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

public static void main(String[] args) {        List
     
       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());        });    }
     

运行结果：

2、list转为Map

public static void main(String[] args) {        List
     
       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
      
        userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1, k2)->k1));        System.out.println(userMap);    }
      
     

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

public static void main(String[] args) {        List
     
       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
      
       > groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));        System.out.println(groupBy);    }
      
     

运行结果：

4、过滤：从集合中过滤出来符合条件的元素

public static void main(String[] args) {        List
     
       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
      
        filterList = userList.stream().filter(user -> user.getName().equals("张三")).collect(Collectors.toList());        filterList.stream().forEach(user ->{            System.out.println(user.getName());        });    }
      
     

运行结果：

5、求和：将集合中的数据按照某个属性求和

public static void main(String[] args) {        List
     
       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);    }
     

运行结果：

 6、从List转为Map，key与value 一 一对应

public static void main(String[] args) {        List
     
       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
      
        userMap = userList.stream().collect(Collectors.toMap(User::getId, user -> user));        System.out.println("toMap：" + JSONArray.toJSONString(userMap));    }
      
     

运行结果：