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Roosevelt Hettinger

Roosevelt Hettinger

11 months ago

Initializing ArrayList in Java: Examples and Methods

Sometimes you want to create an ArrayList with values, just like you initialize t at the time of declaration, as shown below:

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int[] primes = {2, 3, 5, 7, 11, 13, 17};

or


String[] names = {"john", "Johnny", "Tom", "Harry"};

but unfortunately, ArrayList doesn't support such kind of declaration in Java. But don't worry, there is a workaround to declare an ArrayList with values e.g. String, integers, floats, or doubles by using the Arrays.asList() method, which is nothing but a shortcut to convert an Array to ArrayList.
 

Declaring ArrayList with values in Java

Here is a code example to show you how to initialize ArrayList at the time of declaration:

ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6));


This is how you declare an ArrayList of Integer values. You can do the same to create an ArrayList with String objects as well, e.g.

ArrayList<String> cities = new ArrayList<>(Arrays.asList("London", "Tokyo", "New York"));

Similarly here is how you should create ArrayList with float values:

ArrayList<Float> floats = new ArrayList<>(Arrays.asList(3.14f, 6.28f, 9.56f));

don't forget the suffix 'f', it's important because by default floating-point numbers are double in Java.

Some programmer's also like to declare a List with values in one line as:

List<Integer> listOfInts = Arrays.asList(1, 2, 3);


This is Ok to print values, but it's not an ArrayList. You cannot add or remove elements into this list but when you create an ArrayList like new ArrayList(Arrays.asList()), you get a regular ArrayList object, which allows you to add, remove and set values.

Here is a nice summary of code examples of how to make an ArrayList of values in Java:
 

How to declare ArrayList with values in Java

That's all about how to declare an ArrayList with values in Java. You can use this technique to declare an ArrayList of integers, String, or any other object. It's truly useful for testing and demo purpose, but I have also used this to create an ArrayList of an initial set of fixed values.

Further Reading
If you a beginner in Java and just started learning, then you can read Head First Java 2nd Edition, but if you are an experienced Java developer and looking forward to taking your knowledge of Java Collections framework to next level then read Java Generics and Collections from Maurice Naftalin.

#java 

Initializing ArrayList in Java: Examples and Methods
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