Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 339

tianCount to be static , making martianCount classwide data. Every Martian can see the
martianCount as if it were an instance variable of class Martian , but only one copy of the
static martianCount is maintained. This saves space. We save time by having the Mar-
tian constructor increment the static martianCount —there's only one copy, so we do
not have to increment separate copies for each Martian object.
Software Engineering Observation 8.9
Use a static variable when all objects of a class must use the same copy of the variable.
Class Scope
Static variables have class scope —they can be used in all of the class's methods. We can ac-
cess a class's public static members through a reference to any object of the class, or by
qualifying the member name with the class name and a dot ( . ), as in Math.random() . A
class's private static class members can be accessed by client code only through methods
of the class. Actually, static class members exist even when no objects of the class exist —
they're available as soon as the class is loaded into memory at execution time. To access a
public static member when no objects of the class exist (and even when they do), prefix
the class name and a dot ( . ) to the static member, as in Math.PI . To access a private
static member when no objects of the class exist, provide a public static method and
call it by qualifying its name with the class name and a dot.
Software Engineering Observation 8.10
Static class variables and methods exist, and can be used, even if no objects of that class
have been instantiated.
static Methods Cannot Directly Access Instance Variables and Instance Methods
A static method cannot access a class's instance variables and instance methods, because
a static method can be called even when no objects of the class have been instantiated.
For the same reason, the this reference cannot be used in a static method. The this ref-
erence must refer to a specific object of the class, and when a static method is called, there
might not be any objects of its class in memory.
Common Programming Error 8.5
A compilation error occurs if a static method calls an instance method in the same class
by using only the method name. Similarly, a compilation error occurs if a static method
attempts to access an instance variable in the same class by using only the variable name.
Common Programming Error 8.6
Referring to this in a static method is a compilation error.
Tracking the Number of Employee Objects That Have Been Created
Our next program declares two classes— Employee (Fig. 8.12) and EmployeeTest
(Fig. 8.13). Class Employee declares private static variable count (Fig. 8.12, line 7) and
public static method getCount (lines 36-39). The static variable count maintains a
count of the number of objects of class Employee that have been created so far. This class