Comparable and Comparator are two interfaces provided by Java Core API. From their names, we can tell they may be used for comparing stuff in some way. But what exactly are they and what is the difference between them? The following are two examples for answering this question. The simple examples compare two HDTV’s size. How to use Comparable vs. Comparator is obvious after reading the code.
1. Comparable
Comparable is implemented by a class in order to be able to comparing object of itself with some other objects. The class itself must implement the interface in order to be able to compare its instance(s). The method required for implementation is compareTo(). Here is an example:
class HDTV implements Comparable<HDTV> { private int size; private String brand; public HDTV(int size, String brand) { this.size = size; this.brand = brand; } public int getSize() { return size; } public void setSize(int size) { this.size = size; } public String getBrand() { return brand; } public void setBrand(String brand) { this.brand = brand; } @Override public int compareTo(HDTV tv) { if (this.getSize() > tv.getSize()) return 1; else if (this.getSize() < tv.getSize()) return -1; else return 0; } } public class Main { public static void main(String[] args) { HDTV tv1 = new HDTV(55, "Samsung"); HDTV tv2 = new HDTV(60, "Sony"); if (tv1.compareTo(tv2) > 0) { System.out.println(tv1.getBrand() + " is better."); } else { System.out.println(tv2.getBrand() + " is better."); } } } |
Sony is better.
2. Comparator
In some situations, you may not want to change a class and make it comparable. In such cases, Comparator can be used if you want to compare objects based on certain attributes/fields. For example, 2 persons can be compared based on `height` or `age` etc. (this can not be done using comparable.)
The method required to implement is compare(). Now let’s use another way to compare those TV by size. One common use of Comparator is sorting. Both Collections and Arrays classes provide a sort method which use a Comparator.
import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; class HDTV { private int size; private String brand; public HDTV(int size, String brand) { this.size = size; this.brand = brand; } public int getSize() { return size; } public void setSize(int size) { this.size = size; } public String getBrand() { return brand; } public void setBrand(String brand) { this.brand = brand; } } class SizeComparator implements Comparator<HDTV> { @Override public int compare(HDTV tv1, HDTV tv2) { int tv1Size = tv1.getSize(); int tv2Size = tv2.getSize(); if (tv1Size > tv2Size) { return 1; } else if (tv1Size < tv2Size) { return -1; } else { return 0; } } } public class Main { public static void main(String[] args) { HDTV tv1 = new HDTV(55, "Samsung"); HDTV tv2 = new HDTV(60, "Sony"); HDTV tv3 = new HDTV(42, "Panasonic"); ArrayList<HDTV> al = new ArrayList<HDTV>(); al.add(tv1); al.add(tv2); al.add(tv3); Collections.sort(al, new SizeComparator()); for (HDTV a : al) { System.out.println(a.getBrand()); } } } |
Output:
Panasonic Samsung Sony
Often we may use Collections.reverseOrder() method to get a descending order Comparator. Like the following:
ArrayList<Integer> al = new ArrayList<Integer>(); al.add(3); al.add(1); al.add(2); System.out.println(al); Collections.sort(al); System.out.println(al); Comparator<Integer> comparator = Collections.reverseOrder(); Collections.sort(al,comparator); System.out.println(al); |
Output:
[3,1,2] [1,2,3] [3,2,1]
3. When to use Which?
In brief, a class that implements Comparable will be comparable, which means it instances can be compared with each other.
A class that implements Comparator will be used in mainly two situations: 1) It can be passed to a sort method, such as Collections.sort() or Arrays.sort(), to allow precise control over the sort order and 2) It can also be used to control the order of certain data structures, such as sorted sets (e.g. TreeSet) or sorted maps (e.g., TreeMap).
For example, to create a TreeSet. We can either pass the constructor a comparator or make the object class comparable.
Approach 1 – TreeSet(Comparator comparator)
class Dog { int size; Dog(int s) { size = s; } } class SizeComparator implements Comparator<Dog> { @Override public int compare(Dog d1, Dog d2) { return d1.size - d2.size; } } public class ImpComparable { public static void main(String[] args) { TreeSet<Dog> d = new TreeSet<Dog>(new SizeComparator()); // pass comparator d.add(new Dog(1)); d.add(new Dog(2)); d.add(new Dog(1)); } } |
Approach 2 – Implement Comparable
class Dog implements Comparable<Dog>{ int size; Dog(int s) { size = s; } @Override public int compareTo(Dog o) { return o.size - this.size; } } public class ImpComparable { public static void main(String[] args) { TreeSet<Dog> d = new TreeSet<Dog>(); d.add(new Dog(1)); d.add(new Dog(2)); d.add(new Dog(1)); } } |
References:
1. Comparable
2. Comparator
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I have confusion about this concept. But your blog is very helpful and easy to understand , Many thanks for sharing.
This is very Useful.You’ve taught well.
TestArraylist.java
===================================
package Compare;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import Util.TestUtil;
/*
* author:
* date:
* purpose:
* Binary Search using collections
*
* */
public class TestArraylist {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
ArrayList ar=new ArrayList();
ar.add(11);
ar.add(22);
ar.add(2);
ar.add(23);
ar.add(5);
int searchval=23;
// sorting List
Collections.sort(ar);
for(int arr:ar){
System.out.print(“arrrr “+arr);
}
// method call
int result=TestUtil.BinarySrchArrayList(ar,searchval);
System.out.println(“result outside “+result);
if(result>=0){
System.out.println(“result “+result);
}
else{
System.out.println(“result Not exist “);
}
}
}
TestComparableSrch.java
============================
package Compare;
import java.util.ArrayList;
import java.util.Collections;
import Util.TestUtil;
/*
* author:
* date:
* purpose:
* Binary Search using comparable
*
* */
public class TestComparableSrch {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
int searchval=2;
EmployeeComp e1= new EmployeeComp(4, “john”, 24);
EmployeeComp e2= new EmployeeComp(3, “muir”, 20);
EmployeeComp e3= new EmployeeComp(5, “rose”, 21);
EmployeeComp e4= new EmployeeComp(1, “victor”, 22);
ArrayList arrlist=new ArrayList();
arrlist.add(e1);
arrlist.add(e2);
arrlist.add(e3);
arrlist.add(e4);
// sorting
Collections.sort(arrlist);
for(EmployeeComp e:arrlist){
System.out.println(“eeee “+e.getEmpName());
}
String result=TestUtil.BinaryComparableArrayList(arrlist,searchval);
System.out.println(“result “+result);
}
}
TestComparatorSrch.java
======================================
package Compare;
import java.util.ArrayList;
import java.util.Collections;
import Util.TestUtil;
/*
* author:
* date:
* purpose:
* Binary Search using comparator
*
* */
public class TestComparatorSrch {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
String searchval=”muirs”;
EmployeeComparator e1= new EmployeeComparator(4, “john”, 24);
EmployeeComparator e2= new EmployeeComparator(3, “rose”, 20);
EmployeeComparator e3= new EmployeeComparator(5, “muir”, 21);
EmployeeComparator e4= new EmployeeComparator(1, “victor”, 22);
ArrayList arrlist=new ArrayList();
arrlist.add(e1);
arrlist.add(e2);
arrlist.add(e3);
arrlist.add(e4);
// sorting
Collections.sort(arrlist,new EmployeeComparator());
for(EmployeeComparator e:arrlist){
System.out.println(“eeee “+e.getEmpName());
}
String result=TestUtil.BinaryComparatorArrayList(arrlist,searchval);
System.out.println(“result “+result);
}
}
TestUtil.java
====================
package Util;
import java.util.ArrayList;
import Compare.EmployeeComp;
import Compare.EmployeeComparator;
//Util method for the business logic
public class TestUtil {
static int RecordNotExist=-1;
public static int BinarySrchArray(int[]arr,int searchval){
//System.out.println(“searchval”+searchval);
if(arr!=null){
//System.out.println(“arr.length”+arr.length);
int low=0;
int high=arr.length-1;
int mid=(low+high)/2;
int out;
while(low<=high){
if(arr[mid]==searchval){
//System.out.println("hi "+searchval);
out=arr[mid];
return mid;
}
else if(arr[mid]<searchval){
low=mid+1;
}
else{
high=mid-1;
}
mid=(low+high)/2;
}
}
return RecordNotExist;
}
//arraylist
public static int BinarySrchArrayList(ArrayList arr,int searchval){
//System.out.println(“searchval”+searchval);
if(arr!=null){
//System.out.println(“arr.length”+arr.length);
int low=0;
int high=arr.size()-1;
int mid=(low+high)/2;
int out;
while(low<=high){
if(arr.get(mid)==searchval){
//System.out.println("hi "+searchval);
out=arr.get(mid);
return mid;
}
else if(arr.get(mid)<searchval){
low=mid+1;
}
else{
high=mid-1;
}
mid=(low+high)/2;
}
}
return RecordNotExist;
}
// comparable
public static String BinaryComparableArrayList(ArrayList arr,int searchval){
//System.out.println(“searchval”+searchval);
if(arr!=null){
//System.out.println(“arr.length”+arr.length);
int low=0;
int high=arr.size()-1;
int mid=(low+high)/2;
int out;
while(low<=high){
if(arr.get(mid).getEmpId()==searchval){
//System.out.println("hi "+searchval);
out=arr.get(mid).getEmpId();
return arr.get(mid).getEmpName();
}
else if(arr.get(mid).getEmpId()<searchval){
low=mid+1;
}
else{
high=mid-1;
}
mid=(low+high)/2;
}
}
return "RecordNotExist";
}
//Comparator
public static String BinaryComparatorArrayList(ArrayList arr,String searchval){
//System.out.println(“searchval”+searchval);
if(arr!=null){
//System.out.println(“arr.length”+arr.length);
int low=0;
int high=arr.size()-1;
int mid=(low+high)/2;
int out;
while(low<=high){
if(arr.get(mid).getEmpName().equals(searchval)){
//System.out.println("hi "+searchval);
out=arr.get(mid).getEmpId();
System.out.println("hi "+out);
return arr.get(mid).getEmpName();
}
else if(arr.get(mid).getEmpName().compareTo(searchval)<0){
low=mid+1;
}
else{
high=mid-1;
}
mid=(low+high)/2;
}
}
return "NameNotExist";
}
}
EmployeeComp.java
============================
package Compare;
/*
* author:
* date:
* purpose:
* Implementation using comparable interface for sorting employee objects
*
* */
public class EmployeeComp implements Comparable {
private int empId;
private String empName;
private int age;
public EmployeeComp(int id,String name,int age){
this.empId=id;
this.empName=name;
this.age=age;
}
public int getEmpId() {
return empId;
}
public void setEmpId(int empId) {
this.empId = empId;
}
public String getEmpName() {
return empName;
}
public void setEmpName(String empName) {
this.empName = empName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public int compareTo(EmployeeComp o) {
// TODO Auto-generated method stub
return this.empId-o.empId;
}
}
EmployeeComparator.java
==============================
package Compare;
import java.util.Comparator;
/*
* author:
* date:
* purpose:
* Implementation using comparator interface for sorting employee objects
*
* */
public class EmployeeComparator implements Comparator {
private int empId;
private String empName;
private int age;
public EmployeeComparator(){
}
public EmployeeComparator(int id,String name,int age){
this.empId=id;
this.empName=name;
this.age=age;
}
public int getEmpId() {
return empId;
}
public void setEmpId(int empId) {
this.empId = empId;
}
public String getEmpName() {
return empName;
}
public void setEmpName(String empName) {
this.empName = empName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public int compare(EmployeeComparator object1, EmployeeComparator object2) {
// TODO Auto-generated method stub
return object1.getEmpName().compareTo(object2.getEmpName());
}
}
TestBinaryArraySearch:
================================
package Compare;
import java.util.Arrays;
import Util.TestUtil;
/*
* author:
* date:
* purpose:
* Binary search using Arrays
*
* */
public class TestBinaryArraySearch {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
int [] arr={1,22,6,3,9};
int searchval=1;
// sorting array
Arrays.sort(arr);
for(int i=0;i=0){
System.out.println(“result “+result);
}
else{
System.out.println(“result Not exist “);
}
}
}
thank for the detailed explanation,,i was able to get the following
As approach #3, we can implement the comparator as an anonymous class for the treemap. Its more reasonable if the only place we will use the comparator is one time only.
TreeSet d = new TreeSet(new Comparator(){
@Override
public int compare(Dog d1, Dog d2) {
return d1.size – d2.size;
}
});
we can add comparator logic with ananymous method way ..
package com.venu.comparator;
import java.util.*;
import java.io.*;
class Simple{
public static void main(String args[]){
ArrayList al=new ArrayList();
al.add(new Student(101,”Vijay”,23));
al.add(new Student(106,”Ajay”,27));
al.add(new Student(105,”Jai”,21));
System.out.println(“Sorting by Name…”);
Collections.sort(al,new Comparator() {
@Override
public int compare(Student o1, Student o2) {
// TODO Auto-generated method stub
return o1.name.compareTo(o2.name);
}
});
Iterator abs = al.iterator();
while(abs.hasNext()){
Student x = (Student) abs.next();
System.out.println(“the values ” + x.age + “the names” + x.name + “the roll no” + x.rollno);
}
Collections.sort(al,new Comparator() {
@Override
public int compare(Student o1, Student o2) {
// TODO Auto-generated method stub
if(o1.age == o2.age)
return 0;
else if(o1.age >o2.age)
return 1;
else
return -1;
}
});
Iterator agecmp = al.iterator();
while(agecmp.hasNext()){
Student x = (Student) agecmp.next();
System.out.println(x.rollno+” “+x.name+” “+x.age);
}
}
}
//POJO Class
package com.venu.comparator;
class Student{
int rollno;
String name;
int age;
public int getRollno() {
return rollno;
}
public void setRollno(int rollno) {
this.rollno = rollno;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
}
Comparable defines the natural ordering for the class objects….If we need any other ordering then we have to define comparator. Since comparator is not tied to a class so we can have several comparators depicting different sort (based on the fields) sequences for the same class…
See a post about differences here – http://netjs.blogspot.com/2015/10/difference-between-comparable-and-comparator-java.html
thank you it is useful post.i also refer very helpful and useful article about–Sorting using Comparator in Java
Please visit this article———-
http://www.mindstick.com/Articles/a4872d1a-6316-42b8-b431-7b48107cb7db/Sorting%20using%20Comparator%20in%20Java#.VekynJc0Xcc
http://www.mkyong.com/java/java-object-sorting-example-comparable-and-comparator/
Hi
What I feel, that when you are creating new class and which needs to be sorted or compared, it should be implemented by Comparable and if we have some pre-developed class and need to add functinality for compare or sorting, then we need to create a class implemented by Comprator and use this class as a parameter in sort method.
Thanks good explaination….Here another blog also explained nice please go through this blog http://adnjavainterview.blogspot.in/2014/06/difference-between-comparable.html
hi and one more doubt is Collections.sort(al, new SizeComparator());
what is this actually doing
my guess sizecomparatorobject.compare(obj1 of al,obj2 of al)
by the use of result returned by the above compare method (i.e)1,-1,0 how sorting will be done?
hi my doubt about this tutorial is
why we are going for comparator/comparable interface? it is possible for us to define such comparision method inside our class and can call it by the use of instance of its own.
class SizeComparator implements Comparator {
@Override
public int compare(HDTV tv1, HDTV tv2) {
int tv1Size = tv1.getSize();
int tv2Size = tv2.getSize();
if (tv1Size > tv2Size) {
return 1;
} else if (tv1Size < tv2Size) {
return -1;
} else {
return 0;
}
}
}
instead of this it is possible to have our own method?
good example
alert(0);
True. Changed.
Under Heading 2.Comparator
“Comparator is capable of comparing two DIFFERENT types of objects.”
EDIT: Comparator is capable if comparing objects based on different attributes. e.g. 2 men can be compared based on `name` or `age` etc. (this can not be done using comparable. ) Even in the example shown above, author provides example where object of SAME type (HDTV) is being used.
Two ways for sorting – using comparator and comparable. For detailed explanation is here
Nice one.