What is a method reference in Java8? Quick explanation and samples.

Method reference – Lambda expression, which calls known method, written in shorter, more compact way.

This is how we can define it in a short way. There are 4 types of method references:

1. Reference to a static method
2. Reference to an instance method of a particular object
3. Reference to an instance method of an arbitrary object of a particular type
4. Reference to a constructor

While it is almost the same thing as Lambda expressions, lets reuse and extend a bit the same example from my previous post about Lambda expressions. Here is a quick sample for each of reference types:

• Reference to a static method

ContainingClass::staticMethodName

import java.util.function.Function;

public class StaticReferenceSample {

    private static int doSquare(int a) {
        return a * a;
    }

    public static void main(String[] args) {

        Function<Integer, Integer> sf = StaticReferenceSample::doSquare;

        System.out.println("Square of 5: " + sf.apply(5));

    }

}

StaticReferenceSample::doSquare is a reference to a static method “int doSquare(int a)”. As we see, it is not necessary to provide an argument, it is passed implicitly

• Reference to an instance method of a particular object

containingObject::instanceMethodName

public class InstnceReferenceSample {

    public static void main(String[] args) {

        List<String> candidates = new ArrayList<String>();

        List<String> participants = new ArrayList<String>();

        candidates.add("John");
        candidates.add("Peter");
        candidates.add("Matt");

        candidates.forEach(participants::add);

        System.out.println("Participants:");
        participants.forEach(System.out::println);

    }
}

In this example there two types of references combined. participants::add stands for an instance method reference. And System.out::println stands for a static reference as “out” is an static member of System class.

• Reference to an instance method of an arbitrary object of a particular type

ContainingType::methodName

public class ArbitraryObjectRefSample {


    public static void main(String[] args) {

        List<Participant> participants = new ArrayList<Participant>();

        participants.add(new Participant("John"));
        participants.add(new Participant("Peter"));
        participants.add(new Participant("Matt"));

        System.out.println("Participants:");
        participants.forEach(Participant::print);

    }

    private static class Participant {

        private String name;

        public Participant(String name) {
            this.name = name;
        }

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

}

Here Participant::print is a reference to an instance of “participants” method “print” of type “Participant”. The essential difference between a static reference, instance method reference and instance method of an arbitrary type is that the latter invokes a method onto the current object and the first two – pass the current object into the method.

An alternative for non Lambda and Method reference syntax for relevant code snippet would be like this:

//...
  System.out.println("Participants:");
  for (Participant part : participants){
      part.print();
  }
//...

• Reference to a constructor

ClassName::new

Reference to object constructor works the same way as to static method, just referencing it with a name “new”.

public class ArbitraryObjectRefSample {

public static void main(String[] args) {

List<String> candidates = new ArrayList<String>();
List<Participant> participants = new ArrayList<Participant>();

Function<String,Participant> participant=Participant::new;

candidates.add("John");
candidates.add("Peter");
candidates.add("Matt");

candidates.forEach(s ->participants.add(participant.apply(s)));

System.out.println("Participants:");
participants.forEach(Participant::print);

}

private static class Participant {

private String name;

public Participant(String name) {
this.name = name;
}

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

}

In this sample via interface Function<String,Participant> we create function to construct new object of Participant.

For more information about Java method references please refer to documentation.