如何使用 Java 8 流将列表的元素映射到它们的索引?

2022-01-22 00:00:00 lambda java-8 java java-stream

有一个字符串列表，我需要构造一个对象列表，它们实际上是对(字符串，它在列表中的位置).目前我有这样的代码使用谷歌收藏:

Having a list of strings, I need to construct a list of objects which are effectively pairs (string, its position in the list). Currently I have such code using google collections:

public Robots(List<String> names) {
    ImmutableList.Builder<Robot> builder = ImmutableList.builder();
    for (int i = 0; i < names.size(); i++) {
        builder.add(new Robot(i, names.get(i)));
    }
    this.list = builder.build();
}

我想使用 Java 8 流来执行此操作.如果没有索引，我可以这样做:

I would like to do this using Java 8 streams. If there was no index, I could just do:

public Robots(List<String> names) {
    this.list = names.stream()
            .map(Robot::new) // no index here
            .collect(collectingAndThen(
                    Collectors.toList(),
                    Collections::unmodifiableList
            ));
}

要获得索引，我必须这样做:

To get the index, I would have to do something like this: 

public Robots(List<String> names) {
    AtomicInteger integer = new AtomicInteger(0);
    this.list = names.stream()
            .map(string -> new Robot(integer.getAndIncrement(), string))
            .collect(collectingAndThen(
                    Collectors.toList(),
                    Collections::unmodifiableList
            ));
}

但是，文档说映射函数应该是无状态的，但 AtomicInteger 实际上是它的状态.

However, the documentation says that mapping function should be stateless, but the AtomicInteger is effectively its state.

有没有办法将顺序流的元素映射到它们在流中的位置?

Is there a way to map elements of the sequential stream to their positions in the stream?

推荐答案

你可以这样做:

public Robots(List<String> names) {
    this.list = IntStream.range(0, names.size())
                         .mapToObj(i -> new Robot(i, names.get(i)))
                         .collect(collectingAndThen(toList(), Collections::unmodifiableList));
}

但是，根据列表的底层实现，它可能效率不高.你可以从 IntStream 中获取一个迭代器；然后在 mapToObj 中调用 next().

However it may not be as efficient depending on the underlying implementation of the list. You could grab an iterator from the IntStream; then calling next() in the mapToObj.

作为替代方案，proton-pack 库为流定义了 zipWithIndex 功能:

As an alternative, the proton-pack library defines the zipWithIndex functionality for streams:

 this.list = StreamUtils.zipWithIndex(names.stream())
                        .map(i -> new Robot(i.getIndex(), i.getValue()))
                        .collect(collectingAndThen(toList(), Collections::unmodifiableList));

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