Joe Chung
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====== Static Class Variables and Methods, What's This? ======

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===== Static Class Variables and Methods =====

  * Static class members (Class variables and methods that are declared //static//)
    * Belong to the class itself and are __independent of any one instance (object) of the class.__
      * //Instance// variables belong to //instances// of the class, i.e., objects of the the class.
      * //Instance// methods belong to //instances// of the class.
        * Can't call an //instance// method without creating an instance (object) of the class first.

<code>
   MagicEightBall m8 = new MagicEightBall(); // create MagicEightBall instance (object) m8
   System.out.println("Your answer: " + m8.getAnswer(); // call the getAnswer() instance method of m8
</code>

  * Static variables and methods are //non-instance// variables and methods.
    * If you want all the instances of a class to share data, use //static variables// (or static constants), which are also known as //class variables.//
    * This allows for useful public static constants in Java such as //Math.PI//.
    * //Static methods// can be called without creating an instance (object) of the class.
      * This allows for useful methods in Java such as the static methods of the //Math// class, e.g. //Math.random()//, //Math.sqrt()//, //Math.pow()//, etc.
        * After all, it would be inconvenient to have to create //Math// class objects just to use methods like //random()// and //sqrt()// or constants like //PI//.

==== Static class variables ====

  * A static class variable is added using the //static// keyword.

  * Point/Rectangle/TestRectangle static example:
    * In the example, a static int //pointCount// variable is added to a //Point// class:

<code>
   public class Point {

      private  int   _x;
      private  int   _y;

      // Use static pointCount variable to track number of Point objects created:
      private static int pointCount = 0;

      // Constructors:  static pointCount gets incremented in every constructor:
      public Point( int x, int y ) {
         setX( x );
         setY( y );
         pointCount++;
      }

      public Point() {
         pointCount++;
      }
      ...
      ...
      // Instance methods can access static variables
      public int getPointCount() {
         return pointCount;
      }
</code>


  * Also, a static //rectangleCount// variable is added to a //Rectangle// class that includes the above //Point// class:

<code>
   public class Rectangle {

      private Point _point;
      private int _width;
      private int _height;

      // Use static rectangleCount variable to track number of Rectangle objects
      // created:
      private static int rectangleCount = 0;

      // Constructors: static rectangleCount gets incremented in every
      // constructor:
      public Rectangle( int point_x, int point_y, int width, int height )
      {
         _point = new Point( point_x, point_y );
            // Must call Point class constructor to create _point
         setWidth( width );
         setHeight( height );
         rectangleCount++;
      }
      ...
      ...
      // Instance methods can access static variables
      public int getRectangleCount() {
         return rectangleCount;
      }
</code>

  * In //TestRectangle.java//, the //pointCount// and //rectangleCount// static variables are accessed to show how static class variables work:

<code>
   // Use Point class static pointCount variable:
   System.out.println( box.getPoint().getPointCount() +
                       " points so far." );

   // Use Rectangle class static rectangleCount variable:
   System.out.println( box.getRectangleCount() +
                       " rectangles so far." );

</code> 

==== Static class methods ====

  * When a class contains a //static// method, an instance (object) of the class is not needed to use the method.
    * For example, with a //static// version of the //getPointCount()// method called //sgetPointCount()//), we can access the //pointCount// static variable using //Point.sgetPointCount()//.
      * The "Point" in //Point.sgetPointCount()// is the class name, not the name of a //Point// class object.
    * **Restriction:** With a static method, you can only access static variables of the same class.

<code>
   public class Point {
      ...
      private static int pointCount = 0;
      ...
      ...
      // static version of getPointCount:
      public static int sgetPointCount() {
         return pointCount;
      }
      ...
   }



   public class TestRectangle {
      ...
      ...
      // Use Point class' static sgetPointCount method:
      System.out.println( Point.sgetPointCount() + " points so far." );
      ...
      ...
   }
</code>

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===== What's "this"? =====

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  * The "this" reference variable

  * "this" is the name of a reference variable that an object can use to refer to itself.
    * Can be used in only non-static (instance) methods.

  * Common use of "this" is to overcome "shadowing" in "set" methods:
    * See http://www.leepoint.net/data/variables/60shadow-variables.html.

<code>
   public class Rectangle {
      private int width;
      private int height;

   
      // setHeight method of Rectangle class:
      public void setHeight( int height ) {
         height = height; // ??? Shadowing: Which height is which ???
      }
   }







   public class Rectangle {
      private int width;
      private int height;

   
      // Avoid shadowing using "this":
      public void setHeight( int height ) {
         this.height = height;
      
         // "This" Rectangle's instance variable height
         // is equal to the value of the input parameter
         // variable height.
      }
   }
</code>

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