Can a main method be overloaded?

Yes. You can have any number of main methods with different method signature and implementation in the class.

What is the arguement of main method?

main method accepts an array of String object as arguement.

Why is the main method declared static?

main method is called by the JVM even before the instantiation of the class hence it is declared as static.

What is the return type of the main method?

Main method doesn't return anything hence declared void.

Should a main method be compulsorily declared in all java classes?

No not required. main method should be defined only if the source class is a java application.

How to define a constant variable in Java?

The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.
static final int PI = 2.14; is an example for constant.

What are instance variables?

Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.