Implementation of Collection.stream()

Question

I have been working on JDK 1.8 for a few days now where I came across some piece of code which was similar to this:

List<Integer> list = Arrays.asList(1,2,3,4,5);
list.stream();

Now, easy and clean as it may appear to the people who have been working with streams (java.util.stream), I could not find the the actual class that implements the java.util.Collection.stream() method.

I have the following questions, when I say list.stream():

1. Where do I get the java.util.stream.Stream from?
2. How did they implement it without actually "disturbing" the existing collections?(assuming that they did not touch them)

I did try to look through the documentations of java.util.AbstractCollection and java.util.AbstractList but was unable to find it.

Answer 1

Java 8 allows the definition of default methods in interfaces.

Collection<E> then defines :

default Stream<E> stream() {
    return StreamSupport.stream(spliterator(), false);
}

That's how they added it.

Answer 2

As others pointed out, the .stream() method is implemented as a default method in the Collection interface itself, as you can see it in the method signature in the official documentation:

default Stream<E> stream()

How the stream interface is implemented is an implementation detail of the collection. However, implementing the same heavy Stream interface for every collection would be a lot of work and duplication, so they use an intermediate abstraction called Spliterator.

This SO thread on .stream() might be worth reading as well.