Static Class Members

• Sometimes only 1 copy of a particular variable should be shared by all objects of a class.
• A static variable is used for this reason (and others).
• A static variable represents "class-wide" information.
• Static members have class scope:
  • A class's public static class members can be accessed via:
    • any object of that class
    • the class name, using the dot notation (e.g., Integer.parseInt is a public static method of the Integer class).
  • A class's private static class members can be accessed via members of the class.
• Static class members exist even when no objects of that class exist (e.g., Integer.parseInt).
• To access a public or friendly static class member when no objects exist,

  prefix the class name and the dot operator to the class member (instance variable or method).

• To access a private static class member when no objects exist,

  a public or friendly static method must be used, and must be invoked by prefixing its name with the class name and the dot operator (e.g., Integer.parseInt() ).

• In the applet below, there may be many employee objects, but only 1 instance variable, count, which holds the number of employee objects currently existing.
• It is initialized to 0 by default.
• Since it is private, it must be accessed via a method.
• Constructors increment count; the finalizer decrements count.
• count can always be accessed via the expression: Employee.getCount().
• To illustrate the finalizer method, the applet removes the employees via the statements:

  e1 = null;
  e2 = null;

  to force a garbage collection, the garbage collection method is invoked explicitly:

  System.gc();

• The System output follows:

  Employee object constructor: Susan Baker
  Employee object constructor: Bob Jones
  Employee object finalizer: Susan Baker
  Employee object finalizer: Bob Jones

• A static method cannot access non-static members. Why?
• A static method has no this reference. Why?