Reference to Static Method (Method References)

What is a reference to static methods mean?

A static method reference refers to the static method for a class. We can use a method reference to call the static methods directly.

Syntax

The syntax for referencing a static method is as follows.

ClassName::staticMethodName

Example

Let us write a simple example that contains four files

1. Book - a POJO class with getters and setters. You can replace the class with Java record , also.
2. BookRepository - for database transactions.
3. BookRepositoryImpl - An implementation class that injects fake data.
4. ReferenceToStaticMethod - This is where the actual magic happens.

Code

public class Book {
  private final String title;
  private final String author;
  private final int year;
  private final int copiesSoldInMillions;
  private final double rating;
  private final double costInEuros;

  public Book(
      String title,
      String author,
      int year,
      int copiesSoldInMillions,
      double rating,
      double costInEuros) {
    this.title = title;
    this.author = author;
    this.year = year;
    this.copiesSoldInMillions = copiesSoldInMillions;
    this.rating = rating;
    this.costInEuros = costInEuros;
  }

  public Double getCostInEuros() {
    return costInEuros;
  }

  public double getRating() {
    return rating;
  }

  public String getTitle() {
    return title;
  }

  @Override
  public String toString() {
    return "commons.Book{ title="
        + title
        + ", author="
        + author
        + ", copiesSoldInMillions="
        + copiesSoldInMillions
        + ", costInEuros="
        + costInEuros
        + "}";
  }
}

import java.util.List;

public interface BookRepository {
  List<Book> getAllBooks();
}

import java.util.Arrays;
import java.util.List;

public class BookRepositoryImpl implements BookRepository {

  @Override
  public List<Book> getAllBooks() {
    return Arrays.asList(
        new Book("Don Quixote", "Miguel de Cervantes", 1605, 500, 3.9, 9.99),
        new Book("A Tale of Two Cities", "Charles Dickens", 1859, 200, 3.9, 10.0),
        new Book("The Lord of the Rings", "J.R.R. Tolkien", 2001, 150, 4.0, 12.50),
        new Book("The Little Prince", "Antoine de Saint-Exupery", 2016, 142, 4.4, 5.0),
        new Book("The Dream of the Red Chamber", "Cao Xueqin", 1791, 100, 4.2, 10.0));
  }
}

import java.util.List;

public class ReferenceToStaticMethod {

  public static int compareByTitle(Book first, Book second) {
    return first.getTitle().compareTo(second.getTitle());
  }

  public static int compareByCost(Book first, Book second) {
    return first.getCostInEuros().compareTo(second.getCostInEuros());
  }

  public static void main(String[] args) {
    BookRepositoryImpl bookRepositoryImpl = new BookRepositoryImpl();

    // Inflate fake books
    List<Book> books = bookRepositoryImpl.getAllBooks();

    // Sort based on price
    System.out.println("SORT BASED ON PRICE: ");
    books.sort(ReferenceToStaticMethod::compareByCost);
    books.stream()
        .map(book -> book.getTitle() + " -> " + book.getCostInEuros())
        .forEach(System.out::println);

    
    System.out.println("---------");

    // Sort based on price
    System.out.println("SORT BASED ON TITLES: ");
    books.sort(ReferenceToStaticMethod::compareByTitle);
    books.stream()
        .map(book -> book.getTitle() + " -> " + book.getRating())
        .forEach(System.out::println);
  }
}

Explanation

Book, BookRepository, and BookRepositoryImpl files are self-explanatory. So let's see whats happening in ReferenceToStaticMethod class.

Lines 5 to 7 contain a static method compareByTitle(), which sorts the list of Book objects based on title.

Lines 9 to 11 contain a static method compareByCost(), which sorts the list of Book objects based on costInEuros per each book.

In line 14, we created an instance of BookRepositoryImpl class. We use this reference instance to call .getAllBooks() the method in line 17 to load the fake data into a collection of objects List<Book>.

Line 20 and 28 contains we are calling the static methods we declared inside the class to sort the list of Book objects.

The following explanation is the same for code snippet lines from "21 to 23" and "29 to 31".

On line 21, we chained the collection of book objects stored in books variable with .stream(), which converts all the collections of Book objects into a Stream of Book objects.

Line 22 contains an intermediate stream operation .map(...) forms stream of Book objects based on the parameters that are included inside the method, in this case title and costInEuros.

Line 23 contains a simple .forEach a terminal operation for printing elements on the console.