Should we use type cast for the object.toArray()?

Question

String[] a = c.toArray(new String[0]);

First: Do I need type cast here? (I think we should write like (String[])c.toArray(); BUT I have seen it as just c.toArray() without using type cast. Is this valid?

Second: Also why we write new String[0]?

Answer 1

The type cast is usually only needed if you're using pre-generics Java. If you look at the docs for Collection.toArray(T[]) you'll see that it knows that the type of the array which is returned is the same as the type of array passed in. So, you can write:

List<String> list = new ArrayList<String>();
list.add("Foo");
String[] array = list.toArray(new String[0]);

You pass in the array to tell the collection what result type you want, which may not be the same as the type in the collection. For example, if you have a List<OutputStream> you may want to convert that to an Object[], a Stream[] or a ByteArrayOutputStream[] - obviously the latter is only going to work if every element actually is a ByteArrayOutputStream. It also means that if you already have an array of the right type and size, the contents can be copied into that instead of creating a new array.

A previous version of this answer was inaccurate, by the way - if you use the overload which doesn't take any parameters, you always get back an Object[] which can't be cast:

List<String> list = new ArrayList<String>();
list.add("Foo");

// toArray doesn't know the type of array to create
// due to type erasure
Object[] array = list.toArray();

// This cast will fail at execution time
String[] stringArray = (String[]) arrray;

EDIT: I've just noticed this is also mentioned in erdemoo's answer, but it can't hurt to have it here too :)