java stream流中的collect()方法详解

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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<R, ? super T> 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<R, R> 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<String> stream = Stream.of("hello", "world", "helloworld");         // 最原始和基础的方式         /*         List<String> 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<String> listDetail = stream.collect(                 () -> {                     ArrayList<String> 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<String> 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<String> stream = Stream.of("hello", "world", "helloworld");         // 这样的写法兼具灵活和简单         ArrayList<String> list = stream.collect(Collectors.toCollection(ArrayList::new));         TreeSet<String> treeSet = stream.collect(Collectors.toCollection(TreeSet::new));         String s = stream.collect(Collectors.joining()); // 拼接成字符串         HashMap<String, String> map = stream.collect(HashMap::new, (x, y) -> {             x.put(y, y); // 自己做自己的key         }, HashMap::putAll);     } }

　

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