Java 8 - Method references

This article is in continuation to my other posts on Functional Interfaces, static and default methods and Lambda expressions.

Method references are the special form of Lambda expression. When your lambda expression are doing nothing other than invoking existing behaviour (method), you can achieve same by referring it by name.

:: is used to refer to a method.
Method type arguments are infered by JRE at runtime from context it is defined.

Types of method references

Static method reference
Instance method reference of particular object
Instance method reference of an arbitrary object of particular type
Constructor reference

Static method reference

When you refer static method of Containing class. e.g. ClassName::someStaticMethodName

class MethodReferenceExample {
  public static int compareByAge(Employee first, Employee second) {
    return Integer.compare(first.age, second.age);
  }
}

Comparator compareByAge = MethodReferenceExample::compareByAge;

Instance method reference of particular object

When you refer to the instance method of particular object e.g. containingObjectReference::someInstanceMethodName

static class MyComparator {
  public int compareByFirstName(User first, User second) {
    return first.getFirstName().compareTo(second.getFirstName());
  }
  
  public int compareByLastName(User first, User second) {
    return first.getLastName().compareTo(second.getLastName());
}

private static void instanceMethodReference() {
  System.err.println("Instance method reference");
  List<User> users = Arrays.asList(new User("Gaurav", "Mazra"),
      new User("Arnav", "Singh"), new User("Daniel", "Verma"));
  MyComparator comparator = new MyComparator();
  System.out.println(users);
  Collections.sort(users, comparator::compareByFirstName);
  System.out.println(users);
}

Instance method reference of an arbitrary object of particular type

When you refer to instance method of some class with ClassName. e.g. ClassName::someInstanceMethod;

Comparator<String> stringIgnoreCase = String::compareToIgnoreCase;
//this is equivalent to
Comparator<String> stringComparator = (first, second) -> first.compareToIgnoreCase(second);

Constructor reference

When you refer to constructor of some class in lambda. e.g. ClassName::new

Function<String, Job> jobCreator = Job::new;
//the above function is equivalent to
Function<String, Job> jobCreator2 = (jobName) -> return new Job(jobName);

You can find the full example on github.

You can also view my other article on Java 8

Java 8 - Functional Interfaces Java 8 - Lambda expressions Java 8 - Default and Static methods in Interfaces Java 8 - Filtering with Predicates Java 8 - Interating Collections with #forEach, Consumer and BiConsumer

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