Sorting using Comparable in java with example


Table of Contents

Program to sort on the basis of the name using Comparable.

  • Define an Employee class which should implement Comparable interface and create two variables name and id.  Provide corresponding getter/setter and parameterized constructor. Here we are going to sort employee object on basis of Id.
  • Override compareTo () method and provide logic.
  • @Override
    public int compareTo(Object o) {
            Employee employee = (Employee) o;
            String name = employee.name;
            return this.name.compareTo(name);
    }
    
  • Override compareTo () method and provide logic.
  • Create a couple of employee object using parameterized constructor and add into list.
  • List emplist = new ArrayList();
    emplist.add(new Employee("John", 101));
    emplist.add(new Employee("Eric", 106));
    emplist.add(new Employee("Arya", 103));
    emplist.add(new Employee("Sansa", 105));
    emplist.add(new Employee("Cersei", 102));
    
  • Use Collections.sort(emplist) the method which will internally call the compareTo() method(Check here how String compareTo() method internally works in java). Here Collections is a class and sort() is a static method. Use Java 8 forEach() method and print the name and id.

Comparable example in java:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
class Employee implements Comparable {
 private String name;
 private int id;
 public Employee(String string, int i) {
  this.name = string;
  this.id = i;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public int getId() {
  return id;
 }
 public void setId(int id) {
  this.id = id;
 }
 @Override 
 public int compareTo(Object o) {
  Employee employee = (Employee) o;
  String name = employee.name;
  return this.name.compareTo(name);
 }
}
public class SortingByNameUsingComprable {
 public static void main(String[] args) {
  List emplist = new ArrayList();
  emplist.add(new Employee("John", 101));
  emplist.add(new Employee("Eric", 106));
  emplist.add(new Employee("Arya", 103));
  emplist.add(new Employee("Sansa", 105));
  emplist.add(new Employee("Cersei", 102));
  Collections.sort(emplist);
  emplist.forEach(empolyee -> 
          System.out.println(empolyee.getName() + " " + empolyee.getId()));
 }
}
Arya 103
Cersei 102
Eric 106
John 101
Sansa 105

Program to sort on the basis of the id using Comparable.

For sorting on basis of id we need to change compareTo() logic as below. The rest of the steps would be the same.

@Override public int compareTo(Object o) {
 Employee employee = (Employee) o;
 return this.getId() - employee.getId();
}
import java.lang.reflect.Field;
import java.util.*;
 
class Employee implements Comparable {
    String name;
    int id;
 
    public Employee(String string, int i) {
        this.name = string;
        this.id = i;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
    }
 
    @Override
    public int compareTo(Object o) {
        Employee employee = (Employee) o;
        return this.getId() - employee.getId();
 
    }
}
 
public class SortingByIdUsingComprable {
    public static void main(String[] args) {
        List emplist = new ArrayList();
        emplist.add(new Employee("John", 101));
        emplist.add(new Employee("Eric", 106));
        emplist.add(new Employee("Arya", 103));
        emplist.add(new Employee("Sansa", 105));
        emplist.add(new Employee("Cersei", 102));
 
        Collections.sort(emplist);
        emplist.forEach(empolyee -> System.out.println(empolyee.getName() + "   " + empolyee.getId()));
    }
}
John 101
Cersei 102
Arya 103
Sansa 105
Eric 106

Learn more about collection in java


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