Jul 24, 2021by @kodeazy

what is comparable in java and how it works?

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java

comparable is an interface used to sort objects of user defined class.
It is found in java.lang package.
public void sort(List list) method of collection interface is used to sort the objects.
For Example Collections.sort(al); here al is an object.

compareTo(Object) method Compares current object with the specified object for order to sort in ascending or descending order as per the requirment.

public int compareTo(Employee e){
    if(salary>e.salary)
    return 1;
    else if(salary<e.salary)
    return -1;
    else
    return 0;

}

compareTo(Object) method returns positive or negitive or zero as above.
Above method returns Objects in Asc order

To print in Desc order just modify the method return statments from 1 to-1 and -1 to 1 as below in compareTo(Object) method

public int compareTo(Employee e){
    if(salary>e.salary)
    return -1;
    else if(salary<e.salary)
    return 1;
    else
    return 0;

}

Below is the Example to Sort and display employees data based on their salary in Ascending Order

import java.util.*;
class Employee implements Comparable<Employee>{
int age;
int salary;
String name;
Employee(int age,int salary,String name){
    this.age=age;
    this.salary=salary;
    this.name=name;

}
public int compareTo(Employee e){
    if(salary>e.salary)
    return 1;
    else if(salary<e.salary)
    return -1;
    else
    return 0;

}
}
class ComparableExample{
public static void main(String args[]){
    List<Employee> al=new ArrayList<Employee>();
    al.add(new Employee(27, 103, "Suresh"));
    al.add(new Employee(23, 102, "Ramesh"));
    al.add(new Employee(21, 105, "Mahesh"));
    al.add(new Employee(29, 104, "Gopi"));
    Collections.sort(al);
    Iterator<Employee> it=al.iterator();
    System.out.println("Details after sorting");
    while(it.hasNext()){
        Employee e=(Employee)it.next();
        System.out.println("Name  :"+e.name+" salary :"+e.salary+" Age :"+e.age);
    }
}
}

Output:

Name  :Ramesh salary :102 Age :23
Name  :Suresh salary :103 Age :27
Name  :Gopi   salary :104 Age :29  
Name  :Mahesh salary :105 Age :21