stream的基本用法

stream交集、差集、并集、去重并集

//String[] 成list 
String[] strArr = targetHeadCost.getCurrentCostChangeReason().split(",");
List<String> strList = Arrays.stream(strArr).collect(Collectors.toList());

//list 转成String 并使用逗号隔开
List<String> reasonList = model.getCurrentCostChangeReason();
if (null != reasonList && reasonList.size() > 0) {
    String reasonStr = String.join(",", reasonList);
    targetHeadCost.setCurrentCostChangeReason(reasonStr);
}
@Test
   public void LambTest() {  //lamb 表达式处理list 集合
      /*一般有filter 操作时，不用并行流parallelStream ,如果用的话可能会导致线程安全问题
　　     判断对象要重写hash*/
 
      List<String> list1 = new ArrayList();
      list1.add("1111");
      list1.add("2222");
      list1.add("3333");
 
      List<String> list2 = new ArrayList();
      list2.add("3333");
      list2.add("4444");
      list2.add("5555");
      
 
      // 交集
      List<String> intersection = list1.stream().filter(item -> list2.contains(item)).collect(Collectors.toList());
      System.out.println("---得到交集 intersection---");
      intersection.parallelStream().forEach(System.out::println);
 
      //同双冒号写法
      intersection.parallelStream().forEach(x->{
         System.out.println("x ="+x);
      });
 
      // 差集 (list1 - list2)
      List<String> reduce1 = list1.stream().filter(item -> !list2.contains(item)).collect(Collectors.toList());
      System.out.println("---得到差集 reduce1 (list1 - list2)---");
      reduce1.parallelStream().forEach(System.out::println);
 
      // 差集 (list2 - list1)
      List<String> reduce2 = list2.stream().filter(item -> !list1.contains(item)).collect(Collectors.toList());
      System.out.println("---得到差集 reduce2 (list2 - list1)---");
      reduce2.parallelStream().forEach(System.out::println);
 
      // 并集
      List<String> listAll = list1.parallelStream().collect(Collectors.toList());
      List<String> listAll2 = list2.parallelStream().collect(Collectors.toList());
      listAll.addAll(listAll2);
      System.out.println("---得到并集 listAll---");
      listAll.parallelStream().forEach(System.out::println);
 
      // 去重并集
      List<String> listAllDistinct = listAll.stream().distinct().collect(Collectors.toList());
      System.out.println("---得到去重并集 listAllDistinct---");
      listAllDistinct.parallelStream().forEach(System.out::println);
 
      System.out.println("---原来的List1---");
      list1.parallelStream().forEach(System.out::println);
      System.out.println("---原来的List2---");
      list2.parallelStream().forEach(System.out::println);
 
   }

---

stream中list转map

• 方式一

List<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        Map<String, String> map = list.stream().collect(Collectors.toMap(Function.identity(),Function.identity()));
        map.forEach((x, y) -> System.out.println("key:" + x +", value:"+ y));

• 方式二

List<UserBean> userList = new ArrayList<>();
		UserBean userBean = new UserBean("张三","热爱学习", 21);
		userList.add(userBean);
		
		 userBean = new UserBean("张三2","热爱学习2", 25);
		userList.add(userBean);
		
		 userBean = new UserBean("张三33","热爱学习3", 27);
		userList.add(userBean);
		
		 userBean = new UserBean("张三4","热爱学习2", 26);
		userList.add(userBean);
		
		System.out.println(userList);
		Map<String,Integer>  map = 	    
        userList.stream().collect(Collectors.toMap(UserBean::getUserName, UserBean::getAge, 
        (key1,key2) ->key2));
		
		System.out.println(map);

排序

List<People> peopleList = new ArrayList<>();
        People lihua1 = People.builder().id(1).name("lihua1").build();
        People lihua2 = People.builder().id(2).name("lihua2").build();
        People lihua3 = People.builder().id(3).name("lihua3").build();
        People lihua4 = People.builder().id(4).name("lihua4").build();

        Collections.addAll(peopleList, lihua1, lihua2, lihua3, lihua4);

        System.out.println("--------------------顺序输出-------------------");

        peopleList.stream().sorted(Comparator.comparing(People::getId))
                .forEach(System.out::println);

        System.out.println("--------------------逆序输出-------------------");

        peopleList.stream().sorted(Comparator.comparing(People::getId).reversed())
                .forEach(System.out::println);

        System.out.println("--------------------求最小值-------------------");

        People peopleMin = peopleLst.stream().min(Comparator.comparing(People::getId))
                .orElse(null);
        System.out.println(peopleMin);

        System.out.println("--------------------求最大值--------------------");

        People peopleMax = peopleList.stream().max(Comparator.comparing(People::getId))
                .orElse(null);
        System.out.println(peopleMax);