[英]How to map elements of the list to their indices using Java 8 streams?
Having a list of strings, I need to construct a list of objects which are effectively pairs (string, its position in the list)
. 有了一个字符串列表,我需要构建一个有效对
(string, its position in the list)
。 Currently I have such code using google collections: 目前我有使用google集合的代码:
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();
}
I would like to do this using Java 8 streams. 我想用Java 8流做到这一点。 If there was no index, I could just do:
如果没有索引,我可以这样做:
public Robots(List<String> names) {
this.list = names.stream()
.map(Robot::new) // no index here
.collect(collectingAndThen(
Collectors.toList(),
Collections::unmodifiableList
));
}
To get the index, I would have to do something like this: 要获得索引,我必须做这样的事情:
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
));
}
However, the documentation says that mapping function should be stateless, but the AtomicInteger
is effectively its state. 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。
Is there a way to map elements of the sequential stream to their positions in the stream? 有没有办法将顺序流的元素映射到它们在流中的位置?
You could do something like this: 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));
}
However it may not be as efficient depending on the underlying implementation of the list. 但是,根据列表的底层实现,它可能效率不高。 You could grab an iterator from the
IntStream
; 你可以从
IntStream
获取一个迭代器; then calling next()
in the mapToObj
. 然后在
mapToObj
调用next()
。
As an alternative, the proton-pack library defines the zipWithIndex
functionality for streams: 作为替代方案, proton-pack库为流定义
zipWithIndex
功能:
this.list = StreamUtils.zipWithIndex(names.stream())
.map(i -> new Robot(i.getIndex(), i.getValue()))
.collect(collectingAndThen(toList(), Collections::unmodifiableList));
The easiest way is to stream indices: 最简单的方法是流索引:
List<Robot> robots = IntStream.range(0, names.size())
.mapToObj(i -> new Robot(i, names.get(i))
.collect(toList());
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