Why comparable is not working on set?

Question

Here is my files

User.java

public class User implements Comparable<User> {

private String name;
private int age;

public User(String name, int age) {
    super();
    this.name = name;
    this.age = age;
}

public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public int getAge() {
    return age;
}
public void setAge(int age) {
    this.age = age;
}

@Override
public String toString() {
    return "User [name=" + name + ", age=" + age + "]";
}
/**
 * Sorting by name
 */
@Override
public int compareTo(User user) {
    return this.name.compareTo(user.name);
}
}

Test.java

public class Test {
public static void main(String[] args) {

    User u1=new User("Shanaya", 22);
    User u2=new User("Jhon", 27);
    User u3=new User("Arya", 20);

    Set<User> set=new HashSet<>();
    set.add(u1);
    set.add(u2);
    set.add(u3);

    System.out.println(set);  //by name sorting not working here
}

Overriden compareTo() is here to compare objects "by names" only Still output of program is "by age" comparison

output: [User [name=Arya, age=20], User [name=Shanaya, age=22], User [name=Jhon, age=27]]

There is no guarantee that the items in a `HashSet` will be sorted, you need to explicitly sort them. — StaticBeagle, Jun 04 '18 at 02:17
Possible duplicate of [Sorting Values of Set](https://stackoverflow.com/questions/4165832/sorting-values-of-set) — Shubhendu Pramanik, Jun 04 '18 at 02:21
Note that if you implement compareTo(), you should also implement hashCode() and equals() accordingly. — Dorian Gray, Jun 04 '18 at 05:18

score 3 · Answer 1 · answered Jun 04 '18 at 02:18

The problem here is that a HashSet is not an ordered set in Java, and it won't make use of your implementation of the Comparable interface. Try using an ordered set like TreeSet instead:

User u1 = new User("Shanaya", 22);
User u2 = new User("Jhon", 27);
User u3 = new User("Arya", 20);

Set<User> set = new TreeSet<>();
set.add(u1);
set.add(u2);
set.add(u3);

for (User user : set) {
    System.out.println(user);
}

Why comparable is not working on set?

1 Answers1