How peek() and allMatch() works together in Java 8 Stream API

Question

I found a quiz about Java 8 Stream API of peek method as below

Arrays.asList("Fred", "Jim", "Sheila")
      .stream()
      .peek(System.out::println)
      .allMatch(s -> s.startsWith("F"));

The output is

Fred
Jim

I am confused how this stream works? My expected result should be

Fred
Jim
Sheila

The peek() method is an intermediate operation and it processes each element in Stream. Can anyone explain me this.

Answer 1

It's a stream optimization known as short-circuiting. Essentially, what happens is that allMatch prevents the execution of unnecessary intermediate operations on the stream, because there is no point in performing them when the final result is known.

It's as though this happened:

take"Fred"
peek("Fred")
evaluate("Fred".startsWith("F"))
decide whether the result of allMatch() is known for sure: Not yet

take"Jim"
peek("Jim")
evaluate("Jim".startsWith("F"))
decide whether the result of allMatch() is known for sure: Yes

When "Jim".startsWith("F") is evaluated, the result of allMatch(s -> s.startsWith("F")) is known for certain. It doesn't matter what values come in the pipeline after "Jim", we know that all values start with "F" is false

This is not specific to the peek/allMatch combination, there are multiple intermediate and terminal short-circuiting operations. java.util.stream package's docs state:

Further, some operations are deemed short-circuiting operations. An intermediate operation is short-circuiting if, when presented with infinite input, it may produce a finite stream as a result. A terminal operation is short-circuiting if, when presented with infinite input, it may terminate in finite time. Having a short-circuiting operation in the pipeline is a necessary, but not sufficient, condition for the processing of an infinite stream to terminate normally in finite time.

Extend this to finite streams, and short-circuiting operations obviate the execution of unnecessary pipeline steps, as in the case of your example.