:: (double colon) operator in Java 8

Question

I was exploring the Java 8 source and found this particular part of code very surprising:

//defined in IntPipeline.java
@Override
public fin         


        

Answer 1

This is a method reference in Java 8. The oracle documentation is here.

As stated in the documentation...

The method reference Person::compareByAge is a reference to a static method.

The following is an example of a reference to an instance method of a particular object:

class ComparisonProvider {
    public int compareByName(Person a, Person b) {
        return a.getName().compareTo(b.getName());
    }

    public int compareByAge(Person a, Person b) {
        return a.getBirthday().compareTo(b.getBirthday());
    }
}

ComparisonProvider myComparisonProvider = new ComparisonProvider();
Arrays.sort(rosterAsArray, myComparisonProvider::compareByName); 

The method reference myComparisonProvider::compareByName invokes the method compareByName that is part of the object myComparisonProvider. The JRE infers the method type arguments, which in this case are (Person, Person).