Creating a Generic Array in Java

Creating a Generic Array in Java

The loose types of Java generics are hard to coerce into the strong types of Java arrays. We'll understand the challenges of using generics with arrays. We'll create an example of a generic array.

1. Introduction

We may wish to use arrays as part of classes or functions that support generics. Due to the way Java handles generics, this can be difficult.

In this tutorial, we'll understand the challenges of using generics with arrays. Then, we'll create an example of a generic array.

We'll also look at where the Java API has solved a similar problem.

2. Considerations When Using Generic Arrays

An important difference between arrays and generics is how they enforce type checking. Specifically, arrays store and check type information at runtime. Generics, however, check for type errors at compile-time and don't have type information at runtime.

Java's syntax suggests we might be able to create a new generic array:

T[] elements = new T[size];

But, if we attempted this, we'd get a compile error.

To understand why, let's consider the following:

public <T> T[] getArray(int size) {
    T[] genericArray = new T[size]; // suppose this is allowed
    return genericArray;
}

As an unbound generic type T resolves to Object, our method at runtime will be:

public Object[] getArray(int size) {
    Object[] genericArray = new Object[size];
    return genericArray;
}

Then, if we call our method and store the result in a String array:

String[] myArray = getArray(5);

The code will compile fine but fail at runtime with a ClassCastException. This is because we've just assigned an Object[] to a String[] reference. Specifically, an implicit cast by the compiler would fail to convert Object[] to our required type String[].

Although we can't initialize generic arrays directly, it is still possible to achieve the equivalent operation if the precise type information is provided by the calling code.

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