Java: how to implement `toArray` for `Collection`

Question

Right now, I have:

    public <T> T[] toArray(T[] old) {
        T[] arr = Arrays.copyOf(old, old.length + size());
        int i = old.length;
        for(E obj : this) {
            arr[i] = old.getClass().getComponentType().cast(obj);
            ++i;
        }
        return arr;
    }

(Note that this does not follow the contract as it was pointed out by axtavt.)

where I get this warning:

Type safety: Unchecked cast from capture#2-of ? to T

Is this still the best / most straightforward way to implement it? Can I somehow code it in a way without that warning? How would I implement it otherwise?

---

Edit: My current solution. First, I really wanted to not have such a warning in toArray itself. Therefore, I coded these little helper functions (read here for a further discussion about these):

@SuppressWarnings("unchecked") static <T> Class<? extends T> classOf(T obj) {
    return (Class<? extends T>) obj.getClass();
}

@SuppressWarnings("unchecked") static <T> Class<? extends T> classOf(T[] array) {
    return (Class<? extends T>) array.getClass().getComponentType();
}

@SuppressWarnings("unchecked") static <T> T[] newArray(Class<T> clazz, int size) {
    return (T[]) Array.newInstance(clazz, size);
}   

Now, my toArray implementation looks like:

    public <T> T[] toArray(T[] array) { 
        int size = size();
        if (array.length < size) { 
            array = newArray(classOf(array), size);
        } else if (array.length > size) {
            array[size] = null;
        }

        int i = 0;
        for (E e : this) {
            array[i] = classOf(array).cast(e);
            i++;
        }
        return array;
    } 

Answer 1

Is this still the best / most straightforward way to implement it? How would I implement it otherwise?

It's not how Josh Bloch did. Have a look at the source of AbstractCollection#toArray(). Here's an extract of relevance from JDK 1.6.0_22.

public <T> T[] toArray(T[] a) {
    // Estimate size of array; be prepared to see more or fewer elements
    int size = size();
    T[] r = a.length >= size 
        ? a 
        : (T[]) Array.newInstance(a.getClass().getComponentType(), size);
    Iterator<E> it = iterator();

    for (int i = 0; i < r.length; i++) {
        if (!it.hasNext()) { // fewer elements than expected
            if (a != r)
                return Arrays.copyOf(r, i);
            r[i] = null; // null-terminate
            return r;
        }
        r[i] = (T) it.next();
    }
    return it.hasNext() ? finishToArray(r, it) : r;
}

The source code is available in src.zip file of the JDK. You can integrate it in any decent IDE like Eclipse/IDEA/Netbeans so that you can see it when you open the AbstractCollection class.

Can I somehow code it in a way without that warning?

No. Use @SuppressWarnings("unchecked") if it is bothering you.

That said, I'd suggest to extend AbstractCollection instead of implementing Collection if possible so that you have at least the basic features already implemented for you.

Answer 2

First of all, if it's supposed to be an implementation of Collection.toArray(), it doesn't follow the contract - you shouldn't keep old elements in the array (see javadoc).

The correct implementation looks like this:

public <T> T[] toArray(T[] array) { 
    int size = size();
    if (array.length < size) { 
        // If array is too small, allocate the new one with the same component type
        array = Array.newInstance(array.getClass().getComponentType(), size);
    } else if (array.length > size) {
        // If array is to large, set the first unassigned element to null
        array[size] = null;
    }

    int i = 0;
    for (E e: this) {
        // No need for checked cast - ArrayStoreException will be thrown 
        // if types are incompatible, just as required
        array[i] = (T) e;
        i++;
    }
    return array;
}