What is the purpose of List<Void>?

Question

I didn't even know this was doable, but I saw while perusing some code online a method with a signature like this:

public List<Void> read( ... ) 

... What? Is there ever a reason to do this? What could this List even hold? As far as I was aware, it's not possible to instantiate a Void object.

Answer 1

It is possible that this method signature was created as a by-product of some generic class.

For example, SwingWorker has two type parameters, one for final result and one for intermediate results. If you just don't want to use any intermediate results, you pass Void as the type parameter, resulting in some methods returning Void - i.e. nothing.

If there were a method List<V> returnAllIntermediateResults() in SwingWorker with Void as the type parameter V, it would have created a method just like you posted in your question.

The code would be perfectly valid. You can instantiate any implementation of the List interface (e.g. ArrayList) with type parameter Void. But the only value a Void type can have is null. So the list could not hold anything else but nulls, if the implementation allows null elements.

Answer 2

One case in which it may be useful is if you wanted to return a collection of return values from a function. Say

static List<T> forEach(Func<A,T> func, List<A> items) {    List<T> ret = new List<T>();    for(int i = 0; i< items.length; i++) {      ret.add(func.call(items[i]);    }    return ret; }  public static void main() {   ...   List<Void> boringResult =     forEach(     new Func<Void, Integer> {@override Void call(Integer i) {...}}); } 

Not that useful but you could see a case where it was required.