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3.6. Abstract Classes and Methods

In Example 3-4, we declared our Circle class to be part of a package named shapes. Suppose we plan to implement a number of shape classes: Rectangle, Square, Ellipse, Triangle, and so on. We can give these shape classes our two basic area() and circumference() methods. Now, to make it easy to work with an array of shapes, it would be helpful if all our shape classes had a common superclass, Shape. If we structure our class hierarchy this way, every shape object, regardless of the actual type of shape it represents, can be assigned to variables, fields, or array elements of type Shape. We want the Shape class to encapsulate whatever features all our shapes have in common (e.g., the area() and circumference() methods). But our generic Shape class doesn't represent any real kind of shape, so it cannot define useful implementations of the methods. Java handles this situation with abstract methods.

Java lets us define a method without implementing it by declaring the method with the abstract modifier. An abstract method has no body; it simply has a signature definition followed by a semicolon.[7] Here are the rules about abstract methods and the abstract classes that contain them:

[7] An abstract method in Java is something like a pure virtual function in C++ (i.e., a virtual function that is declared = 0). In C++, a class that contains a pure virtual function is called an abstract class and cannot be instantiated. The same is true of Java classes that contain abstract methods.

There is an important feature of the rules of abstract methods. If we define the Shape class to have abstractarea() and circumference() methods, any subclass of Shape is required to provide implementations of these methods so it can be instantiated. In other words, every Shape object is guaranteed to have implementations of these methods defined. Example 3-5 shows how this might work. It defines an abstractShape class and two concrete subclasses of it.

Example 3-5. An Abstract Class and Concrete Subclasses

public abstract class Shape {
  public abstract double area();            // Abstract methods: note
  public abstract double circumference();   // semicolon instead of body. 

class Circle extends Shape {
  public static final double PI = 3.14159265358979323846;
  protected double r;                              // Instance data
  public Circle(double r) { this.r = r; }          // Constructor
  public double getRadius() { return r; }          // Accessor
  public double area() { return PI*r*r; }          // Implementations of
  public double circumference() { return 2*PI*r; } // abstract methods. 

class Rectangle extends Shape {
  protected double w, h;                               // Instance data
  public Rectangle(double w, double h) {               // Constructor
    this.w = w;  this.h = h; 
  public double getWidth() { return w; }               // Accessor method
  public double getHeight() { return h; }              // Another accessor
  public double area() { return w*h; }                 // Implementations of
  public double circumference() { return 2*(w + h); }  // abstract methods.

Each abstract method in Shape has a semicolon right after its parentheses. There are no curly braces, and no method body is defined. Using the classes defined in Example 3-5, we can now write code like this:

Shape[] shapes = new Shape[3];          // Create an array to hold shapes
shapes[0] = new Circle(2.0);            // Fill in the array
shapes[1] = new Rectangle(1.0, 3.0);
shapes[2] = new Rectangle(4.0, 2.0);

double total_area = 0;
for(int i = 0; i < shapes.length; i++)
    total_area += shapes[i].area();     // Compute the area of the shapes

There are two important points to notice here:

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