Can a java lambda have more than 1 parameter?

Question

In Java, is it possible to have a lambda accept multiple different types?

I.e: Single variable works:

    Function <Integer, Integer> adder = i -> i + 1;
    System.out.println (adder.apply (10));

Varargs also work:

    Function <Integer [], Integer> multiAdder = ints -> {
        int sum = 0;
        for (Integer i : ints) {
            sum += i;
        }
        return sum;
    };

    //.... 
    System.out.println ((multiAdder.apply (new Integer [] { 1, 2, 3, 4 })));

But I want something that can accept many different types of arguments, e.g:

    Function <String, Integer, Double, Person, String> myLambda = a , b, c, d->  {
    [DO STUFF]
    return "done stuff"
    };

The main use is to have small inline functions inside functions for convenience.

I've looked around google and inspected Java's Function Package, but could not find. Is this possible?

Answer 1

It's possible if you define such a functional interface with multiple type parameters. There is no such built in type. (There are a few limited types with multiple parameters.)

@FunctionalInterface
interface Function6<One, Two, Three, Four, Five, Six> {
    public Six apply(One one, Two two, Three three, Four four, Five five);
}

public static void main(String[] args) throws Exception {
    Function6<String, Integer, Double, Void, List<Float>, Character> func = (a, b, c, d, e) -> 'z';
}

I've called it Function6 here. The name is at your discretion, just try not to clash with existing names in the Java libraries.

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There's also no way to define a variable number of type parameters, if that's what you were asking about.

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Some languages, like Scala, define a number of built in such types, with 1, 2, 3, 4, 5, 6, etc. type parameters.

Answer 2

For something with 2 parameters, you could use BiFunction. If you need more, you can define your own function interface, like so:

@FunctionalInterface
public interface FourParameterFunction<T, U, V, W, R> {
    public R apply(T t, U u, V v, W w);
}

If there is more than one parameter, you need to put parentheses around the argument list, like so:

FourParameterFunction<String, Integer, Double, Person, String> myLambda = (a, b, c, d) -> {
    // do something
    return "done something";
};