Java Comparator interface

Java Comparator interface is used to order the objects of a user-defined class.

This interface is found in java.util package and contains 2 methods compare(Object obj1,Object obj2) and equals(Object element).

It provides multiple sorting sequences, i.e., you can sort the elements on the basis of any data member, for example, rollno, name, age or anything else.

Methods of Java Comparator Interface

Method	Description
public int compare(Object obj1, Object obj2)	It compares the first object with the second object.
public boolean equals(Object obj)	It is used to compare the current object with the specified object.
public boolean equals(Object obj)	It is used to compare the current object with the specified object.

Collections class

Collections class provides static methods for sorting the elements of a collection. If collection elements are of Set or Map, we can use TreeSet or TreeMap. However, we cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements also.

Method of Collections class for sorting List elements

public void sort(List list, Comparator c): is used to sort the elements of List by the given Comparator.

Java Comparator Example (Non-generic Old Style)

Let's see the example of sorting the elements of List on the basis of age and name. In this example, we have created 4 java classes:

Student.java
AgeComparator.java
NameComparator.java
Simple.java

Student.java

This class contains three fields rollno, name and age and a parameterized constructor.

class Student{
int rollno;
String name;
int age;
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
}

AgeComparator.java

This class defines comparison logic based on the age. If the age of the first object is greater than the second, we are returning a positive value. It can be anyone such as 1, 2, 10. If the age of the first object is less than the second object, we are returning a negative value, it can be any negative value, and if the age of both objects is equal, we are returning 0.

import java.util.*;
class AgeComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1=(Student)o1;
Student s2=(Student)o2;

if(s1.age==s2.age)
return 0;
else if(s1.age>s2.age)
return 1;
else
return -1;
}
}

NameComparator.java

This class provides comparison logic based on the name. In such case, we are using the compareTo() method of String class, which internally provides the comparison logic.

import java.util.*;
class NameComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1=(Student)o1;
Student s2=(Student)o2;

return s1.name.compareTo(s2.name);
}
}

Simple.java

In this class, we are printing the values of the object by sorting on the basis of name and age.

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 NameComparator());
Iterator itr=al.iterator();
while(itr.hasNext()){
Student st=(Student)itr.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}

System.out.println("Sorting by age");

Collections.sort(al,new AgeComparator());
Iterator itr2=al.iterator();
while(itr2.hasNext()){
Student st=(Student)itr2.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}


}
}

       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23
       
       Sorting by age       
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java Comparator Example (Generic)

Student.java

class Student{
int rollno;
String name;
int age;
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
}

AgeComparator.java

import java.util.*;
class AgeComparator implements Comparator<Student>{
public int compare(Student s1,Student s2){
if(s1.age==s2.age)
return 0;
else if(s1.age>s2.age)
return 1;
else
return -1;
}
}

NameComparator.java

This class provides comparison logic based on the name. In such case, we are using the compareTo() method of String class, which internally provides the comparison logic.

import java.util.*;
class NameComparator implements Comparator<Student>{
public int compare(Student s1,Student s2){
return s1.name.compareTo(s2.name);
}
}

Simple.java

In this class, we are printing the values of the object by sorting on the basis of name and age.

import java.util.*;
import java.io.*;
class Simple{
public static void main(String args[]){

ArrayList<Student> al=new ArrayList<Student>();
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 NameComparator());
for(Student st: al){
System.out.println(st.rollno+" "+st.name+" "+st.age);
}

System.out.println("Sorting by age");

Collections.sort(al,new AgeComparator());
for(Student st: al){
System.out.println(st.rollno+" "+st.name+" "+st.age);
}
}
}

       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23

       Sorting by age     
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java 8 Comparator interface

Java 8 Comparator interface is a functional interface that contains only one abstract method. Now, we can use the Comparator interface as the assignment target for a lambda expression or method reference.

Methods of Java 8 Comparator Interface

Method	Description
int compare(T o1, T o2)	It compares the first object with second object.
static <T,U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor)	It accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator that compares by that sort key.
static <T,U> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)	It accepts a function that extracts a sort key from a type T, and returns a Comparator that compares by that sort key using the specified Comparator.
static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor)	It accepts a function that extracts a double sort key from a type T, and returns a Comparator that compares by that sort key.
static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor)	It accepts a function that extracts an int sort key from a type T, and returns a Comparator that compares by that sort key.
static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor)	It accepts a function that extracts a long sort key from a type T, and returns a Comparator that compares by that sort key.
boolean equals(Object obj)	It is used to compare the current object with the specified object.
static <T extends Comparable<? super T>> Comparator<T> naturalOrder()	It returns a comparator that compares Comparable objects in natural order.
static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)	It returns a comparator that treats null to be less than non-null elements.
static <T> Comparator<T> nullsLast(Comparator<? super T> comparator)	It returns a comparator that treats null to be greater than non-null elements.
default Comparator<T> reversed()	It returns comparator that contains reverse ordering of the provided comparator.
static <T extends Comparable<? super T>> Comparator<T> reverseOrder()	It returns comparator that contains reverse of natural ordering.
default Comparator<T> thenComparing(Comparator<? super T> other)	It returns a lexicographic-order comparator with another comparator.
default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor)	It returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
default <U> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)	It returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)	It returns a lexicographic-order comparator with a function that extracts a double sort key.
default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor)	It returns a lexicographic-order comparator with a function that extracts a int sort key.
default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor)	It returns a lexicographic-order comparator with a function that extracts a long sort key.

Java 8 Comparator Example

Let's see the example of sorting the elements of List on the basis of age and name.

File: Student.java

class Student {  
   int rollno;  
   String name;  
  int age;  
    Student(int rollno,String name,int age){  
    this.rollno=rollno;  
    this.name=name;  
    this.age=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;
	}

    }  

File: TestSort1.java

    import java.util.*;  
    public class TestSort1{  
    public static void main(String args[]){  
    ArrayList<Student> al=new ArrayList<Student>();  
    al.add(new Student(101,"Vijay",23));  
    al.add(new Student(106,"Ajay",27));  
    al.add(new Student(105,"Jai",21)); 
	//Sorting elements on the basis of name
    Comparator<Student> cm1=Comparator.comparing(Student::getName);
     Collections.sort(al,cm1);
     System.out.println("Sorting by Name");
     for(Student st: al){
    	 System.out.println(st.rollno+" "+st.name+" "+st.age);
    	 }
     //Sorting elements on the basis of age
      Comparator<Student> cm2=Comparator.comparing(Student::getAge);
	 Collections.sort(al,cm2);
     System.out.println("Sorting by Age");
     for(Student st: al){
    	 System.out.println(st.rollno+" "+st.name+" "+st.age);
    	 }  
    }  
    }  

Sorting by Name
106 Ajay 27
105 Jai 21
101 Vijay 23
Sorting by Age
105 Jai 21
101 Vijay 23
106 Ajay 27

Java 8 Comparator Example: nullsFirst() and nullsLast() method

Here, we sort the list of elements that also contains null.

File: Student.java

class Student {  
   int rollno;  
   String name;  
  int age;  
    Student(int rollno,String name,int age){  
    this.rollno=rollno;  
    this.name=name;  
    this.age=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;
	}
    }  

File: TestSort2.java

   import java.util.*;  
    public class TestSort2{  
    public static void main(String args[]){  
    ArrayList<Student> al=new ArrayList<Student>();  
    al.add(new Student(101,"Vijay",23));  
    al.add(new Student(106,"Ajay",27));  
    al.add(new Student(105,null,21));  
    Comparator<Student> cm1=Comparator.comparing(Student::getName,Comparator.nullsFirst(String::compareTo));
     Collections.sort(al,cm1);
     System.out.println("Considers null to be less than non-null");
     for(Student st: al){
    	 System.out.println(st.rollno+" "+st.name+" "+st.age);
    	 }
     Comparator<Student> cm2=Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo));
     Collections.sort(al,cm2);
     System.out.println("Considers null to be greater than non-null");
     for(Student st: al){
    	 System.out.println(st.rollno+" "+st.name+" "+st.age);
    	 }
    }  
    }   

Considers null to be less than non-null
105 null 21
106 Ajay 27
101 Vijay 23
Considers null to be greater than non-null
106 Ajay 27
101 Vijay 23
105 null 21

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