Java 8 how to pass a non-functional method as parameter

Question

I have a class which contains multiple methods:

Class SimpleClass {
    methodA(var1, var2) {
      //body
    }

    methodB(var1, var2) {
      //body
    }
    ....

}

Using java 8 Lambda, can I send one of the above methods as a parameter into another function of some other class? Something like below:

Class Service {
   doService(Method arg) {
     //Invoke passed simple class method here
     arg()
   }

}

Answer 1

If doService has the appropriate signature, you can write:

service.doService(mySimpleClass::methodA);

Full example:

class SimpleClass {
  public void methodA(String a, String b) {
    System.out.println(a + b);
  }
  //other methods
}

class Service {
  public void doService(BiConsumer<String, String> consumer) {
    consumer.accept("Hel", "lo");
  }
}

public static void main(String[] args) {
  SimpleClass sc = new SimpleClass();
  new Service().doService(sc::methodA); //prints Hello
}

Answer 2

Fully working example

import java.util.function.BiConsumer;
class SimpleClass {
    void methodA(String a, String b) {
        System.out.printf("%s %s", a, b);
    }
}
class Service {
    void doService(BiConsumer<String,String> method) {
        method.accept("Hola", "mundo");
    }
}
class Main {
    public static void main( String ... args ) { 
        SimpleClass sc = new SimpleClass();
        Service s = new Service();
        s.doService(sc::methodA);
    }
}

Because there are no function types in Java 8 you have to specify your service signature to accept one of the functional interfaces.

In general terms if the method accepts arguments but doesn't returns a result: it's a Consumer. If it returns a Boolean then it's a Predicate and if it returns some other value it's a Function. There are others like Supplier and others.

In an ideal world we would've written:

class Service {
   void doService( void(String,String) method /*<-- fictional method type*/ ) { 
      method("Hello", "world");
   }
} 

But we have to use those functional interfaces at the moment.

This is a very good read about method references in Java 8