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immutable object

Java Comparable interface

Java Comparable interface

Java Comparable interface provides us facility to sort our custom class object based on natural sorting order means either in ascending order or in descending order.

Comparable interface available in java.lang package and it has only one method public int compareTo().

Comparable provides natural order sorting.

Wen can sort elements  based on single data member only.

Why we need to implement Comparable interface

When we have some requirement to sort Employee record on the basis of their id then using Comparable interface we can sort.If you did not used Comparable interface in your class then you can not call Collections.sort() method on that object.

In simple word i can say it used to allow Collections.sort() to work.

Collections.sort() will be applied only on Comparable implemented classes that is why all wrapper classes are implemented by Comparable interface.

We can not apply Collections.sort() on any non comparable implemented classes.

Working of compareTo(Object)

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

 Method Name  Working
 compareTo(Object)  It will return -1  If comparable object is less than the passed object.
 compareTo(Object)  It will return 0  If comparable object is same as the passed object
 compareTo(Object)   It will return 1  If comparable object is    greater than the passed object


Java Comparable interface Example


How to implement Comparable

public class Employee implements Comparable<Employee> {
 private int id;
 private String name;
 private String address;

 public Employee(int id, String name, String address) { = id; = name;
     this.address = address; 
 @Override public int compareTo(Employee o) {
 if (id > { 
     return 1;
 } else if (id < {
     return -1;
 } else {
     return 0; 
   @Override public String toString() { return "Employee [id=" + id + ", name=" + name + ", address=" + address + "]"; }

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class Test {
  public static void main(String[] args) { 
    List<Employee> list = new ArrayList<>();
    list.add(new Employee(101, "Vikas", "Hyderabad"));
    list.add(new Employee(105, "Sanjit", "Banglore"));
    list.add(new Employee(103, "Yogesh", "Pune"));

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