Comparator

The Comparator interface is used to order objects of an arbitrary class. It is not to be confused with the Comparable interface, which is implemented by the class to be sorted. The Comparator interface is implemented in a separate class.

Syntax

class MyComparator implements Comparator<MyClass> {
  @Override public int compare(MyClass a, MyClass b)
  {
    // Compare logic
    ...
    return result;
  }
}

Applying the Comparator interface to a class, MyComparator, requires the implements keyword (e.g., Comparator<MyClass>). This interface has a .compare() method that returns an int value based on whether two MyClass instances, a and b, can be logically sorted.

A Comparator class can be passed as an argument to methods such as Arrays.sort() and Collections.sort() to specify the sort order, potentially overriding the natural sort order defined by the class’s own .compareTo() method.

Example

The following example showcases the Comparator interface. First, an Employee class is defined:

// Employee.java
public class Employee {
  String firstName;
  String lastName;

  // Constructor sets firstName and lastName
  public Employee(String first, String last)
  {
    this.firstName = first;
    this.lastName = last;
  }

  // User-friendly output when printed.
  public String toString()
  {
    return "( " + lastName + ", " + firstName + " )";
  }
}

The next snippet defines an EmployeeSort class that implements the Comparator interface and overrides its .compare() method to sort based on lastName and then on firstName:

import java.util.*;

// EmployeeSort.java
public class EmployeeSort implements Comparator<Employee> {

  // Implement the Comparator interface
  @Override public int compare(Employee valueA, Employee valueB)
  {
    if (valueA.lastName.compareTo(valueB.lastName) != 0) {
      // If lastNames are different, compare lastName
      return valueA.lastName.compareTo(valueB.lastName);
    } else {
      // If lastNames are the same, compare firstName
      return valueA.firstName.compareTo(valueB.firstName);
    }
  }
}

This last snippet demonstrates the Comparator interface:

import java.util.*;

// SortExample.java
public class SortExample {
  public static void main(String[] args)
  {
    // Set up array with a few Employee classes
    Employee a[] = new Employee[5];
    a[0] = new Employee("Kirk","Douglas");
    a[1] = new Employee("Mel","Brooks");
    a[2] = new Employee("Jane","Fonda");
    a[3] = new Employee("Henry","Fonda");
    a[4] = new Employee("Michael","Douglas");

    // Use .sort() method with Comparable class.
    Arrays.sort(a, new EmployeeSort());

    // Print out the sorted Employees
    for (int i=0; i < a.length; i++) {
      System.out.println(a[i]);
    }
  }
}

This example results in the following output:

( Brooks, Mel )
( Douglas, Kirk )
( Douglas, Michael )
( Fonda, Henry )
( Fonda, Jane )