Java 8 Comparator Changes

Java 8 has made many improvements to the Comparator interface. In this article, I will be explaining the Java 8 Comparator changes.

Comparator.compare

From Java 8 onwards, Comparator is designated as a functional interface. So, you can implement the Comparator.compare method via a lambda expression.

Consider the following Book class:

public class Book {

  String name;
  int numPages;
  
  public Book(String name, int numPages) {
    this.name = name;
    this.numPages = numPages;
  }
  //getters and setters
}

And suppose you have a List of Book objects which needs to be sorted on the basis of the numPages field. Prior to Java 8, you had to create a class that implements the Comparator interface and use it to sort the Book List.

The following code demonstrates this:

public class BookCompartor implements Comparator<Book> {

  public int compare(Book book1, Book book2) {
    return book1.getNumPages() - book2.getNumPages();
  }
}

public class ComparatorBeforeJava8Demo {

  public static void main(String[] args) {
    List<Book> books = new ArrayList<>();
    
    books.add(new Book("The Complete Reference",743));
    books.add(new Book("Head First Java",451));
    books.add(new Book("Design Patterns",612));
    
    System.out.println("Before sorting:");
    books.forEach(book -> System.out.println(book.getName()+","+book.getNumPages()));
    books.sort(new BookCompartor());
    System.out.println("After sorting:");
    books.forEach(book -> System.out.println(book.getName()+","+book.getNumPages()));
  }
}

So, this code uses the List.sort method and passes the BookComparator to it.

This code is cumbersome since you need to create a class that implements the Comparator interface and then use it.

The code above can be written using a lambda expression as follows:

public class ComparatorAfterJava8Demo {

  public static void main(String[] args) {
    List<Book> books = new ArrayList<>();

    books.add(new Book("The Complete Reference", 743));
    books.add(new Book("Head First Java", 451));
    books.add(new Book("Design Patterns", 612));

    System.out.println("Before sorting:");
    books.forEach(book -> System.out.println(book.getName() + "," + book.getNumPages()));

    books.sort((book1, book2) -> book1.getNumPages() - book2.getNumPages());

    System.out.println("After sorting:");
    books.forEach(book -> System.out.println(book.getName() + "," + book.getNumPages()));
  }
}

So, there is no need to create a BookComparator class as done earlier. This code is much simpler and easier to read.

Comparator.comparing

Java 8 has added a new static method called Comparator.comparing to the Comparator interface.  This accepts as parameter a Function that specifies the sort key and returns a Comparator that sorts on the specified key.

The following code demonstrates this method:

List<Book> books = new ArrayList<>();
books.add(new Book("The Complete Reference", 743));
books.add(new Book("Head First Java", 451));
books.add(new Book("Design Patterns", 612));

Comparator<Book> bookComparator = Comparator.comparing(book -> book.getName());
books.sort(bookComparator);

System.out.println("After sorting:");
books.forEach(book -> System.out.println(book.getName() + "," + book.getNumPages()));

This code invokes the Comparator.comparing method with a lambda expression that accepts a Book object and returns the name field. So, this returns a Comparator that sorts the Book objects on the basis of the name field.

So, this code displays the following output:

After sorting:
Design Patterns,612
Head First Java,451
The Complete Reference,743

Comparator.comparingInt

The Comparator.comparingInt is another new static method on the Comparator interface. It is similar to the Comparator.comparing method. Instead of accepting a Function as a parameter, it accepts a ToIntFunction as a parameter. It returns a Comparator that sorts on the basis of an int key returned by the ToIntFunction . For example,  in order to sort the Book objects on the basis of the numPages field, we can use this method as follows:

List<Book> books = new ArrayList<>();
books.add(new Book("The Complete Reference", 743));
books.add(new Book("Head First Java", 451));
books.add(new Book("Design Patterns", 612));

Comparator<Book> bookComparator = Comparator.comparingInt(book -> book.getNumPages());
books.sort(bookComparator);

System.out.println("After sorting:");
books.forEach(book -> System.out.println(book.getName() + "," + book.getNumPages()));

This code prints the following output:

After sorting: 
Head First Java,451 
Design Patterns,612 
The Complete Reference,743

Just like comparingInt, Java 8 has added static methods called comparingLong and comparingDouble which can sort based on a long key and double key respectively.

Comparator.naturalOrder

Compator.naturalOrder is another new static method on the Comparator interface. It returns a Comparator that sorts in the natural order of the underlying Collection.

List<String> fruits = Arrays.asList("StrawBerry", "Apple", "Cherry", "Banana");
Comparator<String> fruitComparator = Comparator.naturalOrder();
fruits.sort(fruitComparator);
System.out.println("After sorting:");
fruits.forEach(fruit -> System.out.println(fruit));

This code creates a List of String values. For Strings, the natural order of sorting is alphabetical order. So, Comparator.naturalOrder sorts the fruits List in alphabetical order.

So, this code displays the following output:

After sorting:
Apple
Banana
Cherry
StrawBerry

Comparator.reversed

Compator.reversed is a default method on the Comparator interface. It returns a Comparator that sorts in the reverse order of the Comparator on which it is invoked.

List<Book> books = new ArrayList<>();
books.add(new Book("The Complete Reference", 743));
books.add(new Book("Head First Java", 451));
books.add(new Book("Design Patterns", 612));

Comparator<Book> bookComparator = Comparator.comparingInt(book -> book.getNumPages());
Comparator<Book> bookComparatorReverse = bookComparator.reversed();
books.sort(bookComparatorReverse);

System.out.println("After sorting:");
books.forEach(book -> System.out.println(book.getName() + "," + book.getNumPages()));

This code first uses the Comparator.comparingInt method which returns a Comparator that sorts in the ascending order of the numPages field. It then invokes the reversed on this Comparator.  The code then uses this to sort the Book List. So, this results in the Book List being sorted in the descending order of the numPages field.

So, this code displays the following output:

After sorting:
The Complete Reference,743
Design Patterns,612
Head First Java,451

Conclusion

So, in this article, we saw some of the Java 8 comparator changes. We saw how to implement the compare method using a lambda expression. We also saw some of the new static and default methods on the Comparator interface.