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  • java8新特性:利用Lambda处理List集合

    Java 8新增的Lambda表达式,我们可以用简洁高效的代码来处理List。

    1、遍历

    复制代码
     public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
    
            userList.stream().forEach(user ->{
                System.out.println(user.getName());
            });
    
        }
    复制代码

    运行结果:

    2、list转为Map

    复制代码
     public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();//存放user对象集合
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
    
            //ID为key,转为Map
            Map<Long,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1, k2)->k1));
            System.out.println(userMap);
    
        }
    复制代码

    运行结果:

    3、将List分组:List里面的对象元素,以某个属性来分组

    复制代码
      public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();//存放user对象集合
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
            User user4 = new User(4L, "张三", 22);
            User user5 = new User(5L, "李四", 20);
            User user6 = new User(6L, "王五", 28);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
            userList.add(user4);
            userList.add(user5);
            userList.add(user6);
    
    
            //根据name来将userList分组
            Map<String, List<User>> groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));
            System.out.println(groupBy);
    
        }
    复制代码

    运行结果:

    4、过滤:从集合中过滤出来符合条件的元素

    复制代码
     public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();//存放user对象集合
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
            User user4 = new User(4L, "张三", 22);
            User user5 = new User(5L, "李四", 20);
            User user6 = new User(6L, "王五", 28);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
            userList.add(user4);
            userList.add(user5);
            userList.add(user6);
    
    
            //取出名字为张三的用户
            List<User> filterList = userList.stream().filter(user -> user.getName().equals("张三")).collect(Collectors.toList());
            filterList.stream().forEach(user ->{
                System.out.println(user.getName());
            });
    
        }
    复制代码

    运行结果:

    5、求和:将集合中的数据按照某个属性求和

    复制代码
     public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();//存放user对象集合
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
            User user4 = new User(4L, "张三", 22);
            User user5 = new User(5L, "李四", 20);
            User user6 = new User(6L, "王五", 28);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
            userList.add(user4);
            userList.add(user5);
            userList.add(user6);
    
    
            //取出名字为张三的用户
            int totalAge = userList.stream().mapToInt(User::getAge).sum();
            System.out.println("和:" + totalAge);
    
        }
    复制代码

    运行结果:

     6、从List转为Map,key与value 一 一对应

    复制代码
     public static void main(String[] args) {
            List<User> userList = Lists.newArrayList();
    
            User user1 = new User(1L, "张三", 24);
            User user2 = new User(2L, "李四", 27);
            User user3 = new User(3L, "王五", 21);
            User user4 = new User(4L, "张三", 22);
            User user5 = new User(5L, "李四", 20);
            User user6 = new User(6L, "王五", 28);
    
            userList.add(user1);
            userList.add(user2);
            userList.add(user3);
            userList.add(user4);
            userList.add(user5);
            userList.add(user6);
    
            Map<Long/*Id*/,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, user -> user));
            System.out.println("toMap:" + JSONArray.toJSONString(userMap));
    
        }
    复制代码

    运行结果:

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  • 原文地址:https://www.cnblogs.com/weizhxa/p/12222000.html
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