Is this a bug in Java, or am I doing something wrong?

Question

I have a Vector which I want to convert to an array, so I wrote my code something like this:
MyObject a = new MyObject("A");
MyObject b = new MyObject("B");
MyObject c = new MyObject("C");
MyObject d = new MyObject("D");
Vector v = new Vector();
v.add(a);
v.add(b);
v.add(c);
v.add(d);
MyObject[] list = new MyObject[v.size()];
list = (MyObject[]) v.toArray();This should work, however it gives me a ClassCastException at runtime. Of course, I found ways around it, like using
v.toArray(list);But using the method this way does not make any sense because according to the API it is not void, but supposedly returns another array of Objects in addition to filling up the array which you pass to it.
Does anyone know the correct way to use the toArray() method, or it's even more mysterious overloaded companion, toArray(Object[] a)?

Answer 1

With regards to the Vector.toArray() method that takes an Object[] as an argument, you would normally use it this way:
Vector vector = <whatever>; // contains only instances of MyClass
MyClass[] array = (MyClass[])vector.toArray(new MyClass[vector.size]);While you're right that if you declare the array first and then simply pass it to the toArray() method that the array will get filled with the appropriate values. However, the array that is returned is not necessarily the array that was passed in. If the array that is passed in is shorter than the number of elements in the Vector, a new array of the same type as the one passed in is allocated and it is that array which is returned. The API only says that an array is returned, not the original one. Here is the source code for the toArray() method for you to look at:
    public synchronized Object[] toArray(Object a[]) {
        if (a.length < elementCount)
            a = (Object[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), elementCount);
     System.arraycopy(elementData, 0, a, 0, elementCount);
        if (a.length > elementCount)
            a[elementCount] = null;
        return a;
    }Shaun