Programming Forums > Application Development > Java

quantalfred · Jul 23rd, 2006, 12:11 AM

I define a class and write the main method without static. I can compile it. But in other class when I initiate one instance of that class (call it aa) and call aa.main(args); (with args the correct input type), I got an error "variable aa might not have been initialized.

I understand that main is called when it is executed. Then does that mean public static is the only way all the main functions can be, at least meaningfully?

lectricpharaoh · Jul 23rd, 2006, 12:42 AM

Quote:

Originally Posted by quantalfred

I define a class and write the main method without static. I can compile it. But in other class when I initiate one instance of that class (call it aa) and call aa.main(args); (with args the correct input type), I got an error "variable aa might not have been initialized.

I understand that main is called when it is executed. Then does that mean public static is the only way all the main functions can be, at least meaningfully?

You can create other versions of main(), with different signatures. However, it is the static one (with a String array as its sole argument) that is called when you 'run' the class. There are two reasons for this.

First, it needs to have the same 'signature', or argument list. This is so the Java VM knows what parameters to use when calling it. Second, and more importantly, it needs to be static so that the VM can call it without first creating any class instances. Non-static methods can only be called using an object of the class. Since the VM won't create classes until you tell it to (barring some startup classes it might create for its own use), you won't have an object to use before main() is called, and thus, it must be a static method.

Having said all that, there is no absolute requirement that a given class have a main() method in the given format, or even a main() method at all. Instead, only the class that is the entry point for your application needs such a method.

Eric the Red · Jul 23rd, 2006, 1:02 AM

Please post what you have so far. Also, include the line number of where the error occurs.

quantalfred · Jul 23rd, 2006, 1:30 AM

Quote:

Originally Posted by Eric the Red

Please post what you have so far. Also, include the line number of where the error occurs.

(Toggle Plain Text)

public class test2{
	public void main(String [] Args){
		System.out.println("hi");
	}
}

in the file test2.java
and

(Toggle Plain Text)

public class test1{
	public static void main(String [] Args){
		test2 aa;
		String[] arg;
		aa.main(arg);
	}
}

in the file test1.java.

test2.java can be compiled but not executed. When test1.java is compiled, the error is what I just posted in my first post.

quantalfred · Jul 23rd, 2006, 1:35 AM

Quote:

Originally Posted by lectricpharaoh

You can create other versions of main(), with different signatures. However, it is the static one (with a String array as its sole argument) that is called when you 'run' the class. There are two reasons for this.

First, it needs to have the same 'signature', or argument list. This is so the Java VM knows what parameters to use when calling it. Second, and more importantly, it needs to be static so that the VM can call it without first creating any class instances. Non-static methods can only be called using an object of the class. Since the VM won't create classes until you tell it to (barring some startup classes it might create for its own use), you won't have an object to use before main() is called, and thus, it must be a static method.

Having said all that, there is no absolute requirement that a given class have a main() method in the given format, or even a main() method at all. Instead, only the class that is the entry point for your application needs such a method.

Thanks, does that mean, the main function is special in the sense that it can not be called in other class. Or if it can be, how can I do it?

titaniumdecoy · Jul 23rd, 2006, 1:34 PM

lectricpharaoh has explained this very well, so I won't go into detail. To put it simply, when you run your Java application, the interpreter looks for a main method which must have the exact signature, public static void main(String[] args). This method will be the entry point for your program; any similar methods (eg, without the "static" keyword) will be treated as ordinary methods.

As for your example code, the reason it does not run has nothing to do with the naming of your methods. Since you have omitted the "static" keyword in the method main() in the test2 class, it is treated as an ordinary method. The errors you are experiencing arise from not having initialized the class or array variables; remember that all variables in Java are references.

(Toggle Plain Text)

public class test1{
	public static void main(String [] Args){
		test2 aa = new test2();
		String[] arg = {};
		aa.main(arg);
	}
}

JimmyJim · Jul 23rd, 2006, 1:38 PM

you can call main like this

(Toggle Plain Text)

class MyClass
{
        public static void main(String[] args)
        {
                 MyClass m = new MyClass();
                 m.main(args) ;
                 //note this wont work, as main is static and therefore is not part 
                 //of the instance of MyClass m.
                 
                 MyClass.main(args);
                 //this should work as you can access static methods and fields 
                 //on a class by using the class name. 
                 //NOTE: THIS IS RECURSIVE IN MY EXAMPLE. 
                 
        }
        ...
}

Calling main is a very bad idea if you ask me.

Its static so that there does not have to be an instance of MyClass to call main. Static method and fields are never part of a instance of that class, but are always there if you call them by the class name. So you could have a static variable that keeps track of the number of instances of that class by incrmenting it in the constructor and decrementing it in the finalize method.

Jul 23rd, 2006, 12:11 AM	#1
quantalfred Newbie Join Date: Jan 2005 Posts: 21 Rep Power: 0	Call the class main function I define a class and write the main method without static. I can compile it. But in other class when I initiate one instance of that class (call it aa) and call aa.main(args); (with args the correct input type), I got an error "variable aa might not have been initialized. I understand that main is called when it is executed. Then does that mean public static is the only way all the main functions can be, at least meaningfully?

Jul 23rd, 2006, 1:02 AM	#3
Eric the Red Hobbyist Programmer Join Date: Feb 2006 Posts: 214 Rep Power: 0	Please post what you have so far. Also, include the line number of where the error occurs. __________________ Death smiles at us all. All a man can do is smile back.

Jul 23rd, 2006, 1:34 PM	#6
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 837 Rep Power: 3	lectricpharaoh has explained this very well, so I won't go into detail. To put it simply, when you run your Java application, the interpreter looks for a main method which must have the exact signature, public static void main(String[] args). This method will be the entry point for your program; any similar methods (eg, without the "static" keyword) will be treated as ordinary methods. As for your example code, the reason it does not run has nothing to do with the naming of your methods. Since you have omitted the "static" keyword in the method main() in the test2 class, it is treated as an ordinary method. The errors you are experiencing arise from not having initialized the class or array variables; remember that all variables in Java are references. (Toggle Plain Text) public class test1{ public static void main(String [] Args){ test2 aa = new test2(); String[] arg = {}; aa.main(arg); } } public class test1{ public static void main(String [] Args){ test2 aa = new test2(); String[] arg = {}; aa.main(arg); } }

Jul 23rd, 2006, 1:38 PM	#7
JimmyJim Programmer Join Date: Jul 2006 Location: England Posts: 43 Rep Power: 0	you can call main like this (Toggle Plain Text) class MyClass { public static void main(String[] args) { MyClass m = new MyClass(); m.main(args) ; //note this wont work, as main is static and therefore is not part //of the instance of MyClass m. MyClass.main(args); //this should work as you can access static methods and fields //on a class by using the class name. //NOTE: THIS IS RECURSIVE IN MY EXAMPLE. } ... } class MyClass { public static void main(String[] args) { MyClass m = new MyClass(); m.main(args) ; //note this wont work, as main is static and therefore is not part //of the instance of MyClass m. MyClass.main(args); //this should work as you can access static methods and fields //on a class by using the class name. //NOTE: THIS IS RECURSIVE IN MY EXAMPLE. } ... } Calling main is a very bad idea if you ask me. Its static so that there does not have to be an instance of MyClass to call main. Static method and fields are never part of a instance of that class, but are always there if you call them by the class name. So you could have a static variable that keeps track of the number of instances of that class by incrmenting it in the constructor and decrementing it in the finalize method.

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Programming Forums > Application Development > Java

Call the class main function