Thread: Java: Why isn't this allowed?

Read section 7.3 in the following tutorial for why this isn't allowed:
http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

This is what the tutorial says:

Originally Posted by tutorial

Similarly, attempting to create an array object whose element type is a type variable causes a compile-time error:

Code:

<T> T[] makeArray(T t) { return new T[100]; // error
}

Since type variables don’t exist at run time, there is no way to determine what the actual array type would be.

I don't agree here. Sure, the type T isn't known, but no T object is created, only references (100 of them) to T objects are created (and set to null).

The only thing I ask is that

Code:

E[] array = new E[100];

should be interpreted as

Code:

E[] array = (E[]) new Object[100];

I don't agree here.

Feel free to email Sun and ask them to change the language to suit your preferences.

Originally Posted by Narf

Feel free to email Sun and ask them to change the language to suit your preferences.

Feel free to not post in this thread unless you have something useful to say.

Originally Posted by Sang-drax

Feel free to not post in this thread unless you have something useful to say.

My suggestion was very serious. I'm sorry that you don't approve of reality, but if you don't like it, you can suggest changes to Sun. But thanks for your rude and insulting response anyway. I always appreciate being wrongly flamed by ..............s.

Originally Posted by Sang-drax

The only thing I ask is that

Code:

E[] array = new E[100];

should be interpreted as

Code:

E[] array = (E[]) new Object[100];

But the (E[]) cast is a no-op. E is (unless it's defined as 'extends XXX') completely replaced by Object in the resulting bytecode. All you're doing is casting an object array to an object array. The only effect of generics are the casts the compiler writes into the code automatically.

Allowing "new E[10]" would lead to the misconception that the runtime type of the array would be E[]. It isn't. It's Object[], which cannot be cast to any more specialized array type. For example, this code throws a class cast exception:
String[] bla = (String[]) new Object[10];

On the other hand, this code is valid, the runtime type being String[]:
String[] bla = new String[10];

Allowing this:
E[] bla = new E[10];
would lead people to believe that the runtime type is E[]. This can be dangerous. And that's why it's not allowed.

Originally Posted by CornedBee

The only effect of generics are the casts the compiler writes into the code automatically.

Thanks!

If I were able to write

Code:

E[] array = new E[10]

it'd be interpreted as

Code:

Object[] array = new Object[10];

But would this be unsafe? I still cannot do:

Code:

Object someObject = ...
array[2] = someObject;

The equals method is not generic. It always takes an Object parameter.