How can I create a generic array in Java?

Due to the implementation of Java generics, you can't have code like this:

public class GenSet {
    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 {
    public Stack(Class clazz, int capacity) {
        array = (T[])Array.newInstance(clazz, capacity);
    }

    private final T[] array;
}

What's going on?

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 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 {

    private E[] a;

    public GenSet(Class 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 {

    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 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.

Stack Overflow Q#529085, score: 789