List of objects that implement this interface can be sorted automatically by sort method of the list interface. This interface has compareTo() method that is used by the sort() method of the list.

In this code Employee class is implementing Comparable interface and have method compareTO(). ComparableDemo.java is showing the use of this interface. This class first makes a list of objects of type Employee and call sort method of java.util.Collections, which internally uses compareTo() method of Employee class and sort the list accordingly.

Employee.java

 public class Employee implements Comparable {

    int EmpID;
    String Ename;
    double Sal;
    static int i;

    public Employee() {
        EmpID = i++;
        Ename = "dont know";
        Sal = 0.0;
    }

    public Employee(String ename, double sal) {
        EmpID = i++;
        Ename = ename;
        Sal = sal;
    }

    public String toString() {
        return "EmpID " + EmpID + "\n" + "Ename " + Ename + "\n" + "Sal" + Sal;
    }

    public int compareTo(Object o1) {
        if (this.Sal == ((Employee) o1).Sal)
            return 0;
        else if ((this.Sal) > ((Employee) o1).Sal)
            return 1;
        else
            return -1;
    }
}

ComparableDemo.java

 import java.util.*;

public class ComparableDemo{

    public static void main(String[] args) {

        List ts1 = new ArrayList();
        ts1.add(new Employee ("Tom",40000.00));
        ts1.add(new Employee ("Harry",20000.00));
        ts1.add(new Employee ("Maggie",50000.00));
        ts1.add(new Employee ("Chris",70000.00));
        Collections.sort(ts1);
        Iterator itr = ts1.iterator();

        while(itr.hasNext()){
            Object element = itr.next();
            System.out.println(element + "\n");
            
        }

    }
}

Output:

EmpID 1
Ename Harry
Sal20000.0

EmpID 0
Ename Tom
Sal40000.0

EmpID 2
Ename Maggie
Sal50000.0

EmpID 3
Ename Chris
Sal70000.0