Implementing compareTo() : Sorting in natural ordering using compareTo()

Defining the criteria for sorting

Suppose we have list of Employees. We have to sort them. By default, you can't sort it, until you define some criteria. Say you define sorting as first name of employee, or you may say sort on the basis of employee id or their salary. So first thing is we have to define some criteria for sorting. This defines "natural ordering" on which sort can be done.

How to define "natural order" in java?

For this, we have to implement Comparable interface, which has compareTo method, which decides how we put natural ordering on the object.

Example:

Employee’s natural ordering would be done according to the employee id. For that, above Employee class must be altered to add the comparing ability as follows.

public class Employee implements Comparable<Employee> {
    private int empId;
    private String name;
    private int age;
    
    /**
     * Compare a given Employee with this object.
     * If employee id of this object is 
     * greater than the received object,
     * then this object is greater than the other.
     */
    public int compareTo(Employee o) {
        return this.empId - o.empId ;
    }
    ….
}

So this compareTo() method does the trick of implementing natural ordering. So if we have 2 Employees, emp1.id – emp2.id is negative, then emp1 falls before emp2 in this ordering. We can say it like this:

emp1.id < emp2.id – emp1 falls before emp2 in natural ordering

emp1.id == emp2.id – emp1 falls with emp2 in natural ordering

emp1.id > emp2.id – emp1 falls after emp2 in natural ordering

 

Sorting the employee list:

 

We’ll write a class to test this natural ordering mechanism. Following class use the Collections.sort(List) method to sort the given list in natural order.

import java.util.*;

public class TestEmployeeSort {
    
    public static void main(String[] args) {     
        List coll = Util.getEmployees();
        Collections.sort(coll); // sort method
        printList(coll);
    }
    
    private static void printList(List<Employee> list) {
        System.out.println("EmpId\tName\tAge");
        for (Employee e: list) {
            System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge());
        }
    }
}

Similarly to sort on the basis of name : write compareTo like this:

public int compareTo(Employee other) {
    return name.compareTo(o2.getName());
//here compareTo() method of String is called
}