Introduction
Class Definition
Data Fields
Methods
Constructors
Instantiation
  Demo 1
  Demo 2
Static Members
  Demo 3
Value&Reference
  Demo 4
Overloading
   Demo 5
Wrappers 
  Demo 6
Autobox/Unbox
   Demo 7
Arrays
  Demo 8
Exceptions
Exercises

In the course of program development a need will often arise for utility methods and constants of various sorts. Object creation uses up memory resources so it is desirable to obtain access to such resources without creating new objects.

We have seen, though, that Java does everything within classes. There are no global variables, for example, as in C/C++ outside of the class definitions.

To provide for such general resources, Java offers static (also called class) data and methods that are contained in a class definition but exist and can be accessed without creating an instance of the class.

Note: We refer to non-static data as instance data since the values belong to a unique instance of the class.

For example, in this class the variable pi is declared static.

The pi data exists and we can access it even when no instance of MyMathStuff is created.

We can access the data directly using the name of the class, as in:

  double x = 2.0 * MyMathStuff.pi;

We could also define static methods that we can call without instantiating the class. For example,

  double x = MyMathStuff.timesPI(2.3);

A static variable or method is also called a class variable or method, since it belongs to the class itself rather than to an instance of that class.

If a class property is also declared final it is referred to as a constant since it cannot be altered:

This, in fact, is how the pi value Math.PI is defined in the Math class. By convention, the names of constants are in uppercase.

The Math class also contains static methods to carry out various mathematical functions without creating a Math object.

Note that class methods can only refer to static data and to the data passed in the argument list. For example, the following class contains a mix of static and non-static data and methods. The static methods cannot refer to the non-static data members of the class because values for those only exist for instances of the class.

           Tech
OOP in Tech Apps
Complex Number
Histogram Class
  Demo
More Wrappers

           Physics
OOP in Physics
Particle Class
Root Finding
  Demo 1
Newton Methods
  Demo 2
Exercises