Java 8 Method Reference (with Examples)

Since Java 8, in simplest words, the method references are a way to refer to methods or constructors without invoking them. They are a shorthand notation of a lambda expression and can be used anywhere a functional interface is expected.

The method references are denoted using ‘Class::methodName‘ type syntax. Let’s learn about different types of available method references in Java 8.

1. Types of Method References

Java 8 allows four types of method references.

Method Reference To	Description	Syntax
Static method	Used to refer to static methods from a class	ClassName::staticMethodName
Instance method from a specific instance	Refer to an instance method using a reference to the supplied object	instance::instanceMethodName
Instance method from the Class type	Invoke the instance method on a reference to an object supplied by the context	ClassName::instanceMethodName
Constructor	Reference to a constructor	ClassName::new

Please note that the method references always utilize the :: operator. Let’s explore each type with examples.

2. Method Reference to a Static Method

This method reference is used when we want to refer to a static method without invoking it. An example to use Math.max() which is a static method.

List<Integer> integers = Arrays.asList(1,12,433,5);
		
Optional<Integer> max = integers.stream().reduce( Math::max ); 

max.ifPresent(value -> System.out.println(value)); 

Output:

433

3. Method Reference to Instance Method of a Particular Object

This type of method reference refers to an instance method of a specific object. It is used when we want to call a method on a particular instance.

Consider the following PrintService class.

public class PrintService {

  public void print(String message) {
      System.out.println(message);
  }
}

We can invoke the print() from an instance of PrintService using the ‘printService::print‘ method reference.

PrintService printService = new PrintService();
List<String> messages = Arrays.asList("Hello", "World", "Method", "References");

// using lambda expression
messages.forEach(message -> printService.print(message));

// iusing method reference
messages.forEach(printService::print);

The method reference is much cleaner and more readable, as the code clearly shows our intention.

4. Method Reference to Instance Method from a Class Type

In this method reference, we do not create any class instance. We can directly use the print() method from the class type PrintService.

List<String> messages = Arrays.asList("Hello", "World", "Method", "References");

messages.forEach(PrintService::print);

5. Method Reference to Constructor

This type of method reference refers to a constructor. It’s used when we want to create a new instance of a class.

For example, to create a new instance of ArrayList, we have use ArrayList::new.

ArrayList<Integer> integers = IntStream
				.range(1, 100)
				.boxed()
				.collect(Collectors.toCollection( ArrayList::new ));

That’s 4 type of method references in java 8 lambda enhancements.

Happy Learning !!