Double Colon (::) Operator in Java 8 Double Colon (::) Operator in Java 8

Method Reference in Java 8

Ashish Lahoti
Java
Page content

The double colon :: operator is introduced in Java 8 for method reference. It is a shorthand syntax for lambda expression that executes one method. You can write more compact and readable code using double colon operator as compare to anonymous classes and lambda expression. We are going to understand its usage with lots of examples.

1. Overview

Prerequisite

You should have a basic understanding of functional interface, lambda expressions and streams to understand these examples, if not then don’t worry, you will learn those concepts as well along the way.

Double colon refers to a method

Double colon :: is basically refers to a single method, and this single method can be a

  1. A Static method ClassName::staticMethodName
    e.g. Integer.parseInteger, Double.parseDouble
  2. An Instance method Object::instanceMethodName
    e.g. System.out::println, String::toUpperCase
  3. A Constructor ClassName::new
  4. A Super method super::parentMethodName

Double colon returns a functional interface

Double colon :: always return a functional interface. There are two ways to use this returned functional interface -

  1. Use this to initialize a functional interface and later execute a function.
    Here we either use java built-in functional interface like Function, Supplier, Consumer, BiFunction or we create our custom functional interface using @FunctionalInterface annotation.
  2. Use this to replace lambda expression in streams.

Don’t worry if it’s looking too complicated. Let’s deep dive into examples to understand them better.

2. Static Method

Syntax
// Lambda expression 
(args) -> ClassName.staticMethodName(args)

// Method Reference
ClassName::staticMethodName

2.1 Calculator

Let’s create two static methods square and multiply in Calculator class.

class Calculator {
	
    public static double square(double num){
       return Math.pow(num, 2);
    }
    public static double multiply(double num1, double num2) {
    	return num1*num2;
    }
}
2.1.1 Initialize Functional Interface

Let’s initialize java built-in Function, BiFunction functional interface and apply it to find square and multiply respectively.
Function interface accepts one argument and returns one argument. Here, it takes one Double arg and return its square.
BiFunction interface accepts two arguments and returns one argument. Here, it takes two Double args and return its multiplication.

// Initialize functional interface
Function<Double, Double> square = Calculator::square;
BiFunction<Double, Double, Double> multiply = Calculator::multiply;

// Execute function
square.apply(3.0);
multiply.apply(2.5, 5.0)

Output

9.0
12.5
2.1.2 Replace Lambda Expression

We generate square of list of Integer by replacing lambda expression with method reference.

List<Integer> numbers = Arrays.asList(1,2,3);

// Lambda expression 
numbers.stream().map(number -> Calculator.square(number)).forEach(number -> System.out.println(number));
		
// Method Reference
numbers.stream().map(Calculator::square).forEach(System.out::println);

Output

1.0
4.0
9.0

2.2 Integer::parseInt

We know that parseInt is a static method in Integer class.

2.2.1 Initialize Functional Interface

Let’s initialize java built-in Function functional interface and apply it to parse String to Integer.
Function interface takes one argument and returns one argument. Here in example, it takes a String argument and returns Integer.

// Initialize functional interface
Function<String, Integer> parseInt = Integer::parseInt;

// Execute function
parseInt.apply("2019");

Output

2019
2.2.2 Replace Lambda Expression

Let’s parse a list of String to Integer using its static parseInt method. We see that how we can replace lambda expression with method reference resulting in same output.

List<String> years = Arrays.asList("2019", "2020", "2021");

// Lambda Expression
years.stream().map(year -> Integer.parseInt(year)).forEach(year -> System.out.println(year));

// Method Reference
years.stream().map(Integer::parseInt).forEach(System.out::println);

Output

2019
2020
2021

3. Instance Method

Syntax
// Lambda expression 
(args) -> object.instanceMethodName(args)

// Method Reference
object::instanceMethodName

3.1 Calculator

Let’s create two instance methods square and multiply in Calculator class.

class Calculator {
	
    public double square(double num){
       return Math.pow(num, 2);
    }
    public double multiply(double num1, double num2) {
    	return num1*num2;
    }
}
3.1.1 Initialize Functional Interface

Let’s initialize java built-in Function, BiFunction functional interface and apply it to find square and multiply respectively.

// Initialize functional interface
Function<Double, Double> square = new Calculator()::square;
BiFunction<Double, Double, Double> multiply = new Calculator()::multiply;

// Execute function
square.apply(3.0);
multiply.apply(2.5, 5.0)

Output

9.0
12.5
3.1.2 Replace Lambda Expression

We generate square of list of Integer by replacing lambda expression with method reference.

List<Integer> numbers = Arrays.asList(1,2,3);

// Lambda expression 
numbers.stream().map(number -> new Calculator().square(number)).forEach(number -> System.out.println(number));
		
// Method Reference
numbers.stream().map(new Calculator()::square).forEach(System.out::println);

Output

1.0
4.0
9.0

3.2 System.out::println

3.2.1 Initialize Functional Interface

Let’s initialize java built-in Supplier functional interface to print a String.
Supplier interface accepts one argument and returns nothing. Here in example, it accepts a String argument and print it.

// Initialize functional interface
Consumer<String> println = System.out::println;

// Execute function
println.accept("Learning Method Reference a.k.a Colon Operator ::");

Output

Learning Method Reference a.k.a Colon Operator ::
3.2.2 Replace Anonymous Class and Lambda Expression

We will print a list of String and see how double colon :: method reference makes the code more concise and readable as compare to anonymous class and lambda expression.

List<String> languages = Arrays.asList("java", "javascript", "css");

// Anonymous Class
languages.forEach(new Consumer<String>() {         
    @Override
    public void accept(String str) {
        System.out.println(str);
    }
});

// Lambda expression 
languages.forEach(str -> System.out.println(str)); 

// Method Reference
languages.forEach(System.out::println);            

Output

java
javascript
css
3.2.3 Print a list of Integer

Let’s print a list of integer using lambda expression and double colon :: method reference.

List<Integer> numbers = Arrays.asList(1,2,3);

// Lambda expression 
numbers.forEach(number->System.out.println(number));

// Method Reference
numbers.forEach(System.out::println);

Output

1
2
3

3.3 String::toUpperCase

3.3.1 Initialize Functional Interface

Let’s initialize java built-in Function functional interface and execute it to change String to UPPERCASE.

// Initialize functional interface
Function<String, String> toUpperCase = String::toUpperCase;

// Execute function
toUpperCase.apply("java");

Output

JAVA
3.3.2 Replace Lambda Expression

Let’s look at the example where we use multiple method references to print the String in uppercase.

List<String> languages = Arrays.asList("java", "javascript", "css");

// Lambda expression
languages.stream().map(str -> str.toUpperCase()).forEach(str -> System.out.println(str));

// Method Reference
languages.stream().map(String::toUpperCase).forEach(System.out::println);          

Output

JAVA
JAVASCRIPT
CSS

4. Constructor

Double colon :: operator can be used to create an instance by calling constructor.

Syntax
ClassName::new

4.1 Create an int[10] array

// Default
int[] array1 = new int[10];

// Method Reference
IntFunction<int[]> arrayMaker = int[]::new;
int[] array2 = arrayMaker.apply(10);

4.2 Create HashMap

// Default
Map map1 = new HashMap();

// Method Reference
Supplier<Map> mapMaker = HashMap::new;
Map map2 = mapMaker.get();

5. Super Method

Syntax
super::parentMethodName

We create square and multiply instance methods in Calculator class and then call those methods in our inherited AdvanceCalculator class using super::parentMethodName to create advance methods squareAndAdd and squareAndMultiply

class Calculator {
	
    public double square(double num){
       return Math.pow(num, 2);
    }
    
    public double multiply(double num1, double num2) {
    	return num1*num2;
    }
}

class AdvanceCalculator extends Calculator {
	
	public double squareAndAdd(double num1, double num2) {
    	Function<Double, Double> square = super::square; 	
    	return square.apply(num1) + square.apply(num2);
    }
	
	public double squareAndMultiply(double num1, double num2) {
    	Function<Double, Double> square = super::square; 
    	BiFunction<Double, Double, Double> multiply = super::multiply;
    	return multiply.apply(square.apply(num1), square.apply(num2));
    }
}

6. Real World Practical example

Let’s create a Class Tutorial with properties name, duration and rating.

class Tutorial {

  private String name;
  private Integer duration;
  private Double rating;

  public Tutorial(String name) {
    this.name = name;
    this.duration = 0;
    this.rating = 0.0;
  }
  
  public Tutorial(String name, Integer duration) {
    this.name = name;
    this.duration = duration;
    this.rating = 0.0;
  }
  
  public Tutorial(String name, Integer duration, Double rating) {
    this.name = name;
    this.duration = duration;
    this.rating = rating;
  }
  
  public String getName() {
    return name;
  }	
  public void setName(String name) {
    this.name = name;
  }
  public Integer getDuration() {
    return duration;
  }
  public void setDuration(Integer duration) {
    this.duration = duration;
  }
  public Double getRating() {
    return rating;
  }
  public void setRating(Double rating) {
    this.rating = rating;
  }
  
  @Override
  public String toString() {
    return "Tutorial[ " + name + "\t- " + duration + "min, rating=" + rating + " ]";
  }

  public static int compareByRating(Tutorial t1, Tutorial t2) {
    return t1.getRating().compareTo(t2.getRating());
  }

  public static int compareByDuration(Tutorial t1, Tutorial t2) {
    return t1.getDuration().compareTo(t2.getDuration());
  }
}

Let’s create a list of tutorials objects:-

// List of tutorials
List<Tutorial> tutorials = Arrays.asList(new Tutorial[] {
    new Tutorial("Streams in Java 8", 30, 4.2),
    new Tutorial("What's new in Java 11", 25, 4.8),
    new Tutorial("Core Java Concepts", 45, 3.5)});

6.1 Print the list of tutorials

// Lambda Expression
tutorials.forEach(tutorial -> System.out.println(tutorial));

// Method Reference
tutorials.forEach(System.out::println);

Output

Tutorial[ Streams in Java 8	- 30min, rating=4.2 ]
Tutorial[ What's new in Java 11	- 25min, rating=4.8 ]
Tutorial[ Core Java Concepts	- 45min, rating=3.5 ]

6.2 Get list of tutorial names in uppercase

// Lambda Expression
tutorials.stream().map(tutorial -> tutorial.getName()).map(name -> name.toUpperCase()).forEach(s -> System.out.println(s));

// Method Reference
tutorials.stream().map(Tutorial::getName).map(String::toUpperCase).forEach(System.out::println);

Output

STREAMS IN JAVA 8
WHAT'S NEW IN JAVA 11
CORE JAVA CONCEPTS

6.3 Sort Tutorials by Rating

// Lambda Expression
tutorials.stream().sorted((tutorial1, tutorial2) -> Tutorial.compareByRating(tutorial1, tutorial2)).forEach(tutorial -> System.out.println(tutorial));

// Method Reference
tutorials.stream().sorted(Tutorial::compareByRating).forEach(System.out::println);	

Output

Tutorial[ Core Java Concepts	- 45min, rating=3.5 ]
Tutorial[ Streams in Java 8	- 30min, rating=4.2 ]
Tutorial[ What's new in Java 11	- 25min, rating=4.8 ]

6.4 Sort Tutorials by Duration

// Lambda Expression
tutorials.stream().sorted((tutorial1, tutorial2) -> Tutorial.compareByDuration(tutorial1, tutorial2)).forEach(tutorial -> System.out.println(tutorial));

// Method Reference
tutorials.stream().sorted(Tutorial::compareByDuration).forEach(System.out::println);	

Output

Tutorial[ What's new in Java 11	- 25min, rating=4.8 ]
Tutorial[ Streams in Java 8	- 30min, rating=4.2 ]
Tutorial[ Core Java Concepts	- 45min, rating=3.5 ]

6.5 Create New Instance of Tutorial

Each constructor method reference returns a Functional Interface. One-arg constructor returns Function and two-arg constructor returns BiFunction functional interface comes default in Java 8 so we are creating TriFunction functional interface on our own to call three-arg constructor.

@FunctionalInterface
interface TriFunction<A, B, C, R> { 
    R apply(A a, B b, C c); 
    default <V> TriFunction<A, B, C, V> andThen( Function<? super R, ? extends V> after) { 
        Objects.requireNonNull(after); 
        return (A a, B b, C c) -> after.apply(apply(a, b, c)); 
    } 
}

// Create an instance from one arg constructor
Function<String, Tutorial> tutorial1 = Tutorial::new;
Tutorial t1 = tutorial1.apply("Tutorial 1");

// Create an instance from two arg constructor
BiFunction<String, Integer, Tutorial> tutorial2 = Tutorial::new; 
Tutorial t2 = tutorial2.apply("Tutorial 2", 25);

// Create an instance from three arg constructor
TriFunction<String, Integer, Double, Tutorial> tutorial3 = Tutorial::new;
Tutorial t3 = tutorial3.apply("Tutorial 3", 30, 4.9);

Arrays.asList(t1, t2, t3).forEach(System.out::println);

Output

Tutorial[ Tutorial 1	- 0min, rating=0.0 ]
Tutorial[ Tutorial 2	- 25min, rating=0.0 ]
Tutorial[ Tutorial 3	- 30min, rating=4.9 ]

7. Conclusion

In this article, we saw how to use double colon operator introduced in Java 8. It is very useful to keep your code concise and readable specially in streams where you can replace lambda expressions with method reference using double colon operator.

Modern IDEs such as Eclipse (Quick Fix Feature) and IntelliJ IDEA (Intention Feature) provide built in support to convert lambda expression to an equivalent method reference.