Static Class Variables and Methods, What's This?

• 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.

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

• 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.

• 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:

   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;
      }

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

   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;
      }

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

   // 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." );

• 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.

   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." );
      ...
      ...
   }

• 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.

   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.
      }
   }