java.lang.Void vs void vs Null

Void is "an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void." (from http://docs.oracle.com/javase/6/docs/api/java/lang/Void.html)

void is a return type signifying no return.

null is the absence of value.

The most common use of Void is for reflection, but that is not the only place where it may be used.

void is a keyword that means that a function does not result a value.

java.lang.Void is a reference type, then the following is valid:

 Void nil = null;

(so far it is not interesting...)

As a result type (a function with a return value of type Void) it means that the function *always * return null (it cannot return anything other than null, because Void has no instances).

 Void function(int a, int b) {
    //do something
    return null;
 }

Why would I like a function that always returns null?

Before the invention of generics, I didn't have an use case for Void.

With generics, there are some interesting cases. For instance, a Future<T> is a holder for the result of an asynchronous operation performed by other thread. Future.get will return the operation value (of type T), and will block until the computation is performed.

But... what if there is nothing to return? Simple: use a Future<Void>. For instance, in Google App Engine the Asyncronous Datastore Service delete operation returns a Future<Void>. When get() is invoked on that future, null is returned after the deletion is complete. One could write a similar example with Callables.

Another use case is a Map without values, i.e. a Map<T,Void>. Such a map behaves like a Set<T>, then it may be useful when there is no equivalent implementation of Set (for instance, there is no WeakHashSet, then one could use a WeakHashMap<T,Void>).