Java 8：如何使用lambda将列表转换为列表 [英] Java 8: How to turn a list into a list of lists using lambda

问题描述

我正在尝试将列表拆分为列表，其中每个列表的最大大小为4.

I'm trying to split a list into a list of list where each list has a maximum size of 4.

我想知道这是怎么回事使用lambdas。

I would like to know how this is possible to do using lambdas.

目前我正在做的方式如下：

Currently the way I'm doing it is as follow:

List<List<Object>> listOfList = new ArrayList<>();

final int MAX_ROW_LENGTH = 4;
int startIndex =0;
while(startIndex <= listToSplit.size() )    
{
    int endIndex = ( ( startIndex+MAX_ROW_LENGTH ) <  listToSplit.size() ) ? startIndex+MAX_ROW_LENGTH : listToSplit.size();
    listOfList.add(new ArrayList<>(listToSplit.subList(startIndex, endIndex)));
    startIndex = startIndex+MAX_ROW_LENGTH;
}

更新

似乎没有一种简单的方法可以使用lambdas来拆分列表。虽然所有答案都非常受欢迎，但它们也是lambda不简化事物的一个很好的例子。

It seems that there isn't a simple way to use lambdas to split lists. While all of the answers are much appreciated, they're also a wonderful example of when lambdas do not simplify things.

推荐答案

尝试这种方法：

static <T> List<List<T>> listSplitter(List<T> incoming, int size) {
    // add validation if needed
    return incoming.stream()
            .collect(Collector.of(
                    ArrayList::new,
                    (accumulator, item) -> {
                        if(accumulator.isEmpty()) {
                            accumulator.add(new ArrayList<>(singletonList(item)));
                        } else {
                            List<T> last = accumulator.get(accumulator.size() - 1);
                            if(last.size() == size) {
                                accumulator.add(new ArrayList<>(singletonList(item)));
                            } else {
                                last.add(item);
                            }
                        }
                    },
                    (li1, li2) -> {
                        li1.addAll(li2);
                        return li1;
                    }
            ));
}
System.out.println(
        listSplitter(
                Arrays.asList(0, 1, 2, 3, 4, 5, 6, 7, 8, 9),
                4
        )
);

另请注意，此代码可以进行优化，而不是：

Also note that this code could be optimized, instead of:

new ArrayList<>(Collections.singletonList(item))

使用这一个：

List<List<T>> newList = new ArrayList<>(size);
newList.add(item);
return newList;

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