Java generics void/Void types

java void return-type
Travis Webb picture Travis Webb · Apr 6, 2011 · Viewed 53.2k times · Source

I am implementing a ResponseHandler for the apache HttpClient package, like so:

new ResponseHandler<int>() {
    public int handleResponse(...) {
        // ... code ...
        return 0;
    }
}

but I'd like for the handleResponse function to return nothing, i.e. void. Is this possible? The following does not compile, since void is not a valid Java type:

new ResponseHandler<void>() {
        public void handleResponse(...) {
            // ... code ...
        }
}

I suppose I could replace void with Void to return a Void object, but that's not really what I want. Question: is it possible to organize this callback situation in such a way that I can return void from handleResponse?

Answer

ILMTitan picture ILMTitan · Apr 6, 2011

The Void type was created for this exact situation: to create a method with a generic return type where a subtype can be "void". Void was designed in such a way that no objects of that type can possibly be created. Thus a method of type Void will always return null (or complete abnormally), which is as close to nothing as you are going to get. You do have to put return null in the method, but this should only be a minor inconvenience.

In short: Do use Void.

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