🧑‍🎓 tltr

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1234

Print is executed multiple times till the first value that matches both filter conditions is found.

❓Question

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//What is the output?
IntStream.range(1, 10)
            .peek(System.out::print)
            .filter(i -> i > 2) //A
            .filter(i -> i > 3) //B
            .findFirst();

🕵️‍♂️ Explanation

Let’s simplify it

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IntStream.range(1, 10);

This won’t do anything because there is no terminal operation which triggers the reading from the source stream.

By adding the peek, which is an intermediate operation, the behaviour won’t change.

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IntStream.range(1, 10).peek(System.out::print);

This findFirst (terminal operation) evaluates the pipeline.

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IntStream.range(1, 10).peek(System.out::print).findFirst();

During the evaluation, the sinks are wrapped (combined) into one Sink. As a result, we get:

1. peek Sink
2. filter A Sink
3. filter B Sink
4. findFirst Sink

While the sink is not cancelled (in our case findFirst has no value) and there is an element in the stream, the pipeline will try to advance, fetch the next element and pass it to the wrapped sink as above (top-down)

✅ Walkthrough

So we will have something like this (since the stream is not empty):

The stream: 1 2 3 4 5 6 7 8 9

• 1 is passed to peek sink
  • peek is executed, 1 is printed
  • 1 passed to filter A sink
    • filter A is executed, false is returned (1>2), the sink is not cancelled, continue the while
• 2 is fetched and passed to peek sink
  • peek is executed, 2 is printed
  • 2 passed to filter A sink
    • filter A is executed, false is returned (2>2), the sink is not cancelled, continue the while
• 3 is fetched and passed to peek sink
  • peek is executed, 3 is printed
  • 3 passed to filter A sink
    • filter A is executed, true is returned (3>2)
  • 3 passed tofilter B sink
    • filter B is executed, false is returned (3>3), the sink is not cancelled, continue the while
• 4 is fetched and passed to peek sink
  • peek is executed, 4 is printed
  • 4 passed to filter A sink
    • filter A is executed, true is returned (4>2)
  • 4 passed to filter B sink
    • filter B is executed, true is returned (4>3)
  • 4 passed to findFirst sink
    • saves the value inside the findFirst which means the sink is cancelled, while interrupted
• 4 returned

Another Example

Let’s move the peek before the findFirst. Almost the same code, and different output

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//What is the output?
IntStream.range(1, 10)
            .filter(i -> i > 2) //A
            .filter(i -> i > 3) //B
            .peek(System.out::print)
            .findFirst();

Output:

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4

The sink:

1. filter A Sink
2. filter B Sink
3. peek Sink
4. findFirst Sink

✅ Walkthrough

So we will have something like this:
The stream: 1 2 3 4 5 6 7 8 9

• 1 is passed to filter A sink
  • filter A is executed, false is returned (1>2), the sink is not cancelled, continue the while
• 2 is fetched and passed to filter A sink
  • filter A is executed, false is returned (2>2), the sink is not cancelled, continue the while
• 3 is fetched and passed to filter A sink
  • filter A is executed, true is returned (3>2)
  • 3 passed tofilter B sink
    • filter B is executed, false is returned (3>3), the sink is not cancelled, continue the while
• 4 is fetched and passed to filter A sink
  • filter A is executed, true is returned (4>2)
  • 4 passed tofilter B sink
    • filter B is executed, true is returned (4>3)
• 4 is passed to peek sink
  • 4 is printed
• 4 is passed to findFirst sink
  • saves the value inside the findFirst which means the sink is cancelled, while interrupted
• 4 returned

📚 Read more

Sink definition
Stream API documentation