listing 1
// A simple class hierarchy. 
 
// A class for two-dimensional objects. 
class TwoDShape { 
  double width; 
  double height; 
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  String style; 
   
  double area() { 
    return width * height / 2; 
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
} 
 
class Shapes { 
  public static void main(String args[]) { 
    Triangle t1 = new Triangle(); 
    Triangle t2 = new Triangle(); 
 
    t1.width = 4.0; 
    t1.height = 4.0; 
    t1.style = "isosceles"; 
 
    t2.width = 8.0; 
    t2.height = 12.0; 
    t2.style = "right"; 
 
    System.out.println("Info for t1: "); 
    t1.showStyle(); 
    t1.showDim(); 
    System.out.println("Area is " + t1.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t2: "); 
    t2.showStyle(); 
    t2.showDim(); 
    System.out.println("Area is " + t2.area()); 
  } 
}

listing 2
// A subclass of TwoDShape for rectangles. 
class Rectangle extends TwoDShape { 
  boolean isSquare() { 
    if(width == height) return true; 
    return false; 
  } 
   
  double area() { 
    return width * height; 
  } 
}

listing 3
// Private members are not inherited.  
 
// This example will not compile. 
 
// A class for two-dimensional objects. 
class TwoDShape { 
  private double width;  // these are 
  private double height; // now private  
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  String style; 
   
  double area() { 
    return width * height / 2; // Error! can't access 
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
}

listing 4
// Use accessor methods to set and get private members. 
 
// A class for two-dimensional objects. 
class TwoDShape { 
  private double width;  // these are 
  private double height; // now private  
 
  // Accessor methods for width and height. 
  double getWidth() { return width; } 
  double getHeight() { return height; } 
  void setWidth(double w) { width = w; } 
  void setHeight(double h) { height = h; } 
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  String style; 
   
  double area() { 
    return getWidth() * getHeight() / 2;  
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
} 
 
class Shapes2 { 
  public static void main(String args[]) { 
    Triangle t1 = new Triangle(); 
    Triangle t2 = new Triangle(); 
 
    t1.setWidth(4.0); 
    t1.setHeight(4.0); 
    t1.style = "isosceles"; 
 
    t2.setWidth(8.0); 
    t2.setHeight(12.0); 
    t2.style = "right"; 
 
    System.out.println("Info for t1: "); 
    t1.showStyle(); 
    t1.showDim(); 
    System.out.println("Area is " + t1.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t2: "); 
    t2.showStyle(); 
    t2.showDim(); 
    System.out.println("Area is " + t2.area()); 
  } 
}

listing 5
// Add a constructor to Triangle. 
 
// A class for two-dimensional objects. 
class TwoDShape { 
  private double width;  // these are 
  private double height; // now private  
 
  // Accessor methods for width and height. 
  double getWidth() { return width; } 
  double getHeight() { return height; } 
  void setWidth(double w) { width = w; } 
  void setHeight(double h) { height = h; } 
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  private String style; 
   
  // Constructor 
  Triangle(String s, double w, double h) { 
    setWidth(w); 
    setHeight(h); 
 
    style = s;  
  } 
 
  double area() { 
    return getWidth() * getHeight() / 2;  
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
} 
 
class Shapes3 { 
  public static void main(String args[]) { 
    Triangle t1 = new Triangle("isosceles", 4.0, 4.0); 
    Triangle t2 = new Triangle("right", 8.0, 12.0); 
 
    System.out.println("Info for t1: "); 
    t1.showStyle(); 
    t1.showDim(); 
    System.out.println("Area is " + t1.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t2: "); 
    t2.showStyle(); 
    t2.showDim(); 
    System.out.println("Area is " + t2.area()); 
  } 
}

listing 6
// Add constructors to TwoDShape. 
class TwoDShape { 
  private double width; 
  private double height; 
 
  // Parameterized constructor. 
  TwoDShape(double w, double h) { 
    width = w; 
    height = h; 
  } 
 
  // Accessor methods for width and height. 
  double getWidth() { return width; } 
  double getHeight() { return height; } 
  void setWidth(double w) { width = w; } 
  void setHeight(double h) { height = h; } 
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  private String style; 
   
  Triangle(String s, double w, double h) { 
    super(w, h); // call superclass constructor 
 
    style = s;  
  } 
 
  double area() { 
    return getWidth() * getHeight() / 2; 
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
} 
 
class Shapes4 { 
  public static void main(String args[]) { 
    Triangle t1 = new Triangle("isosceles", 4.0, 4.0); 
    Triangle t2 = new Triangle("right", 8.0, 12.0); 
 
    System.out.println("Info for t1: "); 
    t1.showStyle(); 
    t1.showDim(); 
    System.out.println("Area is " + t1.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t2: "); 
    t2.showStyle(); 
    t2.showDim(); 
    System.out.println("Area is " + t2.area()); 
  } 
}

listing 7
// Add more constructors to TwoDShape. 
class TwoDShape { 
  private double width; 
  private double height; 
 
  // A default constructor. 
  TwoDShape() { 
    width = height = 0.0; 
  } 
 
  // Parameterized constructor. 
  TwoDShape(double w, double h) { 
    width = w; 
    height = h; 
  } 
 
  // Construct object with equal width and height. 
  TwoDShape(double x) { 
    width = height = x; 
  } 
 
  // Accessor methods for width and height. 
  double getWidth() { return width; } 
  double getHeight() { return height; } 
  void setWidth(double w) { width = w; } 
  void setHeight(double h) { height = h; } 
 
  void showDim() { 
    System.out.println("Width and height are " + 
                       width + " and " + height); 
  } 
} 
 
// A subclass of TwoDShape for triangles. 
class Triangle extends TwoDShape { 
  private String style; 
   
  // A default constructor. 
  Triangle() { 
    super(); 
    style = "null"; 
  } 
 
  // Constructor 
  Triangle(String s, double w, double h) { 
    super(w, h); // call superclass constructor 
 
    style = s;  
  } 
 
  // Construct an isosceles triangle. 
  Triangle(double x) { 
    super(x); // call superclass constructor 
 
    style = "isosceles";  
  } 
 
  double area() { 
    return getWidth() * getHeight() / 2; 
  } 
 
  void showStyle() { 
    System.out.println("Triangle is " + style); 
  } 
} 
 
class Shapes5 { 
  public static void main(String args[]) { 
    Triangle t1 = new Triangle(); 
    Triangle t2 = new Triangle("right", 8.0, 12.0); 
    Triangle t3 = new Triangle(4.0); 
 
    t1 = t2; 
 
    System.out.println("Info for t1: "); 
    t1.showStyle(); 
    t1.showDim(); 
    System.out.println("Area is " + t1.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t2: "); 
    t2.showStyle(); 
    t2.showDim(); 
    System.out.println("Area is " + t2.area()); 
 
    System.out.println(); 
 
    System.out.println("Info for t3: "); 
    t3.showStyle(); 
    t3.showDim(); 
    System.out.println("Area is " + t3.area()); 
 
    System.out.println(); 
  } 
}

listing 8
// Using super to overcome name hiding. 
class A { 
  int i; 
} 
 
// Create a subclass by extending class A. 
class B extends A { 
  int i; // this i hides the i in A 
 
  B(int a, int b) { 
    super.i = a; // i in A 
    i = b; // i in B 
  } 
 
  void show() { 
    System.out.println("i in superclass: " + super.i); 
    System.out.println("i in subclass: " + i); 
  } 
} 
 
class UseSuper { 
  public static void main(String args[]) { 
    B subOb = new B(1, 2); 
 
    subOb.show(); 
  } 
}

listing 9