Is it possible to make a generic functional interface with optional parameter(s)?

I’m trying to create a functional interface in Java with a single method (of course) that can take any type of parameter(s) and return any data type (i.e. a generic method).

This is what I have so far:

Calculator.java

public interface Calculator<T> {
    T operation(T n1, T... n2); //this is where the optional parameter comes in
}

Main.java

public static void main(String[] args) {
    Calculator<Integer> addition = (n1, n2) -> n1 + n2; //this gives an error
}

The error says:

bad operand types for binary operator ‘+’

• Is it possible to create a generic functional interface with optional parameter(s) in Java?
• If so, what am I doing wrong?

Answer

public interface Calculator<T> {
    T operation(T n1, T .. n2); //this is where the optional parameter comes in
}

The error comes from the fact that you are trying to apply the operator + to a Integer and an Array of Integers. The following, for instance

public interface Calculator<T> {
    T operation(T n1, T n2); //this is where the optional parameter comes in
}

would work fine, since you would be applying the operator + to two Integers.

If you want to keep the same Interface then you need to change you code in the main to:

public static void main(String[] args) {
    Calculator<Integer> addition = (n1, n2) -> n1 + Arrays.stream(n2).reduce(0, Integer::sum);
}

Is it possible to create a generic functional interface with optional parameter(s) in Java?

From this SO Thread one can read:

varargs could do that (in a way). Other than that, all variables in the declaration of the method must be supplied. If you want a variable to be optional, you can overload the method using a signature which doesn’t require the parameter.

That being said, what you could do is something like:

public interface Calculator<T> {
    T operation(T ...n);
}

In this way, the method operation can accept 0, 1 … N elements and even null.

Then in your main:

Calculator<Integer> addition = n -> (n == null) ? 0 : 
                                    Arrays.stream(n).reduce(0, Integer::sum);

A running Example:

public class Main {
    public static void main(String[] args) {
        Calculator<Integer> addition = n -> (n == null) ? 0 : Arrays.stream(n).reduce(0, Integer::sum);
        System.out.println(addition.operation(1, 2));
        System.out.println(addition.operation(1));
        System.out.println(addition.operation());
        System.out.println(addition.operation(null));
    }
}

Output:

3 // 1 + 2
1 // 1
0 // empty array 
0 // null