This is comparables, this interfaces compares different objects of the class based on the type of object we give, like in this case age

comparables.java

import java.util.*;;
public class comparable implements Comparable<comparable> {
    private String name;
    private int age;

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

    @Override
    public int compareTo(comparable otherPerson) {
        return this.age - otherPerson.age;
         // return this.name.compareTo(otherPerson.name);
    }

    public String toString() {
        return name + " (" + age + ")";
    }

   
        public static void main(String[] args) {
            List<comparable> people = new ArrayList<>();
            people.add(new comparable("Alice", 25));
            people.add(new comparable("Bob", 20));
            people.add(new comparable("Charlie", 30));
    
            Collections.sort(people);
    
            for (int i = 0; i < people.size(); i++) {
                comparable p = people.get(i);
                System.out.println(p);
            }
        }
        
}