Java Comparator Interface - Custom Sorting for User-Defined Classes

Master the Java Comparator interface to define custom sorting logic for user-defined classes. Essential for sorting elements in collections like TreeSet and TreeMap, Comparator allows you to establish flexible and specific criteria for ordering objects. Dive into examples and tips on using Comparator for advanced sorting in Java.

Java - How to Use Comparator

Both TreeSet and TreeMap store elements in a sorted order. However, the comparator defines the specific criteria for how that order is determined.

Java Comparator Interface

The Comparator interface in Java is part of the java.util package, and it specifies the order of objects for user-defined classes.

Methods of the Comparator Interface

The Comparator interface defines two primary methods: compare() and equals().

The compare() Method

The compare() method compares two elements for order:

Syntax


int compare(Object obj1, Object obj2)

Output


Returns 0 if objects are equal, positive if obj1 > obj2, negative if obj1 < obj2.

By overriding compare(), you can change the order of the objects. For instance, to sort in reverse order, you can create a comparator that inverses the comparison result.

The equals() Method

The equals() method checks whether an object is equal to the invoking comparator:

Syntax


boolean equals(Object obj)

Output


Returns true if obj is a Comparator and uses the same ordering; otherwise, false.

Overriding equals() is generally unnecessary, and most simple comparators will not implement it.

Using Comparator Interface to Sort Custom Objects

In this example, we will use the Comparator interface to sort a custom object, Dog, based on specific criteria.

Example

Syntax


import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

class Dog implements Comparator, Comparable {
private String name;
private int age;

Dog(String n, int a) {
    name = n;
    age = a;
}

public int compareTo(Dog d) {
    return this.name.compareTo(d.name);
}

public int compare(Dog d, Dog d1) {
    return d.age - d1.age;
}

@Override
public String toString() {
    return this.name + "," + this.age;
}
}

public class ComparatorDemo {
public static void main(String args[]) {
    List list = new ArrayList<>();
    list.add(new Dog("Buddy", 5));
    list.add(new Dog("Max", 3));
    list.add(new Dog("Bella", 2));
    list.add(new Dog("Charlie", 4));
    list.add(new Dog("Daisy", 1));

    Collections.sort(list); // Sorts the array list
    System.out.println("Sorted by name:");
    System.out.print(list);
    Collections.sort(list, new Dog());
    System.out.println(" ");
    System.out.println("Sorted by age:");
    System.out.print(list);
}
}

Output


Sorted by name:
[Bella,2, Buddy,5, Charlie,4, Daisy,1, Max,3] 
Sorted by age:
[Daisy,1, Bella,2, Max,3, Charlie,4, Buddy,5]

Note: The sorting method for the Arrays class is similar to that of the Collections class.

Using Comparator Interface for Reverse Sorting

In this example, we will utilize the Comparator interface to sort the Dog objects in reverse order.

Example 2

Syntax


import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

class Dog implements Comparator, Comparable {
private String name;
private int age;

Dog(String n, int a) {
    name = n;
    age = a;
}

public int compare(Dog d, Dog d1) {
    return d.age - d1.age;
}

@Override
public String toString() {
    return this.name + "," + this.age;
}
}

public class ComparatorDemo {
public static void main(String args[]) {
    List list = new ArrayList<>();
    list.add(new Dog("Buddy", 5));
    list.add(new Dog("Max", 3));
    list.add(new Dog("Bella", 2));
    list.add(new Dog("Charlie", 4));
    list.add(new Dog("Daisy", 1));

    Collections.sort(list, Collections.reverseOrder()); // Sorts the array list
    System.out.println("Sorted by name in reverse order:");
    System.out.print(list);
}
}

Output


Sorted by name in reverse order:
[Max,3, Buddy,5, Charlie,4, Bella,2, Daisy,1]

Sorting String Values in Reverse Order

In this final example, we will use the Comparator interface to sort string values in reverse order.

Example 3

Syntax


import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ComparatorDemo {
public static void main(String args[]) {
    List list = new ArrayList<>();
    list.add("Buddy");
    list.add("Max");
    list.add("Bella");
    list.add("Charlie");
    list.add("Daisy");

    Collections.sort(list, Collections.reverseOrder()); // Sorts the array list
    System.out.println("Sorted by name in reverse order:");
    System.out.print(list);
}
}

Output


Sorted by name in reverse order:
[Max, Buddy, Charlie, Bella, Daisy]