对象集合根据其中某个字段去重,一行解决!

妖狐艹你老母 2022-12-23 07:03 169阅读 0赞

>  写业务代码的时候碰到一个业务场景,集合需要根据某个字段去重.
> 
> 自己写的太多,所以到网上找了一个简洁的方案,一行解决了,运行也没有问题.
> 
> 但是不太看得懂代码原理,到网上查了点资料弄明白了,在此记录一下

**核心代码,要用的拿这行去,改下参数名就行了: **

userList = userList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getUserid))), ArrayList::new));

展开的样子:

userList = userList.stream().collect(
            Collectors.collectingAndThen(
                    Collectors.toCollection(
                            () -> new TreeSet<>(
                                    Comparator.comparing(User::getUserid)
                            )
                    ), ArrayList::new
            )
    );

### 我的理解都在注释里面了: ###

**(用到的User类在文章末尾)**

public static void main(String[] args) {
            List<User> userList = new ArrayList<>();
            userList.add(new User("1", "李大锤", "23", "南京"));
            userList.add(new User("2", "张无忌", "18", "西安"));
            userList.add(new User("3", "刘德华", "26", "苏州"));
            userList.add(new User("4", "郭靖", "33", "上海"));
            userList.add(new User("1", "李大锤", "23", "南京"));    //id相同，其他数据也相同
            userList.add(new User("3", "带头大哥", "36", "杭州"));  //id相同，其他数据不同
            System.out.println("原始数据:" + userList);
            //使用stream流,然后可以使用steam中才能用的API
            userList = userList.stream().collect(
                    // collectingAndThen: 第一个参数是 Collectors,第二个参数是Function,用来处理第一个参数
                    Collectors.collectingAndThen(
                            // toCollection: 把TreeSet转为Collectors类
                            Collectors.toCollection(
                                    // 根据userId去重,主要是利用了 TreeSet的有序不重复特性,并且TreeSet能传入比较器,就能根据某一字段去重
                                    () -> new TreeSet<>( Comparator.comparing(
                                            //这里是 User::getUserid 写法的lambda表达式写法
                                            user ->  user.getUserid()
                                    ))
                            )
                    ,  //这里是 ArrayList::new 的lambda表达式写法,将上面的 TreeSet转为ArrayList
                        treeSet -> new ArrayList<>(treeSet)
                    )
            );
            System.out.println("去重后数据:"+ userList);
        }

### 如果还没看懂,可能就要百度下相关API 的用法了. ###

### 这里是看懂之后我自己写的 : ###

public static void main(String[] args) {
            User u1 = new User("1", "李大锤", "23", "南京");
            User u2 = new User("2", "张无忌", "18", "西安");
            User u3 = new User("3", "刘德华", "26", "苏州");
            User u4 = new User("4", "郭靖", "33", "上海");
            User u5 = new User("1", "李大锤", "23", "南京");    //id相同，其他数据也相同
            User u6 = new User("3", "带头大哥", "36", "杭州");  //id相同，其他数据不同
            ArrayList<User> userList = new ArrayList<>();
            userList.add(u1);
            userList.add(u2);
            userList.add(u3);
            userList.add(u4);
            userList.add(u5);
            userList.add(u6);
            System.out.println("原始数据:" + userList);
            userList = new ArrayList<>( userList.stream().collect(Collectors.toCollection(() -> new TreeSet<User>(Comparator.comparing(User::getUserid)))) );
            System.out.println("去重后数据:"+ userList);
    
        }

用到的对象类:

public class User {
        private String userid;
        private String username;
        private String age;
        private String address;
    
        public User(String userid, String username, String age, String address) {
            super();
            this.userid = userid;
            this.username = username;
            this.age = age;
            this.address = address;
        }
    
        public String getUserid() {
            return userid;
        }
        public void setUserid(String userid) {
            this.userid = userid;
        }
        public String getUsername() {
            return username;
        }
        public void setUsername(String username) {
            this.username = username;
        }
        public String getAge() {
            return age;
        }
        public void setAge(String age) {
            this.age = age;
        }
        public String getAddress() {
            return address;
        }
        public void setAddress(String address) {
            this.address = address;
        }
        @Override
        public String toString() {
            return "User [userid=" + userid + ", username=" + username + ", age=" + age + ", address=" + address + "]";
        }
    }