list通过里面对象的某个属性分组、分组求和、去重

 1、实体类和list集合

//实体类
public class User {
    private int id;
    private String name;
    private int age;
    
    public User(int id,String name,int age){
        this.id = id;
        this.name = name;
        this.age = age;
    }
}

//创建list

List<User> users = new ArrayList<>();
users.add(new User(1,"A",8));
users.add(new User(2,"A",7));
users.add(new User(3,"A",6));
users.add(new User(4,"B",7));
users.add(new User(5,"B",6));
users.add(new User(6,"C",6));
users.add(new User(6,"C",6));

2、list 操作

 

/**
 * 1，跟据某个属性(name)分组
 **/
   Map<String,List<User>> collectName = users.stream().collect(Collectors.groupingBy(User::getName));
   System.out.println(collectName);

/**
 * 2，根据某个属性(name)分组，求age的和
 **/
   Map<String,Integer> collectNameSumAge = users.stream().collect(Collectors.groupingBy(User::getName,Collectors.summingInt(User::getAge)));
   System.out.println(collectNameSumAge);

/**
 * 3，取出一组对象的某个属性(name)组成一个新集合
 */
   List<String> names = users.stream().map(User::getName).collect(Collectors.toList());
   System.out.println(names);

/**
 * 4，list去重复
 */
   names = names.stream().distinct().collect(Collectors.toList());
   System.out.println(names);

   users = users.stream().distinct().collect(Collectors.toList());
   System.out.println(users);

/**
 * 5，根据某个属性(name)添加条件过滤数据
 **/
   users = users.stream().filter(user -> user.getName().equals("A")).collect(Collectors.toList());
   System.out.println(users);

/**
 * 6，判断一组对象里面有没有属性值是某个值
 */
   boolean checkName = names.contains("A");
   System.out.println(checkName);

   boolean bool = users.stream().anyMatch(user -> "A".equals(user.getName()));
   System.out.println(bool);