9.7. Polymorphism, Dynamic Binding, and Generic Programming
9.2. Superclasses and Subclasses
In Java terminology, a class C1 extended from another class C2 is called a subclass , and C2 is called a superclass . A superclass is also referred to as a supertype , a parent class, or a base class , and a subclass as a subtype , a child class , an extended class , or a derived class . A subclass inherits accessible data fields and methods from its superclass, and may also add new data fields and methods .
Consider geometric objects. Suppose you want to design the classes to model geometric objects like circles and rectangles. Geometric objects have many common properties and behaviors. They can be drawn in a certain color , filled or unfilled. Thus a general class GeometricObject can be used to model all geometric objects. This class contains the properties color and filled and their appropriate get and set methods. Assume that this class also contains the dateCreated property and the getDateCreated() and toString() methods. The toString() method returns a string representation for the object. Since a circle is a special type of geometric object, it shares common properties and methods with other geometric objects. Thus it makes sense to define the Circle class that extends the GeometricObject class. Likewise, Rectangle can also be declared as a subclass of GeometricObject . Figure 9.1 shows the relationship among these classes. An arrow pointing to the superclass is used to denote the inheritance relationship between the two classes involved.
Figure 9.1. The GeometricObject class is the superclass for Circle and Rectangle .
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The Circle class inherits all accessible data fields and methods from the GeometricObject class. In addition, it has a new data field, radius , and its associated get and set methods. It also contains the getArea() , getPerimeter() , and getDiameter() methods for returning the area, perimeter, and diameter of the circle.
The Rectangle class inherits all accessible data fields and methods from the GeometricObject class. In addition, it has the data fields width and height , and the associated get and set methods. It also contains the getArea() and getPerimeter() methods for returning the area and perimeter of the rectangle.
Note
| | To avoid naming conflict with improved versions of the GeometricObject , Circle , and Rectangle classes introduced in the next chapter, put classes GeometricObject , Circle , and Rectangle into a package named chapter9 . The package name must be mapped to the file system directory structure, so, place the files GeometricObject.java , Circle.java , and Rectangle.java into c:\book\chapter9 . |
The GeometricObject , Circle , and Rectangle classes are shown in Listings 9.1, 9.2, and 9.3.
Listing 9.1. GeometricObject.java
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1 package chapter9; 2 3 public class GeometricObject { 4 private String color = "white" ; 5 private boolean filled; 6 private java.util.Date dateCreated; 7 8 /** Construct a default geometric object */ 9 public GeometricObject() { 10 dateCreated = new java.util.Date(); 11 } 12 13 /** Return color */ 14 public String getColor() { 15 return color; 16 } 17 18 /** Set a new color */ 19 public void setColor(String color) { 20 this .color = color; 21 } 22 23 /** Return filled. Since filled is boolean, 24 so, the get method name is isFilled */ 25 public boolean isFilled() { 26 return filled; 27 } 28 29 /** Set a new filled */ 30 public void setFilled( boolean filled) { 31 this .filled = filled; 32 } 33 34 /** Get dateCreated */ 35 public java.util.Date getDateCreated() { 36 return dateCreated; 37 } 38 39 /** Return a string representation of this object */ 40 public String toString() { 41 return "created on " + dateCreated + "\ncolor: " + color + 42 " and filled: " + filled; 43 } 44 } |
Listing 9.2. Circle.java
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1 package chapter9; 2 3 public class Circle extends GeometricObject { 4 private double radius; 5 6 public Circle() { 7 } 8 9 public Circle( double radius) { 10 this .radius = radius; 11 } 12 13 /** Return radius */ 14 public double getRadius() { 15 return radius; 16 } 17 18 /** Set a new radius */ 19 public void setRadius( double radius) { 20 this .radius = radius; 21 } 22 23 /** Return area */ 24 public double getArea() { 25 return radius * radius * Math.PI; 26 } 27 28 /** Return diameter */ 29 public double getDiameter() { 30 return 2 * radius; 31 } 32 33 /** Return perimeter */ 34 public double getPerimeter() { 35 return 2 * radius * Math.PI; 36 } 37 38 /* Print the circle info */ 39 public void printCircle() { 40 System.out.println( "The circle is created " + getDateCreated() + 41 " and the radius is " + radius); 42 } 43 } |
Listing 9.3. Rectangle.java
1 package chapter9; 2 3 public class Rectangle extends GeometricObject { 4 private double width; 5 private double height; 6 7 public Rectangle() { 8 } 9 10 public Rectangle( double width, double height) { 11 this. width = width; 12 this. height = height; 13 } 14 15 /** Return width */ 16 public double getWidth() { 17 return width; 18 } 19 20 /** Set a new width */ 21 public void setWidth( double width) { 22 this. width = width; 23 } 24 25 /** Return height */ 26 public double getHeight() { 27 return height; 28 } 29 30 /** Set a new height */ 31 public void setHeight( double height) { 32 this. height = height; 33 } 34 35 /** Return area */ 36 public double getArea() { 37 return width * height; 38 } 39 40 /** Return perimeter */ 41 public double getPerimeter() { 42 return 2 * (width + height); 43 } 44 } |
The classes Circle and Rectangle extend the GeometricObject class. The reserved word extends tells the compiler that these classes extend the GeometricObject class, thus inheriting the methods getColor , setColor , isFilled , setFilled , and toString .
The following code in Listing 9.4 creates objects of Circle and Rectangle , and invokes the methods on these objects.
Listing 9.4. TestCircleRectangle.java
1 package chapter9; 2 3 public class TestCircleRectangle { 4 public static void main(String[] args) { 5 Circle circle = new Circle( 1 ); 6 System.out.println( "A circle " + circle.toString() ); 7 System.out.println(circle.getRadius()); 8 System.out.println( "The radius is " + circle.getRadius() ); 9 System.out.println( "The area is " + circle.getArea() ); 10 System.out.println( "The diameter is " + circle.getDiameter() ); 11 12 Rectangle rectangle = new Rectangle( 2 , 4 ); 13 System.out.println( "\nA rectanlge " + rectangle.toString() ); 14 System.out.println( "The area is " + rectangle.getArea() ); 15 System.out.println( "The perimeter is " + 16 rectangle.getPerimeter() ); 17 } 18 } |
Since radius is 1 , color is white , and filled is false by default, the output is as shown in Figure 9.2. Note that TestCircleRectangle.java is stored in c:\book\chapter9 and the TestCircleRectangle class is in package chapter9 . Thus you have to run it from c:\book using the command java chapter9.TestCircleRectangle with the complete class name.
Figure 9.2. The methods in GeometricObject are inherited in Circle and Rectangle .
Note
| | Contrary to the conventional interpretation, a subclass is not a subset of its superclass. In fact, a subclass usually contains more information and functions than its superclass. |
Caution
| | Inheritance is used to model the is-a relationship. Do not blindly extend a class just for the sake of reusing methods. For example, it makes no sense for a Tree class to extend a Person class. A subclass and its superclass must have the is-a relationship. |
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