2.10 Java1620: static members  (Page 3/12)

Condo is analogous to an object

Each condo unit is analogous to an ordinary Java object .

Each set of blueprints delivered to the construction site is roughly analogous to an object of the class named Class . In other words, each set of blueprints describes one or more condo unitsconstructed from that set of blueprints.

When construction is complete

When the construction project is complete, the contractor delivers a set of blueprints for each type of condo unit to the management firm that has beenhired to manage the condo complex. Each set of blueprints continues to be analogous to an object of the class named Class . The blueprints remain at the site of the condo units.

RTTI

Thus, information regarding the construction, wiring, plumbing, air conditioning, etc., for each condo unit (object) continues to be available at the site even after the construction has been completed. (This is somewhat analogous to something called runtime type information and oftenabbreviated as RTTI. A Class object contains RTTI for objects instantiated from that class.)

What are those analogies again?

In the above scenario, each condo unit is (roughly) analogous to an object instantiated from a specific class (set of blueprints).

Each set of blueprints remaining onsite after construction is complete is roughly analogous to a Class object that describes the characteristics of one or more condo units.

What do you care?

Until you get involved in such advanced topics as reflection and introspection , you don't usually have much involvement or much interest in Class objects. They are created automatically, and are primarily used by the Java virtual machine during runtime to help it do thethings that it needs to do.

An exception to that rule

However, there is one area where you will be interested in the use of these Class objects from early on. You will be interested whenever variables or methods in the class definition are declared to be static .

Class variables and class methods

According to the current jargon, declaring a member variable to be static causes it to be a class variable . (Note that local variables cannot be declared static . Only member variables can be declared static .) Similarly, declaring a method to be static causes it to be a class method.

Instance variables and methods

On the other hand, according to the current jargon, not declaring a variable to be static causes it to be an instance variable , and not declaring a method to be static causes it to be an instance method .

In general, we can refer to them as class members and instance members .

What is the difference?

Here are some of the differences between class and instance members insofar as this discussion is concerned.

How many copies of member variables exist?

Every object instantiated from a given class has its own copy of each instance variable defined in the class. (Instance variables are not shared among objects.) However, every object instantiated from a given class shares the same copy of each class variable defined in the class. (It is as though the class variable belongs to the single Class object and not to the individual objects instantiated from that class.)