Java Reusing interface methods

Question

This is the interface I'm using:

interface Command {
    void run(int a, int b, int c);
    void run(int a, int b, int c, int d, int e);
}

void add(Command c){

}

this is execution: (using the first run method)

    add((a, b, c) -> System.out.println(""));

The error i get on the execution is:

    The target type of this expression must be a functional interface

And this only goes away when i comment out the second run method.

I want to be able to execute the same interface using both run methods, and not have to create a new interface just to execute with method 2.

How?

Say I'm implementing your add method. You call it with add((a, b, c) -> System.out.println(""));. What do you expect to happen if, in the implementation of add, I call c.add(1,2,3,4,5);? Why do you expect that? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Nov 12, 2016 at 2:21

Jack · Accepted Answer · 2016-11-12 02:23:11Z

1

A functional interface by design is allowed to have just one method so what you are trying to do can't be done directly, it's clear why: allowing to implement a functional interface with a lambda must create an object which is valid for its declaration, if it has two methods you cant implement them both with a single lambda.

A workaround could be to use variadic arguments, eg:

@FunctionalInterface
interface Command {
 void run(int... args); 
}

add(args -> { int z = args[0] + args[1] + args[2]; });

answered Nov 12, 2016 at 2:23

Jack

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1 Comment

user7148508 Over a year ago

I guess that method works fine, but with that i would have to make an argument an instance of something to call a method: ((Automobile)args[0]).getInCar() is there a way this can be done better?

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