I will try to answer both, please correct me if I am wrong:

Where: If a static method is being called using Classname.method() or using reflection then it doesn’t matter even if you change the return type of the calling method, the same method will still be called.

So JVM probably checks this in one of the native methods of jvm.cpp

methodHandle m (THREAD, init_klass->find_method(vmSymbols::object_initializer_name(),> vmSymbols::void_method_signature()));

if (m.is_null()) { ------ THROW_MSG_0 ………..

Why: Although it’s of useless to return a value from main, as java does not do anything with it but if we try to change the return type of main to int for example, JVM throws

public static int main(String[] args) { return 1;
}

java.lang.NoSuchMethodError: main Exception in thread "main"

So may be Java mandates the use of same signature for entry method main() to maintain a symmetry in all Java programs written.