Question

The toArray method (lets pick the implementation in java.util.ArrayList) is the following:

class ArrayList<E> ....{
    public <T> T[] toArray(T[] a){
        if(a.length < size)
            return (T[]) Arrays.copyof(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if(a.length > size)
            a[size] = null;
        return a;
    }    
}

I am wondering could we use <E> instead of <T> in this case ? like

public E[] toArray(E[] a){
      if(a.length < size)
             return (E[]) Arrays.copyof(elementData, size, a.getClass());
      System.arraycopy(elementData, 0, a, 0, size);
      if(a.length > size)
            a[size] = null;
      return a;
}    

Since the ArrayList class iteself is already generic to <E>, so could we use that instead of a new generic type <T> ?

Answer 1

The point of the <T> is if the array desired is of a base class of E. For example if E is HashMap but the desired array was Map[]. If toArray were locked down to E this would not be possible.

This type of thing is not needed in generic collections / types due to type-erasure. But there is no type-erasure with arrays so the type of the array can be very important.