What is the Java Comparator interface?

The Java Comparator interface is used to provide a custom sorting order for a data structure; this is usually done with user-defined types. The Comparator interface is passed on to a sorting function (i.e., Collections.sort()) to temporarily alter the sorting criteria of a collection:

compare(obj1, obj2)

It is necessary to override this method when using the Comparator interface; the method should return:

  • a negative integer, if obj1 is less than obj2.
  • zero, if obj1 is equal to obj2.
  • a positive integer, if obj1 is greater than obj2.
Declaring a Comparator and passing it to a sorting function.
Declaring a Comparator and passing it to a sorting function.

Code

In the following code, an ArrayList of type Person is sorted on the basis of id using a Comparator:

ComparatorShot.java
Person.java
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class ComparatorShot {
	
	public static void main(String[] args) {
		ArrayList<Person> arr = new ArrayList<>();
		arr.add(new Person(1, "John"));
		arr.add(new Person(5, "Terry"));
		arr.add(new Person(0, "Lukaku"));
		arr.add(new Person(2, "Pablo"));
		
		// Passing Comparator anonymously:
		Collections.sort(arr, new Comparator<Person>() {
			@Override
			public int compare(Person p1, Person p2) {
				// Sorting by IDs:
				
				if(p1.id < p2.id)
					return -1;
				else if(p1.id > p2.id)
					return 1;
				else
					return 0;
			}
		});
		
		// Displaying sorted array:
		arr.forEach(System.out::println);
	}
}

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