Java Comparator Interface

Java Comparator interface is used to sort an array or List of objects based on custom sort order. The custom ordering of items is imposed by implementing Comparator’s compare() method in the objects.

1. When to Use Comparator Interface

Java Comparator interface imposes a total ordering on the objects which may not have a desired natural ordering.

For example, for a List of Employee objects, the natural order may be ordered by employee’s id. But in real-life applications, we may want to sort the list of employees by their first name, date of birth or simply any other such criteria. In such conditions, we need to use Comparator interface.

We can use the Comparator interface in the following situations.

• Sorting the array or list of objects, but NOT in natural order.
• Sorting the array or list of objects where we can not modify the source code to implement Comparable interface.
• Using group by sort on list or array of objects on multiple different fields.

2. Overriding compare() Method

To enable total ordering on objects, we need to create a class that implements the Comparator interface. Then we need to override its compare(T o1, T o2) method.

The compare() compares its two arguments for order. It returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

For a given Employee class, the order by employee name can be imposed by creating a Comparator like below.

import java.util.Comparator;

public class NameSorter implements Comparator<Employee>
{
    @Override
    public int compare(Employee e1, Employee e2) {
        return e1.getName().compareToIgnoreCase( e2.getName() );
    }
}

import java.time.LocalDate;

public class Employee {

    private Long id;
    private String name;
    private LocalDate dob;
}

3. Using Comparator With

3.1. Collections.sort() and Arrays.sort()

1. Use Collections.sort(list, Comparator) method sort a list of objects in order imposed by provided comparator instance.
2. Use Arrays.sort(array, Comparator) method sort an array of objects in order imposed by provided comparator instance.

3.2. Collections.comparing()

This utility method accepts a function that extracts a sort key for the class. This is essentially a field on which the class objects will be sorted.

//Order by name
Comparator.comparing(Employee::getName);
 
//Order by name in reverse order
Comparator.comparing(Employee::getName).reversed();
 
//Order by id field
Comparator.comparing(Employee::getId);
 
//Order by employee age
Comparator.comparing(Employee::getDate);

3.3. Collections.thenComparing()

This utility method is used for group by sort. Using this method, we can chain multiple comparators to sort the list or array of objects on multiple fields.

It is very similar to SQL GROUP BY clause to order rows on different fields.

//Order by name and then by age
Comparator.comparing(Employee::getName)
            .thenComparing(Employee::getDob);
 
//Order by name -> date of birth -> id 
Comparator.comparing(Employee::getName)
            .thenComparing(Employee::getDob)
            .thenComparing(Employee::getId);

Using the above syntax, we can create virtually any sorting logic.

3.4. Collections.reverseOrder()

This utility method returns a comparator that imposes the reverse of the natural ordering or total ordering on a collection of objects that implement the Comparable interface.

//Reverse of natural order as specified in 
//Comparable interface's compareTo() method 
 
Comparator.reversed();
 
//Reverse of order by name
 
Comparator.comparing(Employee::getName).reversed();

3.5. Other Collection Classes

Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps) to provide an ordering that is not natural ordering.

SortedSet<Employee> sortedUniqueEmployees = new TreeSet<Employee>(new NameSorter());

4. Java Comparator Examples

4.1. Sorting List of Custom Objects

Java example to sort a list of employees by name using Comparator.

ArrayList<Employee> list = new ArrayList<>();
         
//Sort in reverse natural order
Collections.sort(list, new NameSorter());

4.2. Sort List in Reverse Order

Java example to sort a list of employees by name using Comparator in reverse order.

ArrayList<Employee> list = new ArrayList<>();
         
Collections.sort(list, Comparator.comparing( Employee::getName ).reversed());

4.3. Group By Sorting

Java example to sort a list of employees on multiple fields i.e. field by field.

ArrayList<Employee> list = new ArrayList<>();
 
Comparator<Employee> groupByComparator = Comparator.comparing(Employee::getName)
                                        .thenComparing(Employee::getDob)
                                        .thenComparing(Employee::getId);
 
Collections.sort(list, groupByComparator);

5. Conclusion

In this tutorial, we learned about Comparator interface of Java collection framework. It helps in imposing a total order on objects without any change to the source code of that class.

We learned to sort list and array of custom objects. We also learned to reverse sort and implement group by sort in Java.

Happy Learning !!