Unneeded generic parameter

Question

Suppose there is a requirement to use a particular generic interface, but the situation does not require making use of one of the generic parameters.

Let's say I need a Callable<T> (which must return a T from its call() method), but on this occasion I don't need the return result, I just want to submit some code to an ExecutorService to "do something"

What's the best option for the type T?

Answer 1

You can use the special Void type:

Callable<Void> callable = new Callable<Void>() {
    @Override
    public Void call() throws Exception {
        // do stuff
        return null;
    }
};

The return statement is required to exit the method. The only value which the compiler accepts is null. Quite handy!