java - Java8 method reference used as Function object to combine functions

Question

Is there a way in Java8 to use a method reference as a Function object to use its methods, something like:

Stream.of("ciao", "hola", "hello")
    .map(String::length.andThen(n -> n * 2))

This question is not related to the Stream, it is used just as example, I would like to have answer about the method reference

Answer 1

You can write a static method to do this:

import java.util.function.*;

class Test {
    public static void main(String[] args) {
        Function<String, Integer> function = combine(String::length, n -> n * 2);
        System.out.println(function.apply("foo"));
    }

    public static <T1, T2, T3> Function<T1, T3> combine(
        Function<T1, T2> first,
        Function<T2, T3> second) {
        return first.andThen(second);
    }
}

You could then put it in a utility class and import it statically.

Alternatively, create a simpler static method which just returns the function it's given, for the sake of the compiler knowing what you're doing:

import java.util.function.*;

class Test {
    public static void main(String[] args) {
        Function<String, Integer> function = asFunction(String::length).andThen(n -> n * 2);
        System.out.println(function.apply("foo"));
    }

    public static <T1, T2> Function<T1, T2> asFunction(Function<T1, T2> function) {
        return function;     
    }
}