java stream流中的collect()方法详解

原文：https://www.cnblogs.com/Lothlorien/p/12024041.html

~~~

public class StreamTest {

    /**

     * stream.collect() 的本质由三个参数构成,

     * 1. Supplier 生产者, 返回最终结果

     * 2. BiConsumer accumulator 累加器

     * 第一个参数是要返回的集合, 第二个参数是遍历过程中的每个元素,

     * 将流中每个被遍历的元素添加到集合中

     * 3. BiConsumer combiner 合并器, 在有并行流的时候才会有用, 一个流时代码不会走到这里

     * 将第二步遍历得到的所有流形成的list都添加到最终的list中,

     * 最后返回list1

     */

    @Test

    public void Test() {

        Stream stream = Stream.of("hello", "world", "helloworld");

        // 最原始和基础的方式

        /*

        List list = stream.collect(

                ()->new ArrayList(),

                (theList, item) -> theList.add(item),

                (list1, list2) -> list1.addAll(list2)

        );

        */

        // 打印出更详尽的过程

        List listDetail = stream.collect(

                () -> {

                    ArrayList arrayList = new ArrayList<>();

                    System.out.println("第一个list诞生, size: " + arrayList.size());

                    return arrayList;

                },

                (theList, item) -> {

                    System.out.println("第二个list的size: " + theList.size());

                    theList.add(item);

                },

                (list1, list2) -> {

                    System.out.println("第三个list1的size: " + list1.size());

                    System.out.println("第四个list2的size: " + list2.size());

                    list1.addAll(list2);

                }

        );

        /* 输出

            第一个list诞生, size: 0

            第二个list的size: 0

            第二个list的size: 1

            第二个list的size: 2

        * */

        // 使用方法引用来传递行为, 更加清晰易懂, new(新建) -> add(累加) -> addAll(合并)

        List list2 = stream.collect(LinkedList::new, LinkedList::add, LinkedList::addAll);

        String concat = stream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append).toString();

        System.out.println(concat);

    }

    @Test

    public void Test2() {

        Stream stream = Stream.of("hello", "world", "helloworld");

        // 这样的写法兼具灵活和简单

        ArrayList list = stream.collect(Collectors.toCollection(ArrayList::new));

        TreeSet treeSet = stream.collect(Collectors.toCollection(TreeSet::new));

        String s = stream.collect(Collectors.joining()); // 拼接成字符串

        HashMap map = stream.collect(HashMap::new, (x, y) -> {

            x.put(y, y); // 自己做自己的key

        }, HashMap::putAll);

    }

}

~~~