java - why does List<String>.toArray() return Object[] and not String[]? how to work around this?

Question

Does anybody know why Java 1.6 has this behaviour:

List<String> list = ArrayList<String>();
String[] arr = (String[]) list.toArray();

And I get a ClassCastException, because it returns Object[] and not String[].

I thought List<T>.toArray() should return T[] - no? Does anyone have an answer why this inconvenience exists in the language? And also how to work around this? How do I get a String[] from List<String> without looping thru the items?

Answer 1

You need to pass in an array so its runtime type can be used as a hint by toArray. Try toArray(new String[0]) instead. You can also pass in a pre-sized array.

To understand, consider what type erasure would have to do to make

new T[4]

work. If Java allowed that, the best it could do post erasure is

new Object[4]

Most toArray implementations use java.lang.reflect.Array to construct an output array of the right type given a type hint passed as a Class.