Why Is Main() in Java Void

Why does Java Specifications want the main method of a program to be void only?

The main is void is specified in JLS 12.1.4 Invoke Test.main:

The method main must be declared public, static, and void.

Sometimes the answer to a why? is because the specification says so (aka Quod Ego Dixit, or Because I Say So (Terry Pratchett)). Sometimes the decisions of a language designer are not available and the only thing we can do is guess. This is also the reason the obvious duplicate (Why is main() in java void?) is closed as primarily opinion-based.

Now as the - to me - obvious reason: JLS 12.8 Program Exit says:

A program terminates all its activity and exits when one of two things happens:

• All the threads that are not daemon threads terminate.
• Some thread invokes the exit method of class Runtime or class System, and the exit operation is not forbidden by the security manager.

This means that the end of the main method does not necessarily mean the end of the program. A return code from a main method has a well-known meaning in languages like C: it is the process exit code. This meaning doesn't apply when the main method may exit long before the process itself ends.

Most non-trivial Java applications have - usually- a short-lived main method that kicks off one or more long-living non-daemon threads. Allowing a return code from a Java main method could imply a meaning that is not actually there: that the return value is the process exit code. So explicitly specifying that main must have a void return type is good thing: it reduces confusion or assigning meaning that isn't there.

Why does the main function in Java reside in a class?

Probably for the same reason you put question marks at the end of a question: that's just how they decided it's done.

The main method is the result of a convention that says "this is how the entry point's method signature should look" which doesn't exempt it from language semantics.

Java does not support methods outside of classes/interfaces and as such it has to be contained in one.

Why can't use Void as the return type for main method

You should use void (lowercase v) not Void object. Void object is not going to get autoboxing like e.g. int/Integer, see Java Language Specification for a list of autoboxing objects.

Void is not a wrapper for void, it is just an object that has a very similar name so it can be used in places where you need to specify a return type (like e.g. Callable<T>), it is just for documentation purposes and to workaround some generic classes return types.

Second use case is in reflection (when you want to check the return value of a void function, you will get Void.TYPE).

Correct line is:

public static void main(String[] args)

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