why can't a function be called from a static context

[Mohit G Gupta]

SCJP1.6 JAVA 6 CERTIFICATION Guide-310-065
by Katherine Sierra (Author), Bert Bates (Author)

  
4. public class Frodo extends Hobbit {
5. public static void main(String[] args) {
6. Short myGold = 7;
7. System.out.println(countGold(myGold, 6));
8. }
9. }
10. class Hobbit {
11. int countGold(int x, int y) { return x + y; }
12. }

What is the result?
A. 13
B. Compilation fails due to multiple errors
C. Compilation fails due to an error on line 6
D. Compilation fails due to an error on line 7
E. Compilation fails due to an error on line 11
Answer:
 D is correct. The Short myGold is autoboxed correctly, but the countGold() method
cannot be invoked from a static context.


  

[Prasad Kharkar]

A static method can be called from any method i.e. the static as well as instance method can call any static method or static variable
A static method cannot call instance method without the help of the object

this is because
instance method are related to the instance and not the class
static method are totally related to the class and they have nothing to do with instance created
static methods and variables are the shared members for all the objects in the class

here I am providing a small program that will clear all your doubts regarding the static and non static members of the class

 
/* A Program to demonstrate the use of Static variables in Java
Developer : Prasad Kharkar
Data	  : 24 january 2010
*/

class StaticDemo
{
	int i =4;
	int j = 5;
	static int k = 6;
	static int l = 7;

	public void m1()
	{
		i++;
		j++;
		System.out.println(i);
		System.out.println(j);
	}
	public static void m2()
	{
		k++;
		l++;
		System.out.println(k);
		System.out.println(l);
	}
	
	public static void main(String [] args)
	{
		StaticDemo o1 = new StaticDemo();
		StaticDemo o2 = new StaticDemo();
		o1.m1();
		o2.m1();
		m2();		// object not required because it is a static method
		m2();
	
	}

}   

[Moguluri Ravi Kiran]

As far as i know

the static methods can be regarded as methods that are outside the class (something similar to main() of c/c++) even though they are inside a class.
so we can not access instance members in the static contexts.

they can be regarded as global methods of class & the instance methods are as local...

---> it is possible to access global methods(static) from local(instance) but oppisite is not true....

for example

  class st
{
  static int x=20;// see this as global thing.
  
  void printX()
  {
    System.out.println(x);// u can access the staic(global)
    main(new String[0]);//method or variable here(in local/instantiable things).
  }

  public static void main(String a[]) // this is also like global thing.
  {
    new st().printX();
    // here we can not go directly
    //by saying printX() directly,i.e we can not access 
    //local(instance) things from global(static)

    printX();// this gives us error..(accessing local like method from global like main(string a[])...
  }

} 
  

infact we made the main(string a[]) static for this purpose , imagine if it is not static - it becomes local to the class only & the JVM can not start the program.

[Sahil Kapoor]

Suppose hypothetically that "It is allowed by JAVA designers" to call non-static methods from static context...then lets see what would happen...but before that make a note about the following point.

1. Basically non-static Methods are used to access the state (Instance variables) of any object. They act according to the state of the object.


Now suppose, you call the non-static method from the static context......the non-static method would demand for certain instance varibales (read above point 1), but instance variables are yet not created , because instance variables are created with the creation of objects. Next question arises in mind is that , what if , we create certain objects and then call method from the static context , then in this case how would a method or JVM comes to know that whose instance variable are you talking about. No object is under consideration of the static method call.

In fact , the methods whose behavior is same regardless of the state of the objects are marked static.

In a nutshell , remember the following points

1) Members means Methods and Instance variables.
2) Non-static Methods can use non-static members. (Ofcourse)
3) Non-static Methods can use static members.
4) Static Methods CANNOT use non-static members.
5) Static Methods can use only static members.

Thanks !!!

[KrishnaPrasad raghavan]

Moguluri Ravi Kiran wrote

the static methods can be regarded as methods that are outside the class (something similar to main() of c/c++) even though they are inside a class.
so we can not access instance members in the static contexts.

I think you are confusing the issue here.

Static method belongs to a class.