In API
"The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void."
java.lang.Void
be used? If the class is "uninstantiable", what use is it?java.lang.Void
and void
? java.lang.Void
is analogous to java.lang.Integer
. Integer
is a way of boxing values of the primitive type int
. Void
is a way of boxing values of the primitive type void
.
"But wait, void
doesn't have any possible values!"
Right! That's what makes java.lang.Void
"uninstantiable". :)
It's a nice feature of the Java type system that every primitive type has a boxed equivalent. int
has Integer
, long
has Long
, byte
has Byte
... and void
has Void
. It would be weird and asymmetrical if Void
didn't exist.
"So what's the difference between java.lang.Void
and void
?"
Easy. void
is a primitive type. Void
is an reference type that inherits from Object
. They're similar in that neither of them has any possible values; but nevertheless they are two very different types, from the type system's point of view.
"But I don't have any use for Void
in my programs."
And I don't have any use for GarbageCollectorMXBean
in mine. Some features don't have non-obscure uses. That's okay.
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 a 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 another 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 . When
. When
get() is invoked on that future,
null` is returned after the deletion is complete. One could write a similar example with Callable s.
Another use case is a Map
without values, ie 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>
).
The only point of Void
is to hold Void.TYPE
, which is sort of like void.class
. If you have a reflective reference to a method that returns void
, and you get its return type, it'll return Void.TYPE
.
You cannot, and should not, use it for anything else.
Void is a AutoBoxing feature (since JDK 1.5) of void.
well its self explanatory that Void is reference whereas void is a primitive type.
So, where the requirement comes to have to use Void ???
One common usage with Generic types where we can't use primitive.
Say, in case of Android
AsyncTaks<Params, Progress, Result>
what if I don't want to get Progress update. I can't use void (primitive type) here we require java.lang.Void
Another example for using Void
is SwingWorker
new SwingWorker<Void, Integer> () {
@Override
protected Void doInBackground(){
...
}
@Override
protected void process(List<Integer> chunk){
...
}
@Override
public void done(){
...
}
}.execute();
Void is useful because sometimes you need to specify the return type of a method outside the method itself.
For example take this java 8 lambda expression , which checks whether an EventResource object has certain properties, using a method called checkBenefitConcertInCentralPark
, passed into the method checkCreatedEvent
:
eventChecker.checkCreatedEvent(TestEvents::checkBenefitConcertInCentralPark);
The checkBenefitConcertInCentralPark
method is defined like this (note the use of Void):
public static Void checkBenefitConcertInCentralPark(EventResource eventResource) {
// JUnit code here...
// assertThat(blablabla :) )
return null; // we can only return null at the end of a method when returning Void
}
and then the checkBenefitConcertInCentralPark
method is passed into the method checkCreatedEvent
.
// Function<EventResource, Void> describes the checkBenefitConcertInCentralPark method
public void checkCreatedEvent(Function<EventResource, Void> function) {
function.apply(this.eventResource);
}
I've personally used it like this:
@FunctionalInterface
interface MyPackagePrivateInterface<T, Next> {
//Returns next
Next compareAndSwap(T prev);
}
@FunctionalInterface
public interface ClientPublicInterface<T> extends MyPackagePrivateInterface<T, Void> {
}
My reasoning is this:
computations based on user input, may create new Type allocations, that your system may need.
The action may KEEP its namesake since in the eyes of the client it is performing what it is saying, but in sublayers within the system an equivalent action is taking place but with completely different Types, Types which ONLY your system should be aware.
This may relate to a principle of "Self similarity" in which a system is composed of smaller components similar to itself, so it may be logical to reuse interfaces with different Types and similar namesake, this system also implies that reification (transforming something abstract into real) is intrinsical to the System state hence the user is not interested in an immediate reading of a return value, but the aim relies on the Object's change of state.
Now this particular example is for atomic operations, and TBH I cannot think of a different situation in which this may be needed, the reason being that native atomic operations are reliant in time-space(space = memory scope) snapshots for them to perform accurately.
This new allocations are not important to the client but only to the system's functionality.
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