUpdate(), executeQuery, etc. are provided by JDBC to interact with the database.

JDBC supports 3 types of statements:
*Statement: *Used for general purpose access to the database and executes a static SQL query at runtime.*PreparedStatement: *Used to provide input parameters to the query during execution.*CallableStatement: *Used to access the database stored procedures and helps in accepting runtime parameters.
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Spring Interview Questions Q1. What is Spring?

Wikipedia defines the Spring framework as “an application framework and inversion of control container for the Java platform. The framework’s core features can be used by any Java application, but there are extensions for building web applications on top of the Java EE platform.” Spring is essentially a lightweight, integrated framework that can be used for developing enterprise applications in java.

Q2. Name the different modules of the Spring framework.

Some of the important Spring Framework modules are:

  • Spring Context – for dependency injection.
  • Spring AOP – for aspect oriented programming.
  • Spring DAO – for database operations using DAO pattern
  • Spring JDBC – for JDBC and DataSource support.
  • Spring ORM – for ORM tools support such as Hibernate
  • Spring Web Module – for creating web applications.
  • Spring MVC – Model-View-Controller implementation for creating web applications, web services etc.
****Q3. List some of the important annotations in annotation-based Spring configuration.

The important annotations are:

  • @Required
  • @Autowired
  • @Qualifier
  • @Resource
  • @PostConstruct
  • @PreDestroy
Q4. Explain Bean in Spring and List the different Scopes of Spring bean.

Beans are objects that form the backbone of a Spring application. They are managed by the Spring IoC container. In other words, a bean is an object that is instantiated, assembled, and managed by a Spring IoC container.

There are five Scopes defined in Spring beans.

  • Singleton: Only one instance of the bean will be created for each container. This is the default scope for the spring beans. While using this scope, make sure spring bean doesn’t have shared instance variables otherwise it might lead to data inconsistency issues because it’s not thread-safe.
  • Prototype: A new instance will be created every time the bean is requested.
  • Request: This is same as prototype scope, however it’s meant to be used for web applications. A new instance of the bean will be created for each HTTP request.
  • Session: A new bean will be created for each HTTP session by the container.
  • Global-session: This is used to create global session beans for Portlet applications.
Q5. Explain the role of DispatcherServlet and ContextLoaderListener.

DispatcherServlet is basically the front controller in the Spring MVC application as it loads the spring bean configuration file and initializes all the beans that have been configured. If annotations are enabled, it also scans the packages to configure any bean annotated with @Component, @Controller, @Repository or @Service annotations.

ContextLoaderListener, on the other hand, is the listener to start up and shut down the WebApplicationContext in Spring root. Some of its important functions includes tying up the lifecycle of Application Context to the lifecycle of the ServletContext and automating the creation of ApplicationContext.

Q6. What are the differences between constructor injection and setter injection?

Q7. What is autowiring in Spring? What are the autowiring modes?

Autowiring enables the programmer to inject the bean automatically. We don’t need to write explicit injection logic. Let’s see the code to inject bean using dependency injection.

The autowiring modes are given below:

Q8. How to handle exceptions in Spring MVC Framework?

Spring MVC Framework provides the following ways to help us achieving robust exception handling.

Controller Based:

We can define exception handler methods in our controller classes. All we need is to annotate these methods with @ExceptionHandler annotation.

Global Exception Handler:

Exception Handling is a cross-cutting concern and Spring provides @ControllerAdvice annotation that we can use with any class to define our global exception handler.

HandlerExceptionResolver implementation:

For generic exceptions, most of the times we serve static pages. Spring Framework provides HandlerExceptionResolver interface that we can implement to create global exception handler. The reason behind this additional way to define global exception handler is that Spring framework also provides default implementation classes that we can define in our spring bean configuration file to get spring framework exception handling benefits.

Q9. What are some of the important Spring annotations which you have used?

Some of the Spring annotations that I have used in my project are:

@Controller – for controller classes in Spring MVC project.

@RequestMapping – for configuring URI mapping in controller handler methods. This is a very important annotation, so you should go through Spring MVC RequestMapping Annotation Examples

@ResponseBody – for sending Object as response, usually for sending XML or JSON data as response.

@PathVariable – for mapping dynamic values from the URI to handler method arguments.

@Autowired – for autowiring dependencies in spring beans.

@Qualifier – with @Autowired annotation to avoid confusion when multiple instances of bean type is present.

@Service – for service classes.

@Scope – for configuring the scope of the spring bean.

@Configuration, @ComponentScan and @Bean – for java based configurations.

AspectJ annotations for configuring aspects and advices , @Aspect, @Before, @After, @Around, @Pointcut, etc.

Q10. How to integrate Spring and Hibernate Frameworks?

We can use Spring ORM module to integrate Spring and Hibernate frameworks if you are using Hibernate 3+ where SessionFactory provides current session, then you should avoid using HibernateTemplate or HibernateDaoSupport classes and better to use DAO pattern with dependency injection for the integration.

Also, Spring ORM provides support for using Spring declarative transaction management, so you should utilize that rather than going for hibernate boiler-plate code for transaction management.** **

Q11. Name the types of transaction management that Spring supports.

Two types of transaction management are supported by Spring. They are:
**Programmatic transaction management: **In this, the transaction is managed with the help of programming. It provides you extreme flexibility, but it is very difficult to maintain.**Declarative transaction management: **In this, transaction management is separated from the business code. Only annotations or XML based configurations are used to manage the transactions.
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Hibernate Interview Questions 1. What is Hibernate Framework?

Object-relational mapping or ORM is the programming technique to map application domain model objects to the relational database tables. Hibernate is Java-based ORM tool that provides a framework for mapping application domain objects to the relational database tables and vice versa.

Hibernate provides a reference implementation of Java Persistence API, that makes it a great choice as ORM tool with benefits of loose coupling. We can use the Hibernate persistence API for CRUD operations. Hibernate framework provide option to map plain old java objects to traditional database tables with the use of JPA annotations as well as XML based configuration.

Similarly, hibernate configurations are flexible and can be done from XML configuration file as well as programmatically.

2. What are the important benefits of using Hibernate Framework?

Some of the important benefits of using hibernate framework are:
Hibernate eliminates all the boiler-plate code that comes with JDBC and takes care of managing resources, so we can focus on business logic.Hibernate framework provides support for XML as well as JPA annotations, that makes our code implementation independent.Hibernate provides a powerful query language (HQL) that is similar to SQL. However, HQL is fully object-oriented and understands concepts like inheritance, polymorphism, and association.Hibernate is an open source project from Red Hat Community and used worldwide. This makes it a better choice than others because learning curve is small and there are tons of online documentation and help is easily available in forums.Hibernate is easy to integrate with other Java EE frameworks, it’s so popular that Spring Framework provides built-in support for integrating hibernate with Spring applications.Hibernate supports lazy initialization using proxy objects and perform actual database queries only when it’s required.Hibernate cache helps us in getting better performance.For database vendor specific feature, hibernate is suitable because we can also execute native sql queries.
Overall hibernate is the best choice in current market for ORM tool, it contains all the features that you will ever need in an ORM tool.

3. Explain Hibernate architecture.

4. What are the differences between get and load methods?

The differences between get() and load() methods are given below.

5. What are the advantages of Hibernate over JDBC?

Some of the important advantages of Hibernate framework over JDBC are:

  1. Hibernate removes a lot of boiler-plate code that comes with JDBC API, the code looks cleaner and readable.
  2. Hibernate supports inheritance, associations, and collections. These features are not present with JDBC API.
  3. Hibernate implicitly provides transaction management, in fact, most of the queries can’t be executed outside transaction. In JDBC API, we need to write code for transaction management using commit and rollback.
  4. JDBC API throws SQLException that is a checked exception, so we need to write a lot of try-catch block code. Most of the times it’s redundant in every JDBC call and used for transaction management. Hibernate wraps JDBC exceptions and throw JDBCException or HibernateException un-checked exception, so we don’t need to write code to handle it. Hibernate built-in transaction management removes the usage of try-catch blocks.
  5. Hibernate Query Language (HQL) is more object-oriented and close to Java programming language. For JDBC, we need to write native SQL queries.
  6. Hibernate supports caching that is better for performance, JDBC queries are not cached hence performance is low.
  7. Hibernate provides option through which we can create database tables too, for JDBC tables must exist in the database.
  8. Hibernate configuration helps us in using JDBC like connection as well as JNDI DataSource for the connection pool. This is a very important feature in enterprise application and completely missing in JDBC API.
  9. Hibernate supports JPA annotations, so the code is independent of the implementation and easily replaceable with other ORM tools. JDBC code is very tightly coupled with the application.

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Java Interview Questions: JSP 1. What are the life-cycle methods for a jsp?

2. What are the JSP implicit objects?

JSP provides 9 implicit objects by default. They are as follows:

3. What are the differences between include directive and include action?

4. How to disable caching on back button of the browser?

<%

response.setHeader(“Cache-Control”,”no-store”);

response.setHeader(“Pragma”,”no-cache”);

response.setHeader (“Expires”, “0”); //prevents caching at the proxy server

%>

5. What are the different tags provided in JSTL?

There are 5 type of JSTL tags.

  1. core tags
  2. sql tags
  3. xml tags
  4. internationalization tags
  5. functions tags
6. How to disable session in JSP?

7. How to delete a Cookie in a JSP?

The following code explains how to delete a Cookie in a JSP :

Cookie mycook = new Cookie("name1","value1");
 
response.addCookie(mycook1);
 
Cookie killmycook = new Cookie("mycook1","value1");
 
killmycook . set MaxAge ( 0 );
 
killmycook . set Path ("/");
 
killmycook . addCookie ( killmycook 1 );

8. Explain the jspDestroy() method.

jspDestry() method is invoked from javax.servlet.jsp.JspPage interface whenever a JSP page is about to be destroyed. Servlets destroy methods can be easily overridden to perform cleanup, like when closing a database connection.

9. How is JSP better than Servlet technology?

JSP is a technology on the server’s side to make content generation simple. They are document-centric, whereas servlets are programs. A Java server page can contain fragments of Java program, which execute and instantiate Java classes. However, they occur inside an HTML template file. It provides the framework for the development of a Web Application.

10. Why should we not configure JSP standard tags in web.xml?

We don’t need to configure JSP standard tags in web.xml because when container loads the web application and find TLD files, it automatically configures them to be used directly in the application JSP pages. We just need to include it in the JSP page using taglib directive.

11. How will you use JSP EL in order to get the HTTP method name?

Using pageContext JSP EL implicit object you can get the request object reference and make use of the dot operator to retrieve the HTTP method name in the JSP page. The JSP EL code for this purpose will look like ${pageContext.request.method}.

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Exception and Thread Java Interview Questions Q1. What is the difference between Error and Exception?

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors you cannot repair them at runtime. Though error can be caught in the catch block but the execution of application will come to a halt and is not recoverable.

While exceptions are conditions that occur because of bad input or human error etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving the user feedback for entering proper values etc.

Q2. How can you handle Java exceptions?

There are five keywords used to handle exceptions in Java:
trycatchfinallythrowthrows## Q3. What are the differences between Checked Exception and Unchecked Exception?

Checked Exception

The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions. Checked exceptions are checked at compile-time.Example: IOException, SQLException etc.#### Unchecked Exception
The classes that extend RuntimeException are known as unchecked exceptions. Unchecked exceptions are not checked at compile-time.Example: ArithmeticException, NullPointerException etc.## Q4. What purpose do the keywords final, finally, and finalize fulfill?

Final:

Final is used to apply restrictions on class, method, and variable. A final class can’t be inherited, final method can’t be overridden and final variable value can’t be changed. Let’s take a look at the example below to understand it better.

class FinalVarExample {
public static void main( String args[])
{
final int a=10;   // Final variable
a=50;             //Error as value can't be changed
}

**Finally **

Finally is used to place important code, it will be executed whether the exception is handled or not. Let’s take a look at the example below to understand it better.

class FinallyExample {
public static void main(String args[]){
try {
int x=100;
}
catch(Exception e) {
System.out.println(e);
}
finally {
System.out.println("finally block is executing");}
}}
}

**Finalize **

Finalize is used to perform clean up processing just before the object is garbage collected. Let’s take a look at the example below to understand it better.

class FinalizeExample {
public void finalize() {
System.out.println("Finalize is called");
}
public static void main(String args[])
{
FinalizeExample f1=new FinalizeExample();
FinalizeExample f2=new FinalizeExample();
f1= NULL;
f2=NULL;
System.gc();
}
}

**Q5. What are the differences between throw and throws? **

Q6. What is exception hierarchy in java?

The hierarchy is as follows:

Throwable is a parent class of all Exception classes. There are two types of Exceptions: Checked exceptions and UncheckedExceptions or RunTimeExceptions. Both type of exceptions extends Exception class whereas errors are further classified into Virtual Machine error and Assertion error.

Q7. How to create a custom Exception?

To create you own exception extend the Exception class or any of its subclasses.

  • class New1Exception extends Exception { } // this will create Checked Exception
  • class NewException extends IOException { } // this will create Checked exception
  • class NewException extends NullPonterExcpetion { } // this will create UnChecked exception
Q8. What are the important methods of Java Exception Class?

Exception and all of it’s subclasses doesn’t provide any specific methods and all of the methods are defined in the base class Throwable.

  1. String getMessage() – This method returns the message String of Throwable and the message can be provided while creating the exception through it’s constructor.
  2. String getLocalizedMessage() – This method is provided so that subclasses can override it to provide locale specific message to the calling program. Throwable class implementation of this method simply use getMessage() method to return the exception message.
  3. Synchronized Throwable getCause() – This method returns the cause of the exception or null id the cause is unknown.
  4. String toString() – This method returns the information about Throwable in String format, the returned String contains the name of Throwable class and localized message.
  5. void printStackTrace() – This method prints the stack trace information to the standard error stream, this method is overloaded and we can pass PrintStream or PrintWriter as an argument to write the stack trace information to the file or stream.
Q9. What are the differences between processes and threads?

Q10. What is a finally block? Is there a case when finally will not execute?

Finally block is a block which always executes a set of statements. It is always associated with a try block regardless of any exception that occurs or not.

Yes, finally will not be executed if the program exits either by calling System.exit() or by causing a fatal error that causes the process to abort.

Q11. What is synchronization?

Synchronization refers to multi-threading. A synchronized block of code can be executed by only one thread at a time. As Java supports execution of multiple threads, two or more threads may access the same fields or objects. Synchronization is a process which keeps all concurrent threads in execution to be in sync. Synchronization avoids memory consistency errors caused due to inconsistent view of shared memory. When a method is declared as synchronized the thread holds the monitor for that method’s object. If another thread is executing the synchronized method the thread is blocked until that thread releases the monitor.

**Q12. Can we write multiple catch blocks under single try block? **

Yes we can have multiple catch blocks under single try block but the approach should be from specific to general. Let’s understand this with a programmatic example.

public class Example {
public static void main(String args[]) {
try {
int a[]= new int[10];
a[10]= 10/0;
}
catch(ArithmeticException e)
{
System.out.println("Arithmetic exception in first catch block");
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("Array index out of bounds in second catch block");
}
catch(Exception e)
{
System.out.println("Any exception in third catch block");
}
}

Q13. What are the important methods of Java Exception Class?

Methods are defined in the base class Throwable. Some of the important methods of Java exception class are stated below.
String getMessage() – This method returns the message String about the exception. The message can be provided through its constructor.**public StackTraceElement[] getStackTrace() – **This method returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack whereas the last element in the array represents the method at the bottom of the call stack.Synchronized Throwable getCause() – This method returns the cause of the exception or null id as represented by a Throwable object.String toString() – This method returns the information in String format. The returned String contains the name of Throwable class and localized message.void printStackTrace() – This method prints the stack trace information to the standard error stream. ## Q14. What is OutOfMemoryError in Java?

OutOfMemoryError is the subclass of java.lang.Error which generally occurs when our JVM runs out of memory.

Q15. What is a Thread?

A thread is the smallest piece of programmed instructions which can be executed independently by a scheduler. In Java, all the programs will have at least one thread which is known as the main thread. This main thread is created by the JVM when the program starts its execution. The main thread is used to invoke the main() of the program.

Q16. What are the two ways to create a thread?

In Java, threads can be created in the following two ways:-

  • By implementing the Runnable interface.
  • By extending the Thread
Q17. What are the different types of garbage collectors in Java?

Garbage collection in Java a program which helps in implicit memory management. Since in Java, using the new keyword you can create objects dynamically, which once created will consume some memory. Once the job is done and there are no more references left to the object, Java using garbage collection destroys the object and relieves the memory occupied by it. Java provides four types of garbage collectors:
Serial Garbage CollectorParallel Garbage CollectorCMS Garbage CollectorG1 Garbage Collector
In case you are facing any challenges with these java interview questions, please comment your problems in the section below. Apart from this Java Interview Questions Blog, if you want to get trained from professionals on this technology, you can opt for structured training from edureka!

So this brings us to the end of the Java interview questions blog. The topics that you learned in this Java Interview Questions blog are the most sought-after skill sets that recruiters look for in a Java Professional. These set of Java Interview Questions will definitely help you ace your job interview. Good luck with your interview!

50+ Java Interview Questions for Programmers

50+ Java Interview Questions for Programmers

This article contains more than 50 Java Interview questions covering all important topics like core Java fundamentals, Java Collection Framework, Java Multithreading and Concurrency, Java IO, JDBC, JVM Internals, Coding Problems, Object-Oriented programming, etc.

This article contains more than 50 Java Interview questions covering all important topics like core Java fundamentals, Java Collection Framework, Java Multithreading and Concurrency, Java IO, JDBC, JVM Internals, Coding Problems, Object-Oriented programming, etc.

Are you ready for your next Java interview?

Hello, guys! Recently, I have been sharing a lot of Java Interview questions and discussion individually, and many of my readers requested to bring them together so that they can have them in the same page and prepare better and this post is the result of that.

This article contains more than 50 Java Interview questions covering all important topics like core Java fundamentals, Java Collection Framework, Java Multithreading and Concurrency, Java IO, JDBC, JVM Internals, Coding ProblemsObject-Oriented programming, etc.

The questions are also picked up from various interviews and they are, by no means, very difficult, and you might have seen them already in your telephonic or face-to-face round of interview.

The questions are also very good to revise important topics like multithreading and collections as I have also shared some useful resources for further learning and improvement like The Complete Java MasterClass to brush up and fill gaps in your Java skills.

So what are we waiting for here is the list of some of the frequently asked Java questions from interviews from both beginner and experienced Java developer of 2 to 5 years experience:

Java Interview Questions and Answers
  1. How Java achieves platform independence? (answer)
  2. hint: bytecode and Java Virtual Machine
  3. What is **ClassLoader** in Java? (answer)
  4. hint: part of JVM that loads bytecodes for classes. You can write your own.
  5. Write a Java program to check if a number is Even or Odd? (answer)
  6. hint: you can use bitwise operator, e.g. bitwise AND &, remember, even the number has zero at the end in binary format and an odd number has 1 in the end.
  7. Difference between **ArrayList** and **HashSet** in Java? (answer)
  8. hint: all differences between List and Set are applicable here, e.g. ordering, duplicates, random search, etc.
  9. What is double checked locking in Singleton? (answer)
  10. hint: two-time check whether instances is initialized or not, first without locking and second with locking.

6) How do you create thread-safe Singleton in Java? (answer)

hint: many ways, e.g. using Enum or by using double-checked locking pattern or using a nested static class.

7) When to use volatile variable in Java? (answer)

hint: when you need to instruct the JVM that a variable can be modified by multiple threads and give hint to JVM that does not cache its value.

8) When to use a transient variable in Java? (answer)

hint: when you want to make a variable non-serializable in a class, which implements the Serializable interface. In other words, you can use it for a variable whose value you don’t want to save. See The Complete Java MasterClass to learn about transient variables in Java.

9) Difference between the transient and volatile variable in Java? (answer)

hint: totally different, one used in the context of serialization while the other is used in concurrency.

10) Difference between Serializable and Externalizable in Java? (answer)

hint: Externalizable gives you more control over the Serialization process.

11) Can we override the private method in Java? (answer)

hint: No, because it’s not visible in the subclass, a primary requirement for overriding a method in Java.

12) Difference between **Hashtable** and **HashMap** in Java? (answer)

hint: several but most important is Hashtable, which is synchronized, while HashMap is not. It’s also legacy and slow as compared to HashMap.

13) Difference between **List**and **Set**in Java? (answer)

hint: List is ordered and allows duplicate. Set is unordered and doesn’t allow duplicate elements.

14) Difference between **ArrayList** and **Vector** in Java (answer)

hint: Many, but most important is that ArrayList is non-synchronized and fast while Vector is synchronized and slow. It’s also legacy class like Hashtable.

15) Difference between **Hashtable** and **ConcurrentHashMap** in Java? (answer)

hint: more scalable

16) How does **ConcurrentHashMap** achieve scalability? (answer)

hint: by dividing the map into segments and only locking during the write operation.

17) Which two methods you will override for an **Object** to be used as **Key** in **HashMap**? (answer)

hint: equals and hashcode

18) Difference between wait and sleep in Java? (answer)

hint: The wait() method releases the lock or monitor, while sleep doesn’t.

19) Difference between **notify** and **notifyAll** in Java? (answer)

hint: notify notifies one random thread is waiting for that lock while notifyAll inform to all threads waiting for a monitor. If you are certain that only one thread is waiting then use notify, or else notifyAll is better. See Threading Essentials Mini-Course by Java Champion Heinz Kabutz to learn more about threading basics.

20) Why you override hashcode, along with **equals()** in Java? (answer)

hint: to be compliant with equals and hashcode contract, which is required if you are planning to store your object into collection classes, e.g. HashMap or ArrayList.

21) What is the load factor of **HashMap** means? (answer)

hint: The threshold that triggers the re-sizing of HashMap is generally 0.75, which means HashMap resize itself if it’s 75 percent full.

22) Difference between **ArrayList** and **LinkedList** in Java? (answer)

hint: same as an array and linked list, one allows random search while other doesn’t. Insertion and deletion easy on the linked list but a search is easy on an array. See Java Fundamentals: Collections Richard Warburton course on Pluralsight to learn more about essential Collection data structure in Java.

23) Difference between **CountDownLatch** and **CyclicBarrier** in Java? (answer)

hint: You can reuse CyclicBarrier after the barrier is broken but you cannot reuse CountDownLatch  after the count reaches to zero.

24) When do you use **Runnable** vs **Thread** in Java? (answer)

hint: always

25) What is the meaning of Enum being type-safe in Java? (answer)

hint: It means you cannot assign an instance of different Enum type to an Enum variable. e.g. if you have a variable like DayOfWeek day then you cannot assign it value from DayOfMonth enum.

26) How does Autoboxing of Integer work in Java? (answer)

hint: using valueOf() method

27) Difference between **PATH** and **Classpath** in Java? (answer)

hint: PATH is used by the operating system while Classpath is used by JVM to locate Java binary, e.g. JAR files or Class files. See Java Fundamentals: The Core Platform to learn more about PATHClasspath, and other Java environment variable.

28) Difference between method overloading and overriding in Java? (answer)

hint: Overriding happens at subclass while overloading happens in the same class. Also, overriding is a runtime activity while overloading is resolved at compile time.

29) How do you prevent a class from being sub-classed in Java? (answer)

hint: just make its constructor private

30) How do you restrict your class from being used by your client? (answer)

hint: make the constructor private or throw an exception from the constructor

31) Difference between **StringBuilder** and **StringBuffer** in Java? (answer)

hint: StringBuilder is not synchronized while StringBuffer is synchronized.

32) Difference between Polymorphism and Inheritance in Java? (answer)

hint: Inheritance allows code reuse and builds the relationship between class, which is required by Polymorphism, which provides dynamic behavior. See Java Fundamentals: Object-Oriented Design to learn more about OOP features.

33) Can we override static method in Java? (answer)

hint: No, because overriding resolves at runtime while static method call is resolved at compile time.

34) Can we access the private method in Java? (answer)

hint: yes, in the same class but not outside the class

35) Difference between interface and abstract class in Java? (answer)

hint: from Java 8, the difference is blurred. However, a Java class can still implement multiple interfaces but can only extend one class.

36) Difference between DOM and SAX parser in Java? (answer)

hint: DOM loads whole XML File in memory while SAX doesn’t. It is an event-based parser and can be used to parse a large file, but DOM is fast and should be preferred for small files.

37) Difference between throw and throws keyword in Java? (answer)

hint: throws declare what exception a method can throw in case of error but throw keyword actually throws an exception. See Java Fundamentals: Exception Handling to learn more about Exception handling in Java.

38) Difference between fail-safe and fail-fast iterators in Java? (answer)

hint: fail-safe doesn’t throw ConcurrentModificationException while fail-fast does whenever they detect an outside change on the underlying collection while iterating over it.

39) Difference between Iterator and Enumeration in Java? (answer)

hint: Iterator also gives you the ability to remove an element while iterating while Enumeration doesn’t allow that.

40) What is **IdentityHashMap** in Java? (answer)

hint: A Map, which uses the == equality operator to check equality instead of the equals() method.

41) What is **String** pool in Java? (answer)

hint: A pool of String literals. Remember it’s moved to heap from perm gen space in JDK 7.

42) Can a **Serializable** class contain a non-serializable field in Java? (answer)

hint: Yes, but you need to make it either static or transient.

43) Difference between this and super in Java? (answer)

hint: this refers to the current instance while super refers to an instance of the superclass.

44) Difference between **Comparator** and **Comparable** in Java? (answer)

hint: Comparator defines custom ordering while Comparable defines the natural order of objects, e.g. the alphabetic order for String. See The Complete Java MasterClass to learn more about sorting in Java.

45) Difference between **java.util.Date** and **java.sql.Date** in Java? (answer)

hint: former contains both date and time while later contains only date part.

46) Why wait and notify method are declared in **Object** class in Java? (answer)

hint: because they require lock which is only available to an object.

47) Why Java doesn’t support multiple inheritances? (answer)

hint: It doesn’t support because of bad experience with C++, but with Java 8, it does in some sense — only multiple inheritances of Type are not supported in Java now.

48) Difference between checked and unchecked Exception in Java? (answer)

hint: In case of checked, you must handle exception using catch block, while in case of unchecked, it’s up to you; compile will not bother you.

49) Difference between Error and Exception in Java? (answer)

hint: I am tired of typing please check the answer

50) Difference between race condition and deadlock in Java? (answer)

hint: both are errors that occur in a concurrent application, one occurs because of thread scheduling while others occur because of poor coding.

Best Java Microservices Interview Questions In 2019

Best Java Microservices Interview Questions In 2019

In this Microservices interview questions article, I have collected the most frequently asked questions by interviewers. These questions are collected after consulting with Microservices Certification Training experts. Let's discuss the best Java Microservices Interview Questions In 2019

Top Microservices Interview Questions

According to Gartner, **microservices **are the new application platform for cloud development. **Microservices **are deployed and managed independently, and once implemented inside containers they have very little interaction with the underlying OS. So, if you are planning to start your career in the **Microservices **and you wish to know the skills related to it, now is the right time to dive in, when the technology is in its nascent state.

In case you have attended any **Microservices interview **in the recent past, do paste those interview questions in the comments section and we’ll answer them ASAP. You can also comment below if you have any **questions **in your mind, which you might face in your Microservices interview.

You may go through this recording of Microservices Interview Questions and **Answers **where our instructor has explained the topics in a detailed manner with examples that will help you to **understand **this concept better.

Q1. List down the advantages of Java Microservices Architecture.

Q2. What do you know about Microservices?

Microservices, aka Microservice Architecture, is an architectural style that structures an application as a collection of small autonomous services, modeled around a **business domain.**In layman terms, you must have seen how bees build their honeycomb by aligning hexagonal wax cells.They initially start with a small section using various materials and continue to build a large beehive out of it.These cells form a pattern resulting in a strong structure which holds together a particular section of the beehive.Here, each cell is independent of the other but it is also correlated with the other cells.This means that damage to one cell does not damage the other cells, so, bees can reconstruct these cells without impacting the complete beehive.

Fig 1: Beehive Representation of Microservices – Microservices Interview Questions

Refer to the above diagram. Here, each hexagonal shape represents an individual service component. Similar to the working of bees, each agile team builds an individual service component with the available frameworks and the chosen technology stack. Just as in a beehive, each service component forms a strong microservice architecture to provide better scalability. Also, issues with each service component can be handled individually by the agile team with no or minimal impact on the entire application.

Q3. What are the features of Microservices?

Fig 3: Features of Microservices – Microservices Interview Questions
Decoupling – Services within a system are largely decoupled. So the application as a whole can be easily built, altered, and scaled
Componentization
– Microservices are treated as independent components that can be easily replaced and upgradedBusiness Capabilities – Microservices are very simple and focus on a single capabilityAutonomy – Developers and teams can work independently of each other, thus increasing speedContinous Delivery – Allows frequent releases of software, through systematic automation of software creation, testing, and approvalResponsibility – Microservices do not focus on applications as projects. Instead, they treat applications as products for which they are responsibleDecentralized Governance – The focus is on using the right tool for the right job. That means there is no standardized pattern or any technology pattern. Developers have the freedom to choose the best useful tools to solve their problemsAgility – Microservices support agile development. Any new feature can be quickly developed and discarded again## Q4. What are the best practices to design Microservices?

The following are the best practices to design microservices:

**Fig 4: **Best Practices to Design Microservices – Microservices Interview Questions

Q5. How does Microservice Architecture work?

A microservice architecture has the following components:

**Fig 5: **Architecture of Microservices – Microservices Interview Questions
Clients – Different users from various devices send requests.Identity Providers – Authenticates user or clients identities and issues security tokens.API Gateway – Handles client requests.Static Content – Houses all the content of the system.Management – Balances services on nodes and identifies failures.Service Discovery – A guide to find the route of communication between microservices.Content Delivery Networks – Distributed network of proxy servers and their data centers.Remote Service – Enables the remote access information that resides on a network of IT devices.## Q6. What are the pros and cons of Microservice Architecture?

Q7. What is the difference between Monolithic, SOA and Microservices Architecture?

**Fig 6: **Comparison Between Monolithic SOA & Microservices – Microservices Interview Questions
Monolithic Architecture is similar to a big container wherein all the software components of an application are assembled together and tightly packaged.A Service-Oriented Architecture is a collection of services which communicate with each other. The communication can involve either simple data passing or it could involve two or more services coordinating some activity.Microservice Architecture is an architectural style that structures an application as a collection of small autonomous services, modeled around a business domain.## Q8. What are the challenges you face while working Microservice Architectures?

Developing a number of smaller microservices sounds easy, but the challenges often faced while developing them are as follows.
Automate the Components: Difficult to automate because there are a number of smaller components. So for each component, we have to follow the stages of Build, Deploy and, Monitor.Perceptibility: Maintaining a large number of components together becomes difficult to deploy, maintain, monitor and identify problems. It requires great perceptibility around all the components.Configuration Management: Maintaining the configurations for the components across the various environments becomes tough sometimes.Debugging: Difficult to find out each and every service for an error. It is essential to maintain centralized logging and dashboards to debug problems.## Q9. What are the key differences between SOA and Microservices Architecture?

The key differences between SOA and microservices are as follows:

Q10. What are the characteristics of Microservices?

You can list down the characteristics of microservices as follows:

**Fig 7: **Characteristics of Microservices – Microservices Interview Questions

Q11. What is Domain Driven Design?

**Fig 8: **Principles of DDD – Microservices Interview Questions

Q12. Why there is a need for Domain Driven Design (DDD)?

**Fig 9: **Factors Why we need DDD – Microservices Interview Questions

Q13. What is Ubiquitous language?

If you have to define the** Ubiquitous Language (UL)**, then it is a common language used by developers and users of a specific domain through which the domain can be explained easily.

The ubiquitous language has to be crystal clear so that it brings all the team members on the same page and also translates in such a way that a machine can understand.

Q14. What is Cohesion?

The degree to which the elements inside a module belong together is said to be cohesion.

Q15. What is Coupling?

The measure of the strength of the dependencies between components is said to be coupling. A good design is always said to have High Cohesion and** Low Coupling**.

Q16. What is REST/RESTful and what are its uses?

Representational State Transfer (REST)/RESTful web services are an architectural style to help computer systems communicate over the internet. This makes microservices easier to understand and implement.

Microservices can be implemented with or without RESTful APIs, but it’s always easier to build loosely coupled microservices using RESTful APIs.

Q17. What do you know about Spring Boot?

It’s a knows fact that spring has become more and more complex as new functionalities have been added. If you have to start a new spring project, then you have to add build path or add maven dependencies, configure application server, add spring configuration. So everything has to be done from scratch.

Spring Boot is the solution to this problem. Using spring boot you can avoid all the boilerplate code and configurations. So basically consider yourself as if you’re baking a cake spring is like the ingredients that are required to make the cake and spring boot is the complete cake in your hand.

**Fig 10: **Factors of Spring Boot – Microservices Interview Questions

Q18. What is an actuator in Spring boot?

Spring Boot actuator provides restful web services to access the current state of running an application in the production environment. With the help of actuator, you can check various metrics and monitor your application.

Q19. What is Spring Cloud?

According to the official website of Spring Cloud, Spring Cloud provides tools for developers to quickly build some of the common patterns in distributed systems (e.g. configuration management, service discovery, circuit breakers, intelligent routing, leadership election, distributed sessions, cluster state).

Q20. What problems are solved by Spring Cloud?

While developing distributed microservices with Spring Boot we face few issues which are solved by Spring Cloud.
**The complexity associated with distributed systems – **This includes network issues, Latency overhead, Bandwidth issues, security issues.**Ability to handle Service Discovery – **Service discovery allows processes and services in a cluster to find each other and communicate.**Solved redundancy issues – **Redundancy issues often occur in distributed systems.**Load balancing – **Improves the distribution of workloads across multiple computing resources, such as a computer cluster, network links, central processing units.**Reduces performance issues – **Reduces performance issues due to various operational overheads.## Q21. What is the use of WebMvcTest annotation in Spring MVC applications?

WebMvcTest annotation is used for unit testing Spring MVC Applications in cases where the test objective is to just focus on Spring MVC Components. In the snapshot shown above, we want to launch only the ToTestController. All other controllers and mappings will not be launched when this unit test is executed.

Q22. Can you give a gist about Rest and Microservices?

REST

Though you can implement microservices in multiple ways, REST over HTTP is a way to implement Microservices. REST is also used in other applications such as web apps, API design, and MVC applications to serve business data.

Microservices

Microservices is an architecture wherein all the components of the system are put into individual components, which can be built, deployed, and scaled individually. There are certain principles and best practices of Microservices that help in building a resilient application.

In a nutshell, you can say that REST is a medium to build Microservices.

Q23. What are different types of Tests for Microservices?

While working with microservices, testing becomes quite complex as there are multiple microservices working together. So, tests are divided into different levels.
At the bottom level, we have technology-facing tests like- unit tests and performance tests. These are completely automated.At the middle level, we have tests for exploratory testing like the stress tests and usability tests.At the **top level, **we have acceptance tests that are few in number. These acceptance tests help stakeholders in understanding and verifying software features.## Q24. What do you understand by Distributed Transaction?

Distributed Transaction is any situation where a single event results in the mutation of two or more separate sources of data which cannot be committed atomically. In the world of microservices, it becomes even more complex as each service is a unit of work and most of the time multiple services have to work together to make a business successful.

Q25. What is an Idempotence and where it is used?

Idempotence is the property of being able to do something twice in such a way that the end result will remain the same i.e. as if it had been done once only.

Usage: Idempotence is used at the remote service, or data source so that, when it receives the instruction more than once, it only processes the instruction once.

Q26. What is Bounded Context?

Bounded Context is a central pattern in Domain-Driven Design. It is the focus of DDD’s strategic design section which is all about dealing with large models and teams. DDD deals with large models by dividing them into different Bounded Contexts and being explicit about their inter-relationships.

Q27. What is Two Factor Authentication?

Two-factor authentication enables the second level of authentication to an account log-in process.

**Fig11: **Representation of Two Factor Authentication – Microservices Interview Questions

So suppose a user has to enter only username and password, then that’s considered a single-factor authentication.

Q28. What are the types of credentials of Two Factor Authentication?

The three types of credentials are:

**Fig 12: **Types of Credentials of Two Factor Authentication – Microservices Interview Questions

Q29. What are Client certificates?

A type of digital certificate that is used by client systems to make authenticated requests to a remote server is known as the** client certificate**. Client certificates play a very important role in many mutual authentication designs, providing strong assurances of a requester’s identity.

Q30. What is the use of PACT in Microservices architecture?

**PACT **is an open source tool to allow testing interactions between service providers and consumers in isolation against the contract made so that the reliability of Microservices integration increases.

Usage in Microservices:

Used to implement Consumer Driven Contract in Microservices.Tests the consumer-driven contracts between consumer and provider of a Microservice.## Q31. What is OAuth?

**OAuth **stands for open authorization protocol. This allows accessing the resources of the resource owner by enabling the client applications on HTTP services such as third-party providers Facebook, GitHub, etc. So with this, you can share resources stored on one site with another site without using their credentials.

Q32. What is Conway’s law?

“Any organization that designs a system (defined broadly) will produce a design whose structure is a copy of the organization’s communication structure.” –*** Mel Conway***

**Fig 13: **Representation of Conway’s Law – Microservices Interview Questions

This law basically tries to convey the fact that, in order for a software module to function, the complete team should communicate well. Therefore the structure of a system reflects the social boundaries of the organization(s) that produced it.

Q33. What do you understand by Contract Testing?

According to Martin Flower,** contract test **is a test at the boundary of an external service which verifies that it meets the contract expected by a consuming service.

Also, contract testing does not test the behavior of the service in depth. Rather, it tests that the inputs & outputs of service calls contain required attributes and the response latency, throughput is within allowed limits.

Q34. What is End to End Microservices Testing?

End-to-end testing validates each and every process in the workflow is functioning properly. This ensures that the system works together as a whole and satisfies all requirements.

In layman terms, you can say that end to end testing is a kind of tests where everything is tested after a particular period.

**Fig 14: **Hierarchy of Tests – Microservices Interview Questions

Q35. What is the use of Container in Microservices?

Containers are a good way to manage microservice based application to develop and deploy them individually*.* You can encapsulate your microservice in a container image along with its dependencies, which then can be used to roll on-demand instances of microservice without any additional efforts required.

**Fig 15: **Representation of Containers and How they are used in Microservices – Microservices Interview Questions

Q36. What is DRY in Microservices architecture?

DRY stands for Don’t Repeat Yourself. It basically promotes the concept of reusing the code. This results in developing and sharing the libraries which in turn result in tight coupling.

Q37. What is a Consumer-Driven Contract (CDC)?

This is basically a pattern for developing Microservices so that they can be used by external systems. When we work on microservices, there is a particular provider who builds it and there are one or more consumers who use Microservice.

Generally, providers specify the interfaces in an XML document. But in Consumer Driven Contract, each consumer of service conveys the interface expected from the Provider.

Q38. What is the role of Web, RESTful APIs in Microservices?

A microservice architecture is based on a concept wherein all its services should be able to interact with each other to build a business functionality. So, to achieve this, each microservice must have an interface. This makes the web API a very important enabler of microservices. Being based on the open networking principles of the Web, RESTful APIs provide the most logical model for building interfaces between the various components of a microservice architecture.

Q39. What do you understand by Semantic monitoring in Microservices architecture?

Semantic monitoring, also known as** synthetic monitoring** combines automated tests with monitoring the application in order to detect business failing factors.

Q40. How can we perform Cross-Functional testing?

Cross-functional testing is a verification of non-functional requirements, i.e. those requirements which cannot be implemented like a normal feature.

Q41. How can we eradicate non-determinism in tests?

Non-Deterministic Tests (NDT) are basically unreliable tests. So, sometimes it may happen that they pass and obviously sometimes they may also fail. As and when they fail, they are made to re-run to pass.

Some ways to remove non-determinism from tests are as follows:
QuarantineAsynchronousRemote ServicesIsolationTimeResource leaks## Q42. What is the difference between Mock or Stub?

Stub

A dummy object that helps in running the test.Provides fixed behavior under certain conditions which can be hard-coded.Any other behavior of the stub is never tested.
For example, for an empty stack, you can create a stub that just returns true for empty() method. So, this does not care whether there is an element in the stack or not.

Mock

A dummy object in which certain properties are set initially.The behavior of this object depends on the set properties.The object’s behavior can also be tested.
For example, for a Customer object, you can mock it by setting name and age. You can set age as 12 and then test for isAdult() method that will return true for age greater than 18. So, your Mock Customer object works for the specified condition.

Q43. What do you know about Mike Cohn’s Test Pyramid?

Mike Cohn provided a model called Test Pyramid. This describes the kind of automated tests required for software development.

**Fig 16: **Mike Cohn’s Test Pyramid – Microservices Interview Questions

As per pyramid, the number of tests at first layer should be highest. At service layer, the number of tests should be less than at the unit test level, but more than at the end-to-end level.

Q44. What is the purpose of Docker?

Docker provides a container environment that can be used to host any application. In this, the software application and the dependencies which support it are tightly-packaged together.

So, this packaged product is called a Container and since it is done by Docker, it is called Docker container!

Q45. What is Canary Releasing?

Canary Releasing is a technique to reduce the risk of introducing a new software version in production. This is done by slowly rolling out the change to a small subset of users before giving it out to the entire infrastructure, i.e. making it available to everybody.

Q46. What do you mean by Continuous Integration (CI)?

Continuous Integration (CI) is the process of automating the build and testing of code every time a team member commits changes to version control. This encourages developers to share code and unit tests by merging the changes into a shared version control repository after every small task completion.

Q47. What is Continuous Monitoring?

**Continuous monitoring **gets into the depth of monitoring coverage, from in-browser front-end performance metrics, through application performance, and down to host virtualized infrastructure metrics.

Q48. What is the role of an architect in Microservices architecture?

An architect in microservices architecture plays the following roles:
Decides broad strokes about the layout of the overall software system.Helps in deciding the zoning of the components. So, they make sure components are mutually cohesive, but not tightly coupled.Code with developers and learn the challenges faced in day-to-day life.Make recommendations for certain tools and technologies to the team developing microservices.Provide technical governance so that the teams in their technical development follow principles of Microservice.## Q49. Can we create State Machines out of Microservices?

As we know that each Microservice owning its own database is an independently deployable program unit, this, in turn, lets us create a State Machine out of it. So, we can specify different states and events for a particular microservice.

For Example, we can define an Order microservice. An Order can have different states. The transitions of Order states can be independent events in the Order microservice.

Q50. What are Reactive Extensions in Microservices?

Reactive Extensions also are known as Rx. It is a design approach in which we collect results by calling multiple services and then compile a combined response. These calls can be synchronous or asynchronous, blocking or non-blocking. Rx is a very popular tool in distributed systems which works opposite to legacy flows.

Hope these Microservices Interview Questions would help you in your Microservices Architect Interviews.

Introduction to Java String Interview Questions and Answers

Introduction to Java String Interview Questions and Answers

String is one of the most widely used Java Class. Here I am listing some important Java String Interview Questions and Answers.

1. Introduction

The String class is one of the most widely used classes in Java, which prompted language designers to treat it specially. This special behavior makes it one of the hottest topics in Java interviews.

In this tutorial, we’ll go through some of the most common interview questions about String.

2. String Fundamentals

This section consists of questions that concern the String internal structure and memory.

Q1. What is a String in Java?

In Java, a String is represented internally by an array of byte values (or char values before JDK 9).

In versions up to and including Java 8, a String was composed of an immutable array of Unicode characters. However, most characters require only 8 bits (1 byte) to represent them instead of 16 bits (char size).

To improve memory consumption and performance, Java 9 introduced compact Strings. This means that if a String contains only 1-byte characters, it will be represented using Latin-1 encoding. If a String contains at least 1 multi-byte character, it will be represented as 2 bytes per character using UTF-16 encoding.

In C and C++, String is also an array of characters, but in Java, it’s a separate object with its own API.

Q2. How can we create a String object in Java?

java.lang.String defines 13 different ways to create a String. Generally, though, there are two:

  • Through a String literal:
String s = "abc";

  • Through the new keyword:
String s = new String("abc");

All String literals in Java are instances of the String class.

Q3. Is String a Primitive or a Derived Type?

A String is a derived type since it has state and behavior. For example, it has methods like substring(), indexOf(), and _equals(), _which primitives cannot have.

But, since we all use it so often, it has some special characteristics that make it feel like a primitive:

  • While strings are not stored on the call stack like primitives are, they are** stored in a special memory region called the string pool**
  • Like primitives, we can use the _+ _operator on strings
  • And again, like primitives, we can create an instance of a _String _without the _new _keyword

Q4. What are the benefits of strings being immutable?

According to an interview by James Gosling, strings are immutable to improve performance and security.

And actually, we see several benefits to having immutable strings:

  • The string pool is only possible if the strings, once created, are never changed, as they are supposed to be reused
  • The code can safely pass a string to another method, knowing that it can’t be altered by that method
  • Immutably automatically makes this class thread-safe
  • Since this class is thread-safe, there is no need to synchronize common data, which in turn improves performance
  • Since they are guaranteed to not change, their hashcode can be easily cached

Q5. How is a String stored in memory?

According to the JVM Specification, String literals are stored in a runtime constant pool, which is allocated from the JVM’s method area.

Although the method area is logically part of the heap memory, the specification does not dictate the location, memory size, or garbage collection policies. It can be implementation-specific.

This runtime constant pool for a class or interface is constructed when the class or interface is created by the JVM.

Q6. Are interned strings eligible for garbage collection in Java?

Yes, all _String_s in the string pool are eligible for garbage collection if there are no references from the program.

Q7. What is the String constant pool?

The string pool, also known as the String constant pool or the String intern pool, is a special memory region where the JVM stores String instances.

It optimizes application performance by reducing how often and how many strings are allocated:

  • The JVM stores only one copy of a particular String in the pool
  • When creating a new String, the JVM searches in the pool for a String having the same value
  • If found, the JVM returns the reference to that String without allocating any additional memory
  • If not found, then the JVM adds it to the pool (interns it) and returns its reference

Q8. Is String thread-safe? How?

Strings are indeed completely thread-safe because they are immutable. Any class which is immutable automatically qualifies for thread-safety because its immutability guarantees that its instances won’t be changed across multiple threads.

For example, if a thread changes a string’s value, a new String gets created instead of modifying the existing one.

Q9. For which _String _operations is it important to supply a Locale?

The Locale class allows us to differentiate between cultural locales as well as to format our content appropriately.

When it comes to the _String _class, we need it when rendering strings in format or when lower- or upper-casing strings.

In fact, if we forget to do this, we can run into problems with portability, security, and usability.

Q10. What is the underlying character encoding for strings?

According to _String’_s Javadocs for versions up to and including Java 8, Strings are stored in the UTF-16 format internally.

The char data type and java.lang.Character objects are also based on the original Unicode specification, which defined characters as fixed-width 16-bit entities.

Starting with JDK 9, Strings that contain only 1-byte characters use Latin-1 encoding, while Strings with at least 1 multi-byte character use UTF-16 encoding.

3. The String API

In this section, we’ll discuss some questions related to the String API.

Q11. How can we compare two Strings in Java? What’s the difference between str1 == str2 and str1.equals(str2)?

We can compare strings in two different ways: by using equal to operator ( == ) and by using the equals() method.

Both are quite different from each other:

  • **The operator (str1 == str2) **checks for referential equality
  • **The method (str1.equals(str2)) **checks for lexical equality

Though, it’s true that if two strings are lexically equal, then _str1.intern() == str2.intern() _is also true.

Typically, for comparing two Strings for their content, we should always use String.equals.

Q12. How can we split a String in Java?

The String class itself provides us with the _String#__split _method, which accepts a regular expression delimiter. It returns us a String[] array:

String[] parts = "john,peter,mary".split(",");
assertEquals(new String[] { "john", "peter", "mary" }, parts);

One tricky thing about split is that when splitting an empty string, we may get a non-empty array:

assertEquals(new String[] { "" }, "".split(","));

Of course, _split _is just one of many ways to split a Java String.

Q13. What is StringJoiner?

_StringJoiner _is a class introduced in Java 8 for joining separate strings into one, like taking a list of colors and returning them as a comma-delimited string. We can supply a delimiter as well as a prefix and suffix:

StringJoiner joiner = new StringJoiner(",", "[", "]");
joiner.add("Red")
  .add("Green")
  .add("Blue");

assertEquals("[Red,Green,Blue]", joiner.toString());

Q14. Difference between String, StringBuffer and StringBuilder?

Strings are immutable. This means that if we try to change or alter its values, then Java creates an absolutely new _String. _

For example, if we add to a string _str1 _after it has been created:

String str1 = "abc";
str1 = str1 + "def";

Then the JVM, instead of modifying str1, creates an entirely new String.

However, for most of the simple cases, the compiler internally uses StringBuilder and optimizes the above code.

But, for more complex code like loops, it will create an entirely new String, deteriorating performance. This is where StringBuilder and StringBuffer are useful.

Both StringBuilder and StringBuffer in Java create objects that hold a mutable sequence of characters.** StringBuffer is synchronized and therefore thread-safe whereas StringBuilder is not.**

Since the extra synchronization in _StringBuffer _is typically unnecessary, we can often get a performance boost by selecting StringBuilder.

Q15. Why is it safer to store passwords in a char[] array rather than a String?

Since strings are immutable, they don’t allow modification. This behavior keeps us from overwriting, modifying, or zeroing out its contents, making Strings unsuitable for storing sensitive information.

We have to rely on the garbage collector to remove a string’s contents. Moreover, in Java versions 6 and below, strings were stored in PermGen, meaning that once a String was created, it was never garbage collected.

By using a char[] array, we have complete control over that information. We can modify it or wipe it completely without even relying on the garbage collector.

Using char[] over String doesn’t completely secure the information; it’s just an extra measure that reduces an opportunity for the malicious user to gain access to sensitive information.

Q16. What does _String’_s intern() method do?

The method intern() creates an exact copy of a String object in the heap and stores it in the _String _constant pool, which the JVM maintains.

Java automatically interns all strings created using string literals, but if we create a String using the new operator, for example, String str = new String(“abc”), then Java adds it to the heap, just like any other object.

We can call the intern() method to tell the JVM to add it to the string pool if it doesn’t already exist there, and return a reference of that interned string:

String s1 = "Baeldung";
String s2 = new String("Baeldung");
String s3 = new String("Baeldung").intern();

assertThat(s1 == s2).isFalse();
assertThat(s1 == s3).isTrue();

Q17. How can we convert String to Integer and Integer to String in Java?

The most straightforward approach to convert a String to an Integer is by using Integer#parseInt:

int num = Integer.parseInt("22");

To do the reverse, we can use Integer#toString:

String s = Integer.toString(num);

Q18. What is String.format() and how can we use it?

String#format returns a formatted string using the specified format string and arguments.

String title = "Baeldung"; 
String formatted = String.format("Title is %s", title);
assertEquals("Title is Baeldung", formatted);

We also need to remember to specify the user’s _Locale, _unless we are okay with simply accepting the operating system default:

Locale usersLocale = Locale.ITALY;
assertEquals("1.024",
  String.format(usersLocale, "There are %,d shirts to choose from. Good luck.", 1024))

Q19. How can we convert a String to Uppercase and Lowercase?

String implicitly provides String#toUpperCase to change the casing to uppercase.

Though, the Javadocs remind us that we need to specify the user’s L__ocale to ensure correctness:

String s = "Welcome to Baeldung!";
assertEquals("WELCOME TO BAELDUNG!", s.toUpperCase(Locale.US));

Similarly, to convert to lowercase, we have String#toLowerCase:

String s = "Welcome to Baeldung!";
assertEquals("welcome to baeldung!", s.toLowerCase(Locale.UK));

Q20. How can we get a character array from String?

String provides toCharArray, which returns a copy of its internal char array pre-JDK9 (and converts the String to a new char array in JDK9+):

char[] hello = "hello".toCharArray();
assertArrayEquals(new String[] { 'h', 'e', 'l', 'l', 'o' }, hello);

Q21. How would we convert a Java String into a byte array?

By default, the method String#getBytes() encodes a String into a byte array using the platform’s default charset.

And while the API doesn’t require that we specify a charset, we should in order to ensure security and portability:

byte[] byteArray2 = "efgh".getBytes(StandardCharsets.US_ASCII);
byte[] byteArray3 = "ijkl".getBytes("UTF-8");

4. String-Based Algorithms

In this section, we’ll discuss some programming questions related to _String_s.

Q22. How can we check if two Strings are anagrams in Java?

An anagram is a word formed by rearranging the letters of another given word, for example, “car” and “arc”.

To begin, we first check whether both the Strings are of equal length or not.

Then we convert them to char[] array, sort them, and then check for equality.

Q23. How can we count the number of occurrences of a given character in a String?

Java 8 really simplifies aggregation tasks like these:

long count = "hello".chars().filter(ch -> (char)ch == 'l').count();
assertEquals(2, count);

And, there are several other great ways to count the l’s, too, including loops, recursion, regular expressions, and external libraries.

Q24. How can we reverse a String in Java?

There can be many ways to do this, the most straightforward approach being to use the reverse method from StringBuilder (or StringBuffer):

String reversed = new StringBuilder("baeldung").reverse().toString();
assertEquals("gnudleab", reversed);

Q25. How can we check if a String is a palindrome or not?

A palindrome is any sequence of characters that reads the same backward as forward, such as “madam”, “radar” or “level”.

To check if a string is a palindrome, we can start iterating the given string forward and backward in a single loop, one character at a time. The loop exits at the first mismatch.

5. Conclusion

In this article, we went through some of the most prevalent String interview questions.

All the code samples used here are available on GitHub.