Why is the stream with a limit infinite?

Question

I have run the following code in Eclipse:

Stream.generate(() -> "Elsa")
      .filter(n -> n.length() ==4)
      .sorted()
      .limit(2)
      .forEach(System.out::println);

The output is:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

What I was expecting since the limit is two:

Elsa
Elsa

Can someone please explain why this is an infinite stream?

Answer 1

The first thing is that Stream::generate creates an infinite stream. That's why the stream is initially infinite.

You limit the stream to two elements by using Stream::limit , which would make it finite.

However, the problem is that you call sorted() , which tries to consume the whole stream. You need to limit the stream before you sort:

Stream.generate(() -> "Elsa")
    .filter(n -> n.length() == 4)
    .limit(2)
    .sorted()
    .forEach(System.out::println);

The documentation says that Stream::sorted() "is a stateful intermediate operation" . The Streams documentation about a stateful intermediate operation explains it very well :

Stateful operations may need to process the entire input before producing a result. For example, one cannot produce any results from sorting a stream until one has seen all elements of the stream .

^{Emphasis mine.}

There it is. Also note that for all Stream operations, their operation type is mentioned in the Javadocs.

Answer 2

Can someone please explain why this is an infinite stream?

Because the javadoc says that is precisely what Stream.generate() creates:

Returns an infinite sequential unordered stream where each element is generated by the provided Supplier

Then when you combine that with sorted() , you tell it to start a sort on an infinite sequence which will obviously cause the JVM to run out of memory.