Why do the new Java 8 streams return an Object Array on toArray calls?

Question

Whilst I was working on a project involving Java 8's new streams, I noticed that when I called Stream#toArray() on a stream, it return an Object[] instead of a T[]. Surprised as I was, I started digging into the source code of Java 8 and couldn't find any reason why they didn't implement Object[] toArray(); as T[] toArray();. Is there any reasoning behind this, or is it just an (in)consistency?

EDIT 1: I noticed in the answers that a lot of people said this would not be possible, but this code snippet compiles and return the expected result?

import java.util.Arrays;

public class Test<R> {

    private Object[] items;

    public Test(R[] items) {
        this.items = items;
    }

    public R[] toArray() {
        return (R[]) items;
    }

    public static void main(String[] args) {
        Test<Integer> integerTest = new Test<>(new Integer[]{
            1, 2, 3, 4
        });

        System.out.println(Arrays.toString(integerTest.toArray()));
    }

}

Answer 1

Try:

String[] = IntStream.range(0, 10).mapToObj(Object::toString).toArray(String[]::new);

The no-arg toArray() method will just return an Object[], but if you pass an array factory (which can be conveniently represented as an array constructor reference), you can get whatever (compatible) type you like.

Answer 2

This is the same problem that List#toArray() has. Type erasure prevents us from knowing the type of array we should return. Consider the following

class Custom<T> {
    private T t;
    public Custom (T t) {this.t = t;}
    public T[] toArray() {return (T[]) new Object[] {t};} // if not Object[], what type?
}

Custom<String> custom = new Custom("hey");
String[] arr = custom.toArray(); // fails

An Object[] is not a String[] and therefore cannot be assigned to one, regardless of the cast. The same idea applies to Stream and List. Use the overloaded toArray(..) method.