:: (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

:: is called Method Reference. It is basically a reference to a single method. I.e. it refers to an existing method by name.

Short Explanation:
Below is an example of a reference to a static method:

class Hey {
     public static double square(double num){
        return Math.pow(num, 2);
    }
}

Function square = Hey::square;
double ans = square.apply(23d);

square can be passed around just like object references and triggered when needed. In fact, it can be just as easily used as a reference to "normal" methods of objects as static ones. For example:

class Hey {
    public double square(double num) {
        return Math.pow(num, 2);
    }
}

Hey hey = new Hey();
Function square = hey::square;
double ans = square.apply(23d);

Function above is a functional interface. To fully understand ::, it is important to understand functional interfaces as well. Plainly, a functional interface is an interface with just one abstract method.

Examples of functional interfaces include Runnable, Callable, and ActionListener.

Function above is a functional interface with just one method: apply. It takes one argument and produces a result.

---

The reason why ::s are awesome is that:

Method references are expressions which have the same treatment as lambda expressions (...), but instead of providing a method body, they refer an existing method by name.

E.g. instead of writing the lambda body

Function square = (Double x) -> x * x;

You can simply do

Function square = Hey::square;

At runtime, these two square methods behave exactly the same as each other. The bytecode may or may not be the same (though, for the above case, the same bytecode is generated; compile the above and check with javap -c).

The only major criterion to satisfy is: the method you provide should have a similar signature to the method of the functional interface you use as object reference.

The below is illegal:

Supplier p = Hey::square; // illegal

square expects an argument and returns a double. The get method in Supplier returns a value but does not take an argument. Thus, this results in an error.

A method reference refers to the method of a functional interface. (As mentioned, functional interfaces can have only one method each).

Some more examples: the accept method in Consumer takes an input but doesn't return anything.

Consumer b1 = System::exit;   // void exit(int status)
Consumer b2 = Arrays::sort;  // void sort(Object[] a)
Consumer b3 = MyProgram::main; // void main(String... args)

class Hey {
    public double getRandom() {
        return Math.random();
    }
}

Callable call = hey::getRandom;
Supplier call2 = hey::getRandom;
DoubleSupplier sup = hey::getRandom;
// Supplier is functional interface that takes no argument and gives a result

Above, getRandom takes no argument and returns a double. So any functional interface that satisfies the criteria of: take no argument and return double can be used.

Another example:

Set set = new HashSet<>();
set.addAll(Arrays.asList("leo","bale","hanks"));
Predicate pred = set::contains;
boolean exists = pred.test("leo");

In case of parameterized types:

class Param {
    T elem;
    public T get() {
        return elem;
    }

    public void set(T elem) {
        this.elem = elem;
    }

    public static  E returnSame(E elem) {
        return elem;
    }
}

Supplier> obj = Param::new;
Param param = obj.get();
Consumer c = param::set;
Supplier s = param::get;

Function func = Param::returnSame;

Method references can have different styles, but fundamentally they all mean the same thing and can simply be visualized as lambdas:

1. A static method (ClassName::methName)
2. An instance method of a particular object (instanceRef::methName)
3. A super method of a particular object (super::methName)
4. An instance method of an arbitrary object of a particular type (ClassName::methName)
5. A class constructor reference (ClassName::new)
6. An array constructor reference (TypeName[]::new)

For further reference, see http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html.