What's the reason I can't create generic array types in Java?

Question

What's the reason Java doesn't allow us to do

private T[] elements = new T[initialCapacity];

?

I could understand .NET didn't allow us to do that, as in .NET you have value types that at run-time can have different sizes, but in Java all kinds of T will be object references, thus having the same size(correct me if I'm wrong).

What is the reason?

Answer 1

The main reason is due to the fact that arrays in Java are covariant.

There's a good overview here.

Answer 2

Quote:

Arrays of generic types are not allowed because they're not sound. The problem is due to the interaction of Java arrays, which are not statically sound but are dynamically checked, with generics, which are statically sound and not dynamically checked. Here is how you could exploit the loophole:

class Box<T> {
    final T x;
    Box(T x) {
        this.x = x;
    }
}

class Loophole {
    public static void main(String[] args) {
        Box<String>[] bsa = new Box<String>[3];
        Object[] oa = bsa;
        oa[0] = new Box<Integer>(3); // error not caught by array store check
        String s = bsa[0].x; // BOOM!
    }
}

We had proposed to resolve this problem using statically safe arrays (aka Variance) bute that was rejected for Tiger.

-- gafter

(I believe it is Neal Gafter, but am not sure)

See it in context here: http://forums.sun.com/thread.jspa?threadID=457033&forumID=316

Answer 3

The reason this is impossible is that Java implements its Generics purely on the compiler level, and there is only one class file generated for each class. This is called Type Erasure.

At runtime, the compiled class needs to handle all of its uses with the same bytecode. So "new T[capacity]" would have absolutely no idea what type needs to be instanciated.

Answer 4

I like the answer indirectly given by Gafter. However, I propose it is wrong. I changed Gafter's code a little. It compiles and it runs for a while then it bombs where Gafter predicted it would

class Box<T> {
    final T x;
    Box(T x) {
        this.x = x;
    }
}

class Loophole {
    public static < T > T [ ] array ( final T ... values )
    {
    return ( values ) ;
    }

    public static void main(String[] args) {
    Box<String> a = new Box ( "Hello" ) ;
    Box<String> b = new Box ( "World" ) ;
    Box<String> c = new Box ( "!!!!!!!!!!!" ) ;
        Box<String>[] bsa = array ( a , b , c ) ;
    System . out . println ( "I created an array of generics." ) ;
        Object[] oa = bsa;
        oa[0] = new Box<Integer>(3); 
    System . out . println ( "error not caught by array store check" ) ;
    try
        {
        String s = bsa[0].x;
        }
    catch ( ClassCastException cause )
        {
        System . out . println ( "BOOM!" ) ;
        cause . printStackTrace ( ) ;
        }
    }
}

The output is

I created an array of generics.
error not caught by array store check
BOOM!
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at Loophole.main(Box.java:26)

So it appears to me you can create generic array types in java. Did I misunderstand the question?

Answer 5

It's because Java's arrays (unlike generics) contain, at runtime, information about its component type. So you must know the component type when you create the array. Since you don't know what T is at runtime, you can't create the array.

Answer 6

Despite Java arrays are considered like Collections in practice, in theory they are of type java.lang.Object, that's why they don't have generics.

Answer 7

By failing to provide a decent solution, you just end up with something worse IMHO.

The common work around is as follows.

T[] ts = new T[n];

is replaced with

T[] ts = (T[]) new Object[n];

I prefer the first example, however more acedemic types seem to prefer the second, or just prefer not to thing about it.

Most of the examples of why you can't just use an Object[] equally apply to List or Collection (which are supported), so I see them as very poor arguments.

Note: this is one of the reasons the Collections library itself doesn't compile without warnings. If you this usecase cannot be supported without warnings, something is fundermentally broken with the generics model IMHO.