F.10. static Class Members

Every object has its own copy of all the instance variables of the class. In certain cases, only one copy of a particular variable should be shared by all objects of a class. A static field—called a class variable—is used in such cases. A static variable represents classwide information—all objects of the class share the same piece of data. The declaration of a static variable begins with the keyword static.

Let’s motivate static data with an example. Suppose that we have a video game with Martians and other space creatures. Each Martian tends to be brave and willing to attack other space creatures when the Martian is aware that at least four other Martians are present. If fewer than five Martians are present, each of them becomes cowardly. Thus, each Martian needs to know the martianCount. We could endow class Martian with martianCount as an instance variable. If we do this, then every Martian will have a separate copy of the instance variable, and every time we create a new Martian, we’ll have to update the instance variable martianCount in every Martian object. This wastes space with the redundant copies, wastes time in updating the separate copies and is error prone. Instead, we declare martianCount 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 Martian constructor increment the static martianCount—there’s only one copy, so we do not have to increment separate copies for each Martian object.

Static variables have class scope. We can access 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.

A static method cannot access non-static class members, 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 reference 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.