If there are two sets, how to get their intersection and union quickly in java?
1. Give two List sets first

List<String> listA = new ArrayList<String>();
List<String> listB = new ArrayList<String>();
listA.add("A");
listA.add("B");
listB.add("B");
listB.add("C");

2. intersection

listA.retainAll(listB);
System.out.println(listA);

Print results:
[B]

3. join and union

// If we don't do the first step, we will take the union with repeated elements
listA.removeAll(listB);
listA.addAll(listB);
System.out.println(listA);

//Print results:
[A, B, C]

4. difference sets

listA.removeAll(listB);
System.out.println(listA);

Print results:
[A]

5. The data in the list is an object, not a basic data type
The basic data type of java is used above. If it is not a basic data type, we can override the methods in the class as follows:
(1) First, create a class as a generic type in the List:
User.java

package com.list;

public class User {
    private String id;
    private String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    @Override
    public boolean equals(Object obj) {
        User u = (User) obj;
        return id.equals(u.getId()) && name.equals(u.getName());
    }

    @Override
    public int hashCode() {
        return id.hashCode();
    }
}

(2) Take no duplicate union operation:

public class RemoveRepetition {
    public static void main(String[] args) {
        List<User> listA = new ArrayList<User>();
        List<User> listB = new ArrayList<User>();

        User user1 = new User("1", "a");
        User user2 = new User("2", "b");
        User user3 = new User("3", "c");

        listA.add(user1);
        listA.add(user2);
        listB.add(user2);
        listB.add(user3);

        listA.removeAll(listB);
        listA.addAll(listB);

        for(User u : listA) {
            System.out.println(u.getId() + ":" + u.getName());
        }
    }
}

//Print results:
1:a
2:b
3:c

(3) Fetch intersection operation

public class RemoveRepetition {
    public static void main(String[] args) {
        List<User> listA = new ArrayList<User>();
        List<User> listB = new ArrayList<User>();

        User user1 = new User("1", "a");
        User user2 = new User("2", "b");
        User user3 = new User("3", "c");

        listA.add(user1);
        listA.add(user2);
        listB.add(user2);
        listB.add(user3);

        listA.retainAll(listB);

        for(User u : listA) {
            System.out.println(u.getId() + ":" + u.getName());
        }
    }
}

//Print results:
2:b