Generics problem and arrays

Question

Sorry I couldnt think of a more concise title.

My question is why does the following piece of code work:

public abstract class TObjectPool<T> {
protected Object[] availableObjects;

TObjectPool(int size){
    availableObjects = new Object[size];
}

protected class RenderElementPool extends TObjectPool<RenderElement>{
    @Override
    public void fill() {
        for (int i = 0; i < capacity; i++) {
            availableObjects[i] = new RenderElement();   
        }
    }
}

when it will not work making the Object array a generic as so:

public abstract class TObjectPool<T> {
protected T[] availableObjects;

TObjectPool(int size){
    availableObjects = (T[]) new Object[size];
}

When the availableObjects[i] = new RenderElement(); line is executed in this latter example I get a ClassCastException. I understand why it works in the first example but not why it doesn't in the second. availableObjects should be an array of RenderElement and I am trying to give it a RenderElement. What crucial bit of info am I missing?

Thanks for the help.

Update...

Thanks very much for the responses. I thought I understood but I have managed to confuse myself again:

If I add the function:

public void add(int index, T object){
    availableObjects[index] = object;
}

to the TObjectPool class. It will happily work with the T[] array.

So the new TObjectPool and subclassed pool are as follows:

public abstract class TObjectPool<T> {
T[] availableObjects;

TObjectPool(int size){
    availableObjects = (T[])new Object[size];
    capacity = size;
    count = capacity;
    fill();
}

public void add(int index, T object){
    availableObjects[index] = object;
}

protected class RenderElementPool extends TObjectPool<RenderElement>{
@Override
public void fill() {
    for (int i = 0; i < capacity; i++) {
        add(i, new RenderElement()); //this works
        //availableObjects[i] = new RenderElement(); //this gives an exception          
    }
}
}

I know how I can get round this now after reading your responses but I am curious. Can anyone shed some light into this peculiarity?

Thanks again.

Answer 1

The cast fails because Object[] is a super class of T[] for all T, but not a subtype for any T other than Object.

If you have a Class<T> available at runtime, you can use Array.newInstance to create a T[].