Java generics in ArrayList.toArray()

If you look at the implementation of toArray(T[] a) of ArrayList<E> class, it is like:

public <T> T[] toArray(T[] a) {    if (a.length < size)        // Make a new array of a's runtime type, but my contents:        return (T[]) Arrays.copyOf(elementData, size, a.getClass());    System.arraycopy(elementData, 0, a, 0, size);    if (a.length > size)        a[size] = null;    return a;}

Problem with this method is that you need to pass array of the same generic type. Now consider if this method do not take any argument then the implementation would be something similar to:

public <T> T[] toArray() {    T[] t = new T[size]; // compilation error    return Arrays.copyOf(elementData, size, t.getClass());}

But the problem here is that you can not create generic arrays in Java because compiler does not know exactly what T represents. In other words creation of array of a non-reifiable type (JLS §4.7) is not allowed in Java.

Another important quote from Array Store Exception (JLS §10.5):

If the component type of an array were not reifiable (§4.7), the Java Virtual Machine could not perform the store check described in the preceding paragraph. This is why an array creation expression with a non-reifiable element type is forbidden (§15.10.1).

That is why Java has provided overloaded version toArray(T[] a).

I will override the toArray() method to tell it that it will return an array of E.

So instead of overriding toArray(), you should use toArray(T[] a).

Cannot Create Instances of Type Parameters from Java Doc might also be interesting for you.

Generic information is erased at runtime. JVM does not know whether your list is List<String> or List<Integer> (at runtime T in List<T> is resolved as Object), so the only possible array type is Object[].

You can use toArray(T[] array) though - in this case JVM can use the class of a given array, you can see it in the ArrayList implementation:

public <T> T[] toArray(T[] a) {    if (a.length < size)        // Make a new array of a's runtime type, but my contents:        return (T[]) Arrays.copyOf(elementData, size, a.getClass());

If you look at the Javadoc for the List interface, you'll notice a second form of toArray: <T> T[] toArray(T[] a).

In fact, the Javadoc even gives an example of how to do exactly what you want to do:

String[] y = x.toArray(new String[0]);