Java - Stream - 收集每N个元素

Question

我正在努力学习java - stream.我能够做简单的迭代/过滤/地图/收集等.

当我试图收集每个3个元素并打印时,如本例所示,[收集每3个元素并打印等等...]

    List<String> list = Arrays.asList("a","b","c","d","e","f","g","h","i","j");

    int count=0;
    String append="";
    for(String l: list){
        if(count>2){
            System.out.println(append);
            System.out.println("-------------------");
            append="";
            count=0;
        }
        append = append + l;
        count++;
    }
    System.out.println(append);

输出:

abc
-------------------
def
-------------------
ghi
-------------------
j

我没有得到任何线索如何使用流这样做.我应该实现自己的收藏家来实现这一目标吗？

Answer 1

您实际上可以使用a IntStream来模拟列表的分页.

List<String> list = Arrays.asList("a","b","c","d","e","f","g","h","i","j");

int pageSize = 3;

IntStream.range(0, (list.size() + pageSize - 1) / pageSize)
        .mapToObj(i -> list.subList(i * pageSize, Math.min(pageSize * (i + 1), list.size())))
        .forEach(System.out::println);

哪个输出:

[a, b, c]
[d, e, f]
[g, h, i]
[j]

如果你想生成字符串,你可以使用,String.join因为你List<String>直接处理:

.mapToObj(i -> String.join("", list.subList(i * pageSize, Math.min(pageSize * (i + 1), list.size()))))

Answer 2

你可以创建自己的Collector.最简单的方法是打电话Collector.of().

由于您的用例需要按顺序处理值,因此这里的实现不支持并行处理.

public static Collector<String, List<List<String>>, List<List<String>>> blockCollector(int blockSize) {
    return Collector.of(
            ArrayList<List<String>>::new,
            (list, value) -> {
                List<String> block = (list.isEmpty() ? null : list.get(list.size() - 1));
                if (block == null || block.size() == blockSize)
                    list.add(block = new ArrayList<>(blockSize));
                block.add(value);
            },
            (r1, r2) -> { throw new UnsupportedOperationException("Parallel processing not supported"); }
    );
}

测试

List<String> input = Arrays.asList("a","b","c","d","e","f","g","h","i","j");
List<List<String>> output = input.stream().collect(blockCollector(3));
output.forEach(System.out::println);

产量

[a, b, c]
[d, e, f]
[g, h, i]
[j]

Answer 3

如果你的项目中有Guava,你可以使用Iterables.partition方法:

import com.google.common.collect.Iterables;
import com.google.common.collect.Streams;
...

Stream<List<String>> stream = Streams.stream(Iterables.partition(list, 3));

Answer 4

我这样解决了：

    List<String> list = Arrays.asList("a","b","c","d","e","f","g","h","i","j");
    int groupBy = 3;

    AtomicInteger index = new AtomicInteger(0);         
    Map<Integer, List<String>> groups = list.stream()
        .collect(Collectors.groupingBy(cdm -> index.getAndIncrement()/groupBy));

    System.out.println(groups);

它准备一个地图，其中行号为键，行上的字符串在键中。