— Scott McNealy, Sun Microsystems Chairman, President and ceo

of duck is “ + size);
^
If there’s a Duck on 
the heap somewhere, we 
don’t know about it.

---

static 
methods
278
 
chapter 10
Static methods can’t use non-static 
methods, either!
What do non-static methods do? They usually use instance 
variable state to affect the behavior of the method.
A getName() 
method returns the value of the name variable. Whose name? 
The object used to invoke the getName() method.
public class Duck {
private int size;
public static void main (String[] args) {
System.out.println(“Size is “ + getSize());
}
public void setSize(int s) {
size = s;
}
public int getSize() {
return size;
}
}

This won’t compile:

Calling getSize() just postpones 
the inevitable—getSize() uses 
the size instance variable.
File Edit Window Help Jack-in
% javac Duck.java
Duck.java:6: non-static method 
getSize() cannot be referenced 
from a static context
System.out.println(“Size 
of duck is “ + getSize());
^
Back to the same problem... 
whose size?
there are no
Dumb Questions
Q: 
What if you try to call a non-static 
method from a static method, but the 
non-static method doesn’t use any in-
stance variables. Will the compiler allow 
that?
A: 
No. The compiler knows that 
whether you do or do not use instance 
variables in a non-static method, you can. 
And think about the implications... if you 
were allowed to compile a scenario like 
that, then what happens if in the future 
you want to change the implementation 
of that non-static method so that one day 
it does use an instance variable? Or worse, 
what happens if a subclass overrides the 
method and uses an instance variable in 
the overriding version?
Q: