StreamApi是jdk1.8所提供的新特性，基于lambda表达式，对数据流的一系列操作。数据源可以来自很多地方，可以是数组、集合也可以是文件等等。对于Stream它包含两个操作，中间操作和终止操作，终止操作出现之前是不会进行中间操作的，也就是说想要使用StreamAPI就必须要有一个终止操作，否者代码将不执行。

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1. 创建Stream
   1. 通过StreamAPI静态工厂创建

      Streamstream = Stream.of(1,2,3,4,5);

   2. 通过集合Collection默认方法stream()创建

      Listlist = List.of(1,2,3,4,5);
      Streamstream = list.stream();

   3. 通过数组创建

      Integer a[] = new Integer[5];
      Streamstream = Arrays.stream(a);

   4. 通过文件创建文件流

      Streamlines = Files.lines(Paths.get("D://a.txt"));

2. 使用StreamAPI

   public class StreamApp {
   
       private ListdataList;
   
       @Before
       public void before() {
           dataList = new ArrayList<>(List.of("zhangsan", "lisi", "wangwu", "zhaoliu", "gaoyuanyuan", "xietingfeng", "zhaoliu"));
       }
   
       @Test
       public void testMap() {
           // map操作
           dataList.stream().map(User::new).forEach(System.out::println);
       }
   
       @Test
       public void testFilter() {
           // filter操作
           dataList.stream().filter(name ->name.startsWith("z")).forEach(System.out::println);
       }
   
       @Test
       public void testAllMatch() {
           // allMatch操作
           boolean boo = dataList.stream().allMatch(name ->name.matches("\\D+"));
           System.out.println(boo);
       }
   
       @Test
       public void testAnyMatch() {
           // anyMatch操作
           boolean boo = dataList.stream().anyMatch(name ->name.contains("y"));
           System.out.println(boo);
       }
   
       @Test
       public void testCount() {
           //统计操作
           System.out.println(dataList.stream().count());
       }
   
       @Test
       public void testDistinct() {
           //去重操作
           dataList.stream().distinct().forEach(System.out::println);
       }
   
       @Test
       public void testFindAny() {
           //findAny操作
           System.out.println(dataList.stream().findAny().orElse("liuhf"));
       }
   
       @Test
       public void testFindFirst() {
           // findFirst操作
           System.out.println(dataList.stream().findFirst().orElse("liuhf"));
       }
   
   
       @Test
       public void testFlatMap() {
           // 扁平化
           dataList.stream().flatMap(name->Stream.of(name.split("z"))).forEach(System.out::println);
       }
   
       @Test
       public void testForeach(){
           // 遍历
           dataList.stream().forEach(System.out::println);
       }
   
       @Test
       public void testForeachOrder(){
           dataList.stream().forEachOrdered(System.out::println);
       }
   
       @Test
       public void testLimit(){
           // limit操作
           dataList.stream().limit(2).forEach(System.out::println);
       }
   
       @Test
       public void testMax(){
           // 查大值
           System.out.println(dataList.stream().max(Comparator.comparingInt(n ->n.charAt(0))).orElse("liuhf"));
       }
   
       @Test
       public void testMin(){
           // 查最小值
           System.out.println(dataList.stream().min(Comparator.comparingInt(n ->n.charAt(0))).orElse("liuhf"));
       }
   
       @Test
       public void testNoneMatch(){
           // 查不存在
           System.out.println(dataList.stream().noneMatch(name ->name.startsWith("a")));
       }
   
       @Test
       public void testPeek(){
           // peek 操作
           Listnames = dataList.stream().peek(name ->System.out.println(name.charAt(0))).toList();
       }
   
       @Test
       public void testReduce(){
           // 统计
           int val = Stream.of(1, 2, 3, 4, 5).reduce(Integer::sum).orElse(0);
           System.out.println(val);
           int value = Stream.of(1, 2, 3, 4, 5).reduce(0, Integer::sum);
           System.out.println(value);
           Integer reduce = Stream.of(1, 2, 3, 4, 5).reduce(0, Integer::sum, Integer::sum);
           System.out.println(reduce);
       }
   
       @Test
       public void testSorted(){
           // 排序
           dataList.stream().sorted().forEach(System.out::println);
           System.out.println(">>>>>>>>>>>>>>>>>>>");
           dataList.stream().sorted(Comparator.comparing(x->x.charAt(1))).forEach(System.out::println);
       }
   
       @Test
       public void testToArray(){
           //转换为数组
           Object[] array = dataList.stream().toArray();
           for (Object o : array) {
               System.out.println(o);
           }
           System.out.println(">>>>>>>>>>>>>");
           //可以使用构造引用，为了表诉清楚先这样编写
           for (String s : dataList.stream().toArray(x ->new String[x])) {
               System.out.println(s);
           }
       }
   
   
   
   
       @Test
       public void testSkip(){
           //跳过
           dataList.stream().skip(2).forEach(System.out::println);
       }
   
       static class User {
           String name;
   
           public User(String name) {
               this.name = name;
           }
   
           @Override
           public String toString() {
               return "User{" +
                       "name='" + name + '\'' +
                       '}';
           }
       }
   }

3. Stream收集器Collector

   public class CollectorApp {
   
       private Listdata;
   
       @Before
       public void before() {
           data = new ArrayList<>(List.of("1", "2", "3", "4", "5"));
       }
   
       @Test
       public void testAveragingInt() {
           //求平均值
           Double collect = data.stream().collect(Collectors.averagingInt(Integer::valueOf));
           System.out.println(collect);
       }
       @Test
       public void testAveragingDouble() {
           //求平均值
           Double collect = data.stream().collect(Collectors.averagingDouble(Double::valueOf));
           System.out.println(collect);
       }
       @Test
       public void testAveragingLong() {
           //求平均值
           Double collect = data.stream().collect(Collectors.averagingLong(Long::valueOf));
           System.out.println(collect);
       }
   
       @Test
       public void testCollectingAndThen(){
           //收集以及做额外的转换
           Setcollect = data.stream().collect(Collectors.collectingAndThen(Collectors.toSet(), Collections::unmodifiableSet));
       }
   
       @Test
       public void testCounting(){
           Long count = data.stream().collect(Collectors.counting());
           System.out.println(count);
       }
   
       @Test
       public void testGroupingBy(){
           //参数1： 分组函数
           Map>map = data.stream().collect(Collectors.groupingBy(num ->Integer.parseInt(num) % 2 == 0));
           System.out.println(map);
   
           //参数1：分组函数 参数2：收集器类型
           Map>map1 = data.stream().collect(Collectors.groupingBy(num ->Integer.parseInt(num) % 2 == 0, Collectors.toSet()));
           System.out.println(map1);
   
           //参数1：分组函数 参数2：工厂，提供容器类型 参数3：收集器类型
           TreeMap>map2 = data.stream().collect(Collectors.groupingBy(num ->Integer.parseInt(num) % 2 == 0, () ->new TreeMap<>(), Collectors.toSet()));
           System.out.println(map2);
       }
   
       @Test
       public void testJoining(){
           //将元素合并为字符串且无分隔符
           String collect = data.stream().collect(Collectors.joining());
           System.out.println(collect);
   
           String collect1 = data.stream().collect(Collectors.joining(","));
           System.out.println(collect1);
   
           String collect2 = data.stream().collect(Collectors.joining(",", "[", "]"));
           System.out.println(collect2);
       }
   
       @Test
       public void testMapping(){
           // 同stream的map函数，使用频率较少，只在stream的map函数不方便使用是才会选择
           Setcollect = data.stream().collect(Collectors.mapping(Integer::parseInt, Collectors.toSet()));
           System.out.println(collect);
       }
   
       @Test
       public void testMaxBy(){
           // 求大值，同stream的max()
           String max = data.stream().collect(Collectors.maxBy((n1, n2) ->Integer.parseInt(n1) - Integer.parseInt(n2))).orElse("0");
           System.out.println(max);
       }
   
       @Test
       public void testMinBy(){
           // 求最小值，同stream的min()
           String min = data.stream().collect(Collectors.minBy((n1, n2) ->Integer.parseInt(n1) - Integer.parseInt(n2))).orElse("0");
           System.out.println(min);
       }
   
       @Test
       public void testPartitioningBy(){
           //分割，一个特殊分组，只能分出boolean类型的key
           Map>map = data.stream().collect(Collectors.partitioningBy(num ->Integer.parseInt(num) % 2 == 0));
           System.out.println(map);
   
           //提供一个收集器
           Map>map1 = data.stream().collect(Collectors.partitioningBy(num ->Integer.parseInt(num) % 2 == 0,Collectors.toSet()));
           System.out.println(map1);
       }
   
       @Test
       public void testReducing(){
           //同stream函数reduce函数，合并
           String s = data.stream().collect(Collectors.reducing((n1, n2) ->String.valueOf(Integer.parseInt(n1) + Integer.parseInt(n2)))).orElse("0");
           System.out.println(s);
   
           String s1 = data.stream().collect(Collectors.reducing("0",(n1, n2) ->String.valueOf(Integer.parseInt(n1) + Integer.parseInt(n2))));
           System.out.println(s1);
   
           String s2 = data.stream().collect(Collectors.reducing("0",(n)->String.valueOf(Integer.parseInt(n)+1),(n1, n2) ->String.valueOf(Integer.parseInt(n1) + Integer.parseInt(n2))));
           System.out.println(s2);
   
       }
   
       @Test
       public void testSummarizing(){
           //总结
           DoubleSummaryStatistics collect = data.stream().collect(Collectors.summarizingDouble(Double::parseDouble));
           System.out.println(collect);
   
           IntSummaryStatistics collect1 = data.stream().collect(Collectors.summarizingInt(Integer::parseInt));
           System.out.println(collect1);
   
           LongSummaryStatistics collect2 = data.stream().collect(Collectors.summarizingLong(Long::parseLong));
           System.out.println(collect2);
       }
   
       @Test
       public void testSumming(){
   
           //计算总和
           Double aDouble = data.stream().collect(Collectors.summingDouble(Double::parseDouble));
           System.out.println(aDouble);
   
           Integer aInt = data.stream().collect(Collectors.summingInt(Integer::parseInt));
           System.out.println(aInt);
   
           Long aLong = data.stream().collect(Collectors.summingLong(Long::parseLong));
           System.out.println(aLong);
       }
   
       @Test
       public  void testToList(){
           Listcollect = data.stream().collect(Collectors.toList());
       }
   
       @Test
       public void testToSet(){
           Setcollect = data.stream().collect(Collectors.toSet());
       }
   
       @Test
       public void testCollection(){
           LinkedListcollect = data.stream().collect(Collectors.toCollection(LinkedList::new));
       }
   
       @Test
       public void testToMap(){
           Mapcollect1 = data.stream().collect(Collectors.toMap(k ->Integer.parseInt(k) + 1, v ->v));
           Mapcollect2 = data.stream().collect(Collectors.toMap(k ->Integer.parseInt(k) + 1, v ->v,(n1,n2)->String.valueOf(Integer.parseInt(n1)+Integer.parseInt(n2))));
           Mapcollect3 = data.stream().collect(Collectors.toMap(k ->Integer.parseInt(k) + 1, v ->v,(n1,n2)->String.valueOf(Integer.parseInt(n1)+Integer.parseInt(n2)),TreeMap::new));
           System.out.println(collect1);
           System.out.println(collect2);
           System.out.println(collect3);
   
           //toConcurrentMap同toMap一样，只是Map为线程安全的ConcurrentMap
       }
   }

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文章名称：Stream流用法示例-创新互联
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