What is the difference between java.lang.Void and void?

Question

In API

"The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void."

1. What is "uninstantiable" place holder class? When will java.lang.Void be used? If the class is "uninstantiable", what use is it?
2. What is difference between java.lang.Void and void?

Answer 1

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.