How to use Comparable Interface

Reference : http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Comparable.html
This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method.
Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort). Objects that implement this interface can be used as keys in a sorted map or elements in a sorted set, without the need to specify a comparator.

Reference : http://www.java-tips.org/java-se-tips/java.lang/how-to-use-comparable-interface.html

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");

}

}
}