Java 8 peek() 方法的一些疑惑

Stream.of("one", "two", "three", "four").peek(e -> System.out.println(e));

这样不会有任何的输出；

Stream.of("one", "two", "three", "four").peek(e -> System.out.println(e)).collect(Collectors.toList());

这样输出:
one
two
three
four

Stream.of("one", "two", "three", "four")
    .peek(e -> System.out.println("Peeked value: " + e))
    .map(String::toUpperCase)
    .peek(e -> System.out.println("Mapped value: " + e))
    .collect(Collectors.toList());

这样输出：
Peeked value: one
Mapped value: ONE
Peeked value: two
Mapped value: TWO
Peeked value: three
Mapped value: THREE
Peeked value: four
Mapped value: FOUR

知道 peek() 方法接受一个实现了 consumer 接口的类，但是实在是搞不清楚到底它是如何运行的。

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peek不是终结操作，其返回值还是Stream，你这种情况应该用forEach

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简单地说就是懒咯 你没有对流进行任何操作peek就不会执行