How can I create a generic array in Java?

arrays, generics, instantiation, java, reflection

Solution

I have to ask a question in return: is your `GenSet` "checked" or "unchecked"? What does that mean?

Checked: strong typing. `GenSet` knows explicitly what type of objects it contains (i.e. its constructor was explicitly called with a `Class<E>` argument, and methods will throw an exception when they are passed arguments that are not of type `E`. See `Collections.checkedCollection`.

-> in that case, you should write:

public class GenSet<E> {

    private E[] a;

    public GenSet(Class<E> c, int s) {
        // Use Array native method to create array
        // of a type only known at run time
        @SuppressWarnings("unchecked")
        final E[] a = (E[]) Array.newInstance(c, s);
        this.a = a;
    }

    E get(int i) {
        return a[i];
    }
}

Unchecked: weak typing. No type checking is actually done on any of the objects passed as argument.

-> in that case, you should write

public class GenSet<E> {

    private Object[] a;

    public GenSet(int s) {
        a = new Object[s];
    }

    E get(int i) {
        @SuppressWarnings("unchecked")
        final E e = (E) a[i];
        return e;
    }
}

Note that the component type of the array should be the erasure of the type parameter:

public class GenSet<E extends Foo> { // E has an upper bound of Foo

    private Foo[] a; // E erases to Foo, so use Foo[]

    public GenSet(int s) {
        a = new Foo[s];
    }

    ...
}

All of this results from a known, and deliberate, weakness of generics in Java: it was implemented using erasure, so "generic" classes don't know what type argument they were created with at run time, and therefore can not provide type-safety unless some explicit mechanism (type-checking) is implemented.

Problem

Due to the implementation of Java generics, you can't have code like this: ``` public class GenSet<E> { private E a[]; public GenSet() { a = new E[INITIAL_ARRAY_LENGTH]; // Error: generic array creation } } ``` How can I implement this while maintaining type safety? I saw a solution on the Java forums that goes like this: ``` import java.lang.reflect.Array; class Stack<T> { public Stack(Class<T> clazz, int capacity) { array = (T[])Array.newInstance(clazz, capacity); } private final T[] array; } ``` What's going on?

Original source

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