Inheritance and Polymorphism

Objectives

u      To develop a subclass from a superclass through inheritance

u      To invoke the superclass’s constructors and methods using the super keyword

u      To override methods in the subclass

u      To distinguish differences between overriding and overloading

u      To comprehend polymorphism, dynamic binding, and generic programming

u      To describe casting and explain why explicit downcasting is necessary

u      To store, retrieve, and manipulates objects in an ArrayList

 

Superclasses and Subclasses

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Are superclass’s Constructor Inherited?

No. They are not inherited.

They are invoked explicitly or implicitly.

Explicitly using the super keyword.

 

A constructor is used to construct an instance of a class. Unlike properties and methods, a superclass's constructors are not inherited in the subclass. They can only be invoked from the subclasses' constructors, using the keyword super. If the keyword super is not explicitly used, the superclass's no-arg constructor is automatically invoked.

 

Superclass’s Constructor Is Always Invoked

A constructor may invoke an overloaded constructor or its superclass’s constructor. If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor.

 

Using the Keyword super

The keyword super refers to the superclass of the class in which super appears. This keyword can be used in two ways:

·        To call a superclass constructor

·        To call a superclass method

 

Caution:

You must use the keyword super to call the superclass constructor. Invoking a superclass constructor’s name in a subclass causes a syntax error. Java requires that the statement that uses the keyword super appear first in the constructor.

 

Declaring a Subclass

A subclass extends properties and methods from the superclass. You can also:

·        Add new properties

·        Add new methods

·        Override the methods of the superclass

 

Constructor Chaining

Constructing an instance of a class invokes all the superclasses’ constructors along the inheritance chain. This is called constructor chaining.

 

public class Faculty extends Employee {

  public static void main(String[] args) {

    new Faculty();

  }

 

  public Faculty() {

    System.out.println("(4) Faculty's no-arg constructor is invoked");

  }

}

 

class Employee extends Person {

  public Employee() {

    this("(2) Invoke Employee’s overloaded constructor");

    System.out.println("(3) Employee's no-arg constructor is invoked");

  }

 

  public Employee(String s) {

    System.out.println(s);

  }

}

 

class Person {

  public Person() {

    System.out.println("(1) Person's no-arg constructor is invoked");

  }

}

 

Calling Superclass Methods

You could rewrite the printCircle() method in the Circle class as follows:

 

public void printCircle() {

  System.out.println("The circle is created " +

    super.getDateCreated() + " and the radius is " + radius);

}

 

Overriding Methods in the Superclass

A subclass inherits methods from a superclass. Sometimes it is necessary for the subclass to modify the implementation of a method defined in the superclass. This is referred to as method overriding

 

public class Circle extends GeometricObject {

  // Other methods are omitted

 

  /** Override the toString method defined in GeometricObject */

  public String toString() {

    return super.toString() + "\nradius is " + radius;

  }

}

 

An instance method can be overridden only if it is accessible. Thus a private method cannot be overridden, because it is not accessible outside its own class. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.

 

Like an instance method, a static method can be inherited. However, a static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.

 

Overriding vs. Overloading

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The Object Class

Every class in Java is descended from the java.lang.Object class. If no inheritance is specified when a class is defined, the superclass of the class is Object.

 

The toString() method in Object

The toString() method returns a string representation of the object. The default implementation returns a string consisting of a class name of which the object is an instance, the at sign (@), and a number representing this object.

 

Polymorphism, Dynamic Binding and Generic Programming

 

package chapter9;
 
public class PolymorphismDemo {
  public static void main(String[] args) {
    m(new GraduateStudent());
    m(new Student());
    m(new Person());
    m(new Object());
  }
 
  public static void m(Object x) {
    System.out.println(x.toString());
  }
}
 
class GraduateStudent extends Student {
}
 
class Student extends Person {
  public String toString() {
    return "Student";
  }
}
 
class Person extends Object {
  public String toString() {
    return "Person";
  }
}

 

When the method m(Object x) is executed, the argument x’s toString method is invoked. x may be an instance of GraduateStudent, Student, Person, or Object. Classes GraduateStudent, Student, Person, and Object have their own implementation of the toString method. Which implementation is used will be determined dynamically by the Java Virtual Machine at runtime. This capability is known as dynamic binding.

 

 

Dynamic Binding

Dynamic binding works as follows: Suppose an object o is an instance of classes C1, C2, ..., Cn-1, and Cn, where C1 is a subclass of C2, C2 is a subclass of C3, ..., and Cn-1 is a subclass of Cn. That is, Cn is the most general class, and C1 is the most specific class. In Java, Cn is the Object class. If o invokes a method p, the JVM searches the implementation for the method p in C1, C2, ..., Cn-1 and Cn, in this order, until it is found. Once an implementation is found, the search stops and the first-found implementation is invoked.

 

Method Matching vs. Binding

Matching a method signature and binding a method implementation are two issues. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compilation time. A method may be implemented in several subclasses. The Java Virtual Machine dynamically binds the implementation of the method at runtime.

 

 

Generic Programming

Polymorphism allows methods to be used generically for a wide range of object arguments. This is known as generic programming. If a method’s parameter type is a superclass (e.g., Object), you may pass an object to this method of any of the parameter’s subclasses (e.g., Student or String). When an object (e.g., a Student object or a String object) is used in the method, the particular implementation of the method of the object that is invoked (e.g., toString) is determined dynamically.

 

Casting Objects

You have already used the casting operator to convert variables of one primitive type to another. Casting can also be used to convert an object of one class type to another within an inheritance hierarchy. In the preceding section, the statement

m(new Student());

 

assigns the object new Student() to a parameter of the Object type. This statement is equivalent to:

 

Object o = new Student(); // Implicit casting

m(o);

 

 

Why Casting Is Necessary?

Suppose you want to assign the object reference o to a variable of the Student type using the following statement:

 

Student b = o;

 

A compilation error would occur. Why does the statement Object o = new Student() work and the statement Student b = o doesn’t? This is because a Student object is always an instance of Object, but an Object is not necessarily an instance of Student. Even though you can see that o is really a Student object, the compiler is not so clever to know it. To tell the compiler that o is a Student object, use an explicit casting. The syntax is similar to the one used for casting among primitive data types. Enclose the target object type in parentheses and place it before the object to be cast, as follows:

 

Student b = (Student)o; // Explicit casting

 

Casting from
Superclass to Subclass

To help understand casting, you may also consider the analogy of fruit, apple, and orange with the Fruit class as the superclass for Apple and Orange. An apple is a fruit, so you can always safely assign an instance of Apple to a variable for Fruit. However, a fruit is not necessarily an apple, so you have to use explicit casting to assign an instance of Fruit to a variable of Apple.

 

Example: Demonstrating Polymorphism and Casting

This example creates two geometric objects: a circle, and a rectangle, invokes the displayGeometricObject method to display the objects. The displayGeometricObject displays the area and diameter if the object is a circle, and displays area if the object is a rectangle.

 

package chapter9;
 
public class TestPolymorphismCasting {
  /** Main method */
  public static void main(String[] args) {
    // Declare and initialize two objects
    Object object1 = new Circle(1);
    Object object2 = new Rectangle(1, 1);
 
    // Display circle and rectanlge
    displayObject(object1);
    displayObject(object2);
  }
 
  /** A method for displaying an object */
  public static void displayObject(Object object) {
    if (object instanceof Circle) {
      System.out.println("The circle area is " +
        ((Circle)object).getArea());
      System.out.println("The circle diameter is " +
        ((Circle)object).getDiameter());
    }
    else if (object instanceof Rectangle) {
      System.out.println("The rectangle area is " +
        ((Rectangle)object).getArea());
    }
  }
}
 

 

The ArrayList and Vector Classes

You can create an array to store objects. But the array’s size is fixed once the array is created. Java provides the ArrayList class that can be used to store an unlimited number of objects.

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

public class TestArrayList {
  public static void main(String[] args) {
    // Create a list to store cities
    java.util.ArrayList cityList = new java.util.ArrayList();
 
    // Add some cities in the list
    cityList.add("London");
    // cityList now contains [London]
    cityList.add("New York");
    // cityList now contains [London, New York]
    cityList.add("Paris");
    // cityList now contains [London, New York, Paris]
    cityList.add("Toronto");
    // cityList now contains [London, New York, Paris, Toronto]
    cityList.add("Hong Kong");
    // contains [London, New York, Paris, Toronto, Hong Kong]
    cityList.add("Singapore");
    // contains [London, New York, Paris, Toronto,
    //             Hong Kong, Singapore]
 
    System.out.println("List size? " + cityList.size());
    System.out.println("Is Toronto in the list? " +
      cityList.contains("Toronto"));
    System.out.println("The location of New York in the list? "
      + cityList.indexOf("New York"));
    System.out.println("Is the list empty? " +
      cityList.isEmpty()); // Print false
 
    // Insert a new city at index 2
    cityList.add(2, "Beijing");
    // contains [London, New York, Beijing, Paris, Toronto,
    //             Hong Kong, Singapore]
 
    // Remove a city from the list
    cityList.remove("Toronto");
    // contains [London, New York, Beijing, Paris,
    //             Hong Kong, Singapore]
 
    // Remove a city at index 1
    cityList.remove(1);
    // contains [London, Beijing, Paris, Hong Kong, Singapore]
 
    // Display London Beijing Paris Hong Kong Singapore
    for (int i = 0; i < cityList.size(); i++)
      System.out.print(cityList.get(i) + " ");
    System.out.println();
 
    // Create a list to store two circles
    java.util.ArrayList list = new java.util.ArrayList();
 
    // Add two circles
    list.add(new Circle(2));
    list.add(new Circle(3));
 
    // Display the area of the first circle in the list
    System.out.println("The area of the circle? " +
      ((Circle)list.get(0)).getArea());
  }
}