Java 8 Stream - Why is filter method not executing

问题:

This question already has an answer here:

• Why does Java8 Stream generate nothing? 3 answers

I am learning filtering using java stream. But the stream after filtering is not printing anything. I think the filter method is not getting executed. My filtering code is as follows:

Stream.of("d2", "a2", "b1", "b3", "c")
    .filter(s -> {
        s.startsWith("b");
        System.out.println("filter: " + s);
        return true;
    });

There is no compilation error and no exception also. Any suggestion?

回答1:

filter is an intermediate operation, which will be executed only if the Stream pipeline ends in a terminal operation.

For example :

Stream.of("d2", "a2", "b1", "b3", "c")
  .filter(s -> {
        s.startsWith("b");
        System.out.println("filter: " + s);
        return true;
  })
  .forEach (System.out::println);

As it is, your filter method is useless, since it always returns true, and thus performs no filtering.