Function.identity()的使用详解

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Stream<String> stream = Stream.of("This", "is", "a", "test");
Map<String, Integer> map = stream.collect(Collectors.toMap(Function.identity(), String::length));

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static  Function identity() {
    return t -> t;
}

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private static Map<String, Task> taskMap(List<Task> tasks) {
  return tasks.stream().collect(toMap(Task::getTitle, task -> task));
}

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import static java.util.function.Function.identity;

private static Map<String, Task> taskMap(List<Task> tasks) {
  return tasks.stream().collect(toMap(Task::getTitle, identity()));
}

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Arrays.asList("a", "b", "c")
          .stream()
          .map(Function.identity()) // <- This,
          .map(str -> str)          // <- is the same as this.
          .collect(Collectors.toMap(
                       Function.identity(), // <-- And this,
                       str -> str));        // <-- is the same as this.

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List list = new ArrayList<>();
list.add(1);
list.add(2);

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int[] arrayOK = list.stream().mapToInt(i -> i).toArray();

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int[] arrayProblem = list.stream().mapToInt(Function.identity()).toArray();