Java 8: Lambda with variable arguments

What I did was for my own use case was to define a helper method that accepts varargs and then invokes the lambda. My goals were to 1) be able to define a function within a method for succinctness and scoping (i.e., the lambda) and 2) make calls to that lambda very succinct. The original poster may have had similar goals since he mentioned, in one of the comments above, wanting to avoid the verbosity of writing Object[] {...} for every call. Perhaps this will be useful for others.

Step #1: define the helper method:

public static void accept(Consumer<Object[]> invokeMe, Object... args) {
    invokeMe.accept(args);
}

Step #2: define a lambda which can use a varying number of arguments:

Consumer<Object[]> add = args -> {
    int sum = 0;
    for (Object arg : args)
        sum += (int) arg;
    System.out.println(sum);
};

Step #3: invoke the lambda many times - this succinctness was why I wanted the syntactic sugar:

accept(add, 1);
accept(add, 1, 2);
accept(add, 1, 2, 3);
accept(add, 1, 2, 3, 4);
accept(add, 1, 2, 3, 4, 5);
accept(add, 1, 2, 3, 4, 5, 6);

In Java you need to use an array like this.

test((Object[] args) -> me.call(args));

If call takes an array variable args this will work. If not you can use reflection to make the call instead.

The final solution I currently use is defining a hierarchy of interfaces (as stated in the question) and use default methods to avoid failure. Pseudo code looks like this:

interface VarArgsRunnable {
     default void run(Object ... arguments) {
          throw new UnsupportedOperationException("not possible");
     }
     default int getNumberOfArguments() {
          throw new UnsupportedOperationException("unknown");
     }
}

and a interface for four arguments for instance:

@FunctionalInterface
interface VarArgsRunnable4 extends VarArgsRunnable {
     @Override
     default void run(Object ... arguments) {
          assert(arguments.length == 4);
          run(arguments[0], arguments[1], arguments[2], arguments[3]);
     }

     void run(Object arg0, Object arg1, Object arg2, Object arg3, Object arg4);

     @Override
     default int getNumberOfArguments() {
          return 4;
     }
}

Having defined 11 interfaces from VarArgsRunnable0 to VarArgsRunnable10 overloading a method becomes quite easy.

public void myMethod(VarArgsRunnable runnable, Object ... arguments) {
     runnable.run(arguments);
}

Since Java can not compose a Lambda by finding the correct extended functional interface of VarArgsRunnable by using something like instance.myMethod((index, value) -> doSomething(to(index), to(value)), 10, "value") one need to overload the method using the correct interface.

public void myMethod(VarArgsRunnable2 runnable, Object arg0, Object arg1) {
    myMethod((VarArgsRunnable)runnable, combine(arg0, arg1));
}

private static Object [] combine(Object ... values) {
    return values;
}

Since this requires to cast Object to any appropriated type using to(...) one can go for parameterization using Generics in order to avoid this usage.

The to-method looks like this: public static T to(Object value) { return (T)value; //Supress this warning }

The example is lame but I use it to call a method with multiple arguments being a permutation of all potential combinations (for testing purposes) like:

run((index, value) -> doTheTestSequence(index, value), values(10, 11, 12), values("A", "B", "C"));

So this little line runs 6 invocations. So you see this is a neat helper being able to test multiple stuff in a single line instead of defining a lot more or use multiple methods in TestNG and whatever... .

PS: Having no need to use reflections is quite a good thing, since it can not fail and is quite save argument count wise.