Method References

Often it will make sense to separate the code for a lambda into its own method.

interface Drummer {
    void drum();
}

class Main {
    void doDrum() {
        IO.println("ratatatatat");
        IO.println("crash!");
        IO.println("crash!");
        IO.println("ratatatatat");
    }

    void main() {
        Drummer drummer = () -> doDrum();
        drummer.drum();
    }
}

When you have a method that matches¹ the method required by a functional interface you can use a "method reference."

For instance methods this will look like instance::methodName. So inside a method this can be this::methodName but you can also have a variableName::methodName

interface Drummer {
    void drum();
}

class Main {
    void doDrum() {
        IO.println("ratatatatat");
        IO.println("crash!");
        IO.println("crash!");
        IO.println("ratatatatat");
    }

    void main() {
        Drummer drummer = this::doDrum;
        drummer.drum();

        Runnable run = drummer::drum;
    }
}

For static methods it will look like ClassName::doDrum.

interface Drummer {
    void drum();
}

class Main {
    static void doDrum() {
        IO.println("ratatatatat");
        IO.println("crash!");
        IO.println("crash!");
        IO.println("ratatatatat");
    }

    void main() {
        Drummer drummer = Drummer::doDrum;
        drummer.drum();
    }
}

And finally for instance methods where the the functional interface's method takes the instance explicitly as an argument it will look the same as a static method reference.

interface StringTransformer {
    String transform(String s);
}

class Main {
    void main() {
        // Instance method, but looks like a reference
        // to a static method that just so happens to
        // take a String as a first argument
        StringTransformer t = String::toUpperCase;
        IO.println(t.transform("ratatatatat"));
    }
}