Both TreeSet and TreeMap store elements in sorted order. However, it is the comparator that defines precisely what sorted order means.

Java Comparator Interface

In Java, the Comparator interface is a part of java.util package and it defines the order of the objects of user-defined classes.

Methods of Comparator Interface

The Comparator interface defines two methods: compare() and equals(). The compare() method, shown here, compares two elements for order −

The compare() Method

int compare(Object obj1, Object obj2)

obj1 and obj2 are the objects to be compared. This method returns zero if the objects are equal. It returns a positive value if obj1 is greater than obj2. Otherwise, a negative value is returned.

By overriding compare(), you can alter the way that objects are ordered. For example, to sort in a reverse order, you can create a comparator that reverses the outcome of a comparison.

The equals() Method

The equals() method, shown here, tests whether an object equals the invoking comparator −

boolean equals(Object obj)

obj is the object to be tested for equality. The method returns true if obj and the invoking object are both Comparator objects and use the same ordering. Otherwise, it returns false.

Overriding equals() is unnecessary, and most simple comparators will not do so.

Comparator Interface to Sort a Custom Object

In this example, we”re using Comparator interface to sort a custom object Dog based on comparison criterias.

Example

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

class Dog implements Comparator<Dog>, Comparable<Dog> {
   private String name;
   private int age;
   Dog() {
   }

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

   public String getDogName() {
      return name;
   }

   public int getDogAge() {
      return age;
   }

   // Overriding the compareTo method
   public int compareTo(Dog d) {
      return (this.name).compareTo(d.name);
   }

   // Overriding the compare method to sort the age 
   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[]) {
      // Takes a list o Dog objects
      List<Dog> list = new ArrayList<>();

      list.add(new Dog("Shaggy", 3));
      list.add(new Dog("Lacy", 2));
      list.add(new Dog("Roger", 10));
      list.add(new Dog("Tommy", 4));
      list.add(new Dog("Tammy", 1));

      Collections.sort(list);   // Sorts the array list
      System.out.println("Sorted by name:");
      // printing the sorted list of names
      System.out.print(list);

      // Sorts the array list using comparator
      Collections.sort(list, new Dog());
      System.out.println(" ");

      System.out.println("Sorted by age:");
      // printing the sorted list of ages
      System.out.print(list);
   }
}

Output

Sorted by name:
[Lacy,2, Roger,10, Shaggy,3, Tammy,1, Tommy,4] 
Sorted by age:
[Tammy,1, Lacy,2, Shaggy,3, Tommy,4, Roger,10]

Note − Sorting of the Arrays class is as the same as the Collections.

Comparator Interface to Reverse Sort

In this example, we”re using Comparator interface to reverse sort the Dog objects.

Example 2

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

class Dog implements Comparator<Dog>, Comparable<Dog> {
   private String name;
   private int age;
   Dog() {
   }

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

   public String getDogName() {
      return name;
   }

   public int getDogAge() {
      return age;
   }

   // Overriding the compareTo method
   public int compareTo(Dog d) {
      return (this.name).compareTo(d.name);
   }

   // Overriding the compare method to sort the age 
   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[]) {
      // Takes a list o Dog objects
      List<Dog> list = new ArrayList<>();

      list.add(new Dog("Shaggy", 3));
      list.add(new Dog("Lacy", 2));
      list.add(new Dog("Roger", 10));
      list.add(new Dog("Tommy", 4));
      list.add(new Dog("Tammy", 1));

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

Output

Sorted by name in reverse order:
[Tommy,4, Tammy,1, Shaggy,3, Roger,10, Lacy,2]

In this example, we”re using Comparator interface to sort String values in reverse order.

Example 3

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

public class ComparatorDemo {

   public static void main(String args[]) {
      // Takes a list o Dog objects
      List<String> list = new ArrayList<>();

      list.add("Shaggy");
      list.add("Lacy");
      list.add("Roger");
      list.add("Tommy");
      list.add("Tammy");

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

Output

Sorted by name in reverse order:
[Tommy, Tammy, Shaggy, Roger, Lacy]