The title might be too long but i'll explain myself.
Usually, when I creates a main mathod, i put it in the class that i start with.
Lately i have seen other people codes and saw that they put the main mathod in a new class.
I thought about it and this question came in my mind.
When i start the program, does instance of the class that contains the main mathod is created?
So when I create a new instance of the same class in the main mathod, will it create 2 instances or the main is not related to the class itself and the class has no instance in the first place, like statics can't use non-static variable?
When i start the program, does instance of the class that contains the
main mathod is created?
No. There is a reason main() method is public and static. You don't have to create instance of the class in which it is defined. Thus when you execute your program, the class which contains main() will be loaded, initialized and then main() will be executed WITHOUT creating an instance of the enclosing class. That's why you can't access this from main()
When I start the program, does instance of the class that contains the main method created?
No. Because the main method is static. No instance of the enclosing class is automatically created. JLS-12.1 Java Virtual Machine Startup says (in part)
The Java Virtual Machine starts execution by invoking the method main of some specified class, passing it a single argument, which is an array of strings.
Main method signature : public static void(String args[])
Static:
Since main method is static Java virtual Machine can call it without creating any instance of class which contains main method. If main method were not declared static than JVM has to create instance of main Class and since constructor can be overloaded and can have arguments there would not be any certain and consistent way for JVM to find main method in Java.
public:
public in Java can be accessible from outside of that class. Since main method is public in Java, JVM can easily access and execute it.
void: its made void which simply means main is not returning anything.
- Main method must be declared public, static and void in Java
otherwise JVM will not able to run Java program
- JVM throws NoSuchMethodException:main if it doesn't find main method of predefined signature in class which is provided to Java command.
- Main method is entry point for any Core Java program. Execution
starts from main method.
Here's a trick : Up to and including Java 6 it was possible to do this using the Static Initialization Block as was pointed out in the question Printing message on Console without using main() method. For instance using the following code:
class A3{
static{
System.out.println("static block is invoked");
System.exit(0);
}
}
The System.exit(0) lets the program exit before the JVM is looking for the main method, otherwise the following error will be thrown:
Exception in thread "main" java.lang.NoSuchMethodError: main
In Java 7, however, this does not work anymore, even though it compiles, the following error will appear when you try to execute it:
The program compiled successfully, but main class was not found. Main
class should contain method: public static void main (String[] args).
see this and this
