Java static reference variable invoking non-static methods

Question

class A{
 void display(){
 System.out.println("hai");
 }
}
class B {
 static A a;
}
class C{
 public static void main(String args[])
 {
 B.a.display();
 // no compile- time error here.why?
 }
}

Also I know that a will be set to null during runtime. But shouldnt the compiler know that we are accessing a non-static method via a static reference variable? This gives a null pointer exception when executed but why is the compiler not giving an error.

Does this mean a static reference variable behaves exactly like an object reference and thus can invoke any method(static and non-static) of the class?

Edit: I am basically confused with static field's access rules. By definition static fields can only directly access other static fields. So does this not include "invoking" a method using a static reference variable? And display() is accessed before its object is created. Is this valid?

• shmosel: I don't understand the question. If a references an instance of A, why shouldn't you be able to call an instance method?
• n247s: When in separate classes, this will work (why not? ). But when these classes are subclasses (non-static subclasses), you wouldn't be able to declare a static variable in the non-static class in the first place. My guese is that this mechanism is confusing you atm?
• user10435687: So static reference variables can "invoke" any method. They only cant directly "access" non-static right? I assumed we cant invoke non-static methods as the variable is static.
• markspace: No, a static variable (reference) could invoke any member of the class. It's just a normal reference, like any other reference. So an example for us so we can see what it is you are talking about.
• markspace: There's a couple of things you might be confused over. Local variables have to be definitely initialized, and the compiler can tell if they've been initialized because they're local. You'll get a compiler error if you try to use a local variable that hasn't been initialized yet. The other is referring to an instance member directly (not through a reference) from a static method (or other static context). Those are also definitely errors, and the compiler will flag them.

Answer 1

"You keep using that word. I do not think it means what you think it means."

The static A a; in your code says, "a is a (reference to) an object of type A that is shared across all instances of class B. a is uninitialized.".

The compiler will have no context to determine when you are going to call the static main() method of class C, so it can't tell what the state of variable a will be at the time you call C.main();. In fact, you could just as easily have a class D:

class D
{
 public static void main( String args[] )
 {
 B.a = new A();
 C.main( args );
 }
}

which would make the code above completely valid and runnable.

• user10435687: Yes I added the extends A by mistake . Corrected my question