Java Polymorphism

"The Java interface feature of the Java programming language enables the flexibility and agility of Java application development. In this hands-on lab, you are going to explore the concept of Java Interface and abstract class. Generally, the ability to appear in many forms. In object-oriented programming, polymorphism refers to a programming language's ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes. For example, given a base class shape, polymorphism enables the programmer to define different area methods for any number of derived classes, such as circles, rectangles and triangles. No matter what shape an object is, applying the area method to it will return the correct results. Polymorphism is considered to be a requirement of any true object-oriented programming language (OOPL)." Quoted from Webopedia.

In this hands-on lab, you are going to explore polymorphic behaviors in Java programs.

Resources

Download the compressed files projects and unzip in the directory <LAB_UNZIPPED_DIRECTORY>
Java Tip 30: Polymorphism and Java article in javaworld.com
The power of polymorphism article from www2.sys-con.com
The Essence of OOP Using Java, Polymorphism based on overloaded methods article from developer.com

Lab Exercises

Exercise 1: Polymorphic behavior via method overriding
Exercise 2: Polymorphic behavior via abstract class
Exercise 3: Polymorphic behavior via Java interface
Homework

Exercise 1: Polymorphic behavior via method overriding

In this exercise, you are going build MyOnlineShop project first and then add ChildrenBook and Cartoon subclasses of the Book class.

Build and run MyOnlineShop NetBeans project
Add ChildrenBook and Cartoon subclasses

(1.1) Build and run MyOnlineShop NetBeans project

The MyOnlineShop project is the same project that you have built and run as part of Java Inheritance hands-on lab. Here we are going to build it again just to get a sense of how it works.

1. Open MyOnlineShop NetBeans project.

Select File->Open Project (Ctrl+Shift+O). (Figure-1.10 below)

Figure-1.10: Open NetBeans project

Observe that the Open Project dialog box appears.
Browse down to <LAB_UNZIPPED_DIRECTORY>/lab_polymorphism/samples directory.
Select MyOnlineShop.
Click Open Project. (Figure-1.11 below)

Figure-1.11: Open MyOnlineShop project

Observe that the MyOnlineShop project node appears under Projects tab window.

2. Build and run MyOnlineShop project

Right click MyOnlineShop project and select Run.
Observe the result in the Output window of the IDE. (Figure-1.12 below)

Item number 0: Type = myonlineshop.TV, Regular price = 1000.0, Sale price = 800.0
Item number 1: Type = myonlineshop.TV, Regular price = 2000.0, Sale price = 1600.0
Item number 2: Type = myonlineshop.MP3Player, Regular price = 250.0, Sale price = 225.0
Item number 3: Type = myonlineshop.Book, Regular price = 34.0, Sale price = 17.0
Item number 4: Type = myonlineshop.Book, Regular price = 15.0, Sale price = 7.5
totalRegularPrice = 3299.0
totalSalePrice = 2649.5

Figure-1.12: Result

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(1.2) Add ChildrenBook and Cartoon

Suppose you want to add two subclasses of Book class and you will call them, ChildrenBook and Cartoon.

1. Write ChildrenBook.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter ChildrenBook.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated ChildrenBook.java gets displayed in the editor window.
Modify the IDE generated ChildrenBook.java as shown in Code-1.20 below.

package myonlineshop;

public class ChildrenBook extends Book{

    int age; // age this book is written for

    /** Creates a new instance of ChildrenBook */
    public ChildrenBook(double regularPrice,
            String publisher,
            int yearPublished,
            int age) {
        super(100, "Sun press", 2002);
        this.age = age;
    }

    // Override this method
    public double computeSalePrice(){
        return super.getRegularPrice() * 0.3;
    }

}

Code-1.20: ChildrenBook.java

2. Write Cartoon.java. The Electronics class itself is an abstract class because it does not provide implementation of the computeSalePrice() abstract method.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter Cartoon.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated Cartoon.java gets displayed in the editor window.
Modify the IDE generated Cartoon.java as shown in Code-1.21 below.

package myonlineshop;

public class Cartoon extends Book {

    String characterName;

    /** Creates a new instance of Cartoon */
    public Cartoon(double regularPrice,
            String publisher,
            int yearPublished,
            String characterName) {
        super(150, "Sun press", 1978);
        this.characterName = characterName;
    }

    // Override this method
    public double computeSalePrice(){
        return super.getRegularPrice() * 0.4;
    }
}

Code-1.21: Cartoon.java

3. Modify Main.java as shown in Code-1.22 below.

package myonlineshop;

public class Main {

    public static void main(String[] args) {

        // Declare and create Product array of size 5
        Product[] pa = new Product[7];

        // Create object instances
        pa[0] = new TV(1000, "Samsung", 30);
        pa[1] = new TV(2000, "Sony", 50);
        pa[2] = new MP3Player(250, "Apple", "blue");
        pa[3] = new Book(34, "Sun press", 1992);
        pa[4] = new Book(15, "Korea press", 1986);
        pa[5] = new ChildrenBook(15, "Pee Wee press", 1987, 8);
        pa[6] = new Cartoon(14, "Pee Wee press", 1924, "Batman");

        // Compute total regular price and total
        // sale price.
        double totalRegularPrice = 0;
        double totalSalePrice = 0;

        for (int i=0; i<pa.length; i++){

            // Call a method of the super class to get
            // the regular price.
            totalRegularPrice += pa[i].getRegularPrice();

            // Since the sale price is computed differently
            // depending on the product type, overriding
            // method of the object instance of the sub-class
            // gets invoked. This is runtime polymorphic
            // behavior.
            totalSalePrice += pa[i].computeSalePrice();

            System.out.println("Item number " + i +
                    ": Type = " + pa[i].getClass().getName() +
                    ", Regular price = " + pa[i].getRegularPrice() +
                    ", Sale price = " + pa[i].computeSalePrice());
        }
        System.out.println("totalRegularPrice = " + totalRegularPrice);
        System.out.println("totalSalePrice = " + totalSalePrice);
    }

}

Code-1.22: Main.java

4. Build and run MyOnlineShop project

Right click MyOnlineShop project and select Run.
Observe the result in the Output window of the IDE. (Figure-1.23 below)

Figure-1.23: Result

return to top of the exercise

Summary

In this exercise, you built MyOnlineShop project and exercised the polymorphic behavior through method overriding.

return to the top

Exercise 2: Polymorphic behavior via Abstract class

In this exercise, you are going build MyOnlineShopUsingAbstractClass project first and then add ChildrenBook and Cartoon subclasses of the Book class.

Build and run MyOnlineUsingAbstractClass NetBeans project.
Add ChildrenBook and Cartoon subclasses

(2.1) Build and run MyOnlineShop NetBeans project

The MyOnlineShopUsingAbstractClass project is the same project that you have built and run as part of Abstract Class and Java Interface hands-on lab. Here we are going to build it again just to get a sense of how it works.

1. Open MyOnlineShopUsingAbstractClass NetBeans project.

Select File->Open Project (Ctrl+Shift+O).
Observe that the Open Project dialog box appears.
Browse down to <LAB_UNZIPPED_DIRECTORY>/lab_polymorphism/samples directory.
Select MyOnlineShopUsingAbstractClass.
Click Open Project.
Observe that the MyOnlineShopUsingAbstractClass project node appears under Projects tab window.

2. Build and run MyOnlineShopUsingAbstractClass project

Right click MyOnlineShopUsingAbstractClass project and select Run.
Observe the result in the Output window of the IDE. (Figure-2.12 below)

Figure-2.12: Result

return to top of the exercise

(2.2) Add ChildrenBook and Cartoon

Suppose you want to add two subclasses of Book class and you will call them, ChildrenBook and Cartoon.

1. Write ChildrenBook.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter ChildrenBook.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated ChildrenBook.java gets displayed in the editor window.
Modify the IDE generated ChildrenBook.java as shown in Code-1.20 below.

Code-2.21: ChildrenBook.java

2. Write Cartoon.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter Cartoon.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated Cartoon.java gets displayed in the editor window.
Modify the IDE generated Cartoon.java as shown in Code-1.20 below.

Code-2.21: Cartoon.java

4. Modify Main.java as shown in Code-2.24 below.

package myonlineshop;

public class Main {

    public static void main(String[] args) {

        // Declare and create Product array of size 5
        Product[] pa = new Product[7];

        // Create object instances and assign them to
        // the type of Product.
        pa[0] = new TV(1000, "Samsung", 30);
        pa[1] = new TV(2000, "Sony", 50);
        pa[2] = new MP3Player(250, "Apple", "blue");
        pa[3] = new Book(34, "Sun press", 1992);
        pa[4] = new Book(15, "Korea press", 1986);
        pa[5] = new ChildrenBook(15, "Pee Wee press", 1987, 8);
        pa[6] = new Cartoon(14, "Pee Wee press", 1924, "Batman");

        // Compute total regular price and total
        // sale price.
        double totalRegularPrice = 0;
        double totalSalePrice = 0;

        for (int i=0; i<pa.length; i++){

            // Call a method of the super class to get
            // the regular price.
            totalRegularPrice += pa[i].getRegularPrice();

            // Since the sale price is computed differently
            // depending on the product type, overriding (implementation)
            // method of the object instance of the sub-class
            // gets invoked. This is runtime polymorphic
            // behavior.
            totalSalePrice += pa[i].computeSalePrice();

            System.out.println("Item number " + i +
                    ": Type = " + pa[i].getClass().getName() +
                    ", Regular price = " + pa[i].getRegularPrice() +
                    ", Sale price = " + pa[i].computeSalePrice());
        }
        System.out.println("totalRegularPrice = " + totalRegularPrice);
        System.out.println("totalSalePrice = " + totalSalePrice);
    }

}

Code-2.24: Main.java

5. Build and run MyOnlineShopUsingAbstractClass project

Right click MyOnlineShopUsingAbstractClass project and select Run.
Observe the result in the Output window of the IDE. (Figure-2.25 below)

Figure-2.25: Result

return to top of the exercise

Summary

In this exercise, you built MyOnlineShopUsingAbstractClass project and exercised the polymorphic behavior through implementing (or overriding) abstract methods of an abstract class.

Return to the top

Exercise 3: Polymorphic behavior via Java interface

In this exercise, you are going build MyOnlineShopUsingInterface project first and then add ChildrenBook and Cartoon subclasses of the Book class.

Build and run MyOnlineUsingInterface NetBeans project
Add ChildrenBook and Cartoon subclasses

(3.1) Build and run MyOnlineUsingInterface NetBeans Project

The MyOnlineShopUsingInterface project is the same project that you have built and run as part of Abstract Class and Java Interface hands-on lab. Here we are going to build it again just to get a sense of how it works.

1. Open MyOnlineShopUsingInterface NetBeans project.

Select File->Open Project (Ctrl+Shift+O).
Observe that the Open Project dialog box appears.
Browse down to <LAB_UNZIPPED_DIRECTORY>/lab_polymorphism/samples directory.
Select MyOnlineShopUsingInterface.
Click Open Project.
Observe that the MyOnlineShopUsingInterface project node appears under Projects tab window.

2. Build and run MyOnlineShopUsingInterface project

Right click MyOnlineShopUsingInterface project and select Run.
Observe the result in the Output window of the IDE. (Figure-3.11 below)

Figure-3.11: Result

(3.2) Add ChildrenBook and Cartoon

Suppose you want to add two subclasses of Book class and you will call them, ChildrenBook and Cartoon.

1. Write BookInterface.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter BookInterface.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated BookInterface.java gets displayed in the editor window.
Modify the IDE generated BookInterface.java as shown in Code-3.20 below.

package myonlineshopusinginterface;

public interface BookInterface extends ProductInterface {
    public String getPublisher();
    public void setPublisher(String publisher);
    public int getYearPublished();
    public void setYearPublished(int yearPublished);
}

Code-3.20: BookInterface.java

2. Modify Book.java as shown in Code-3.21 below.

package myonlineshopusinginterface;

public class Book extends Product implements BookInterface{

    private String publisher;
    private int yearPublished;

    /** Creates a new instance of Book */
    public Book(double regularPrice,
            String publisher,
            int yearPublished) {
        super(regularPrice);
        this.publisher = publisher;
        this.yearPublished = yearPublished;
    }

    // Override the method
    public double computeSalePrice(){
        return super.getRegularPrice() * 0.5;
    }

    public String getPublisher() {
        return publisher;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    public int getYearPublished() {
        return yearPublished;
    }

    public void setYearPublished(int yearPublished) {
        this.yearPublished = yearPublished;
    }

}

Code-3.21: Book.java

3. Write ChildrenBook.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter ChildrenBook.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated ChildrenBook.java gets displayed in the editor window.
Modify the IDE generated ChildrenBook.java as shown in Code-3.22 below.

package myonlineshopusinginterface;

public class ChildrenBook extends Book{

    int age; // age this book is written for

    /** Creates a new instance of ChildrenBook */
    public ChildrenBook(double regularPrice,
            String publisher,
            int yearPublished,
            int age) {
        super(100, "Sun press", 2002);
        this.age = age;
    }

    // Override this method
    public double computeSalePrice(){
        return super.getRegularPrice() * 0.3;
    }

}

Code-3.22: ChildrenBook.java

2. Write Cartoon.java.

Right click MyOnlineShop project node and select New->Java Class.
Observe Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter Cartoon.
For the Package field, either enter myonlineshop or choose myonlineshop from the drop-down menu.
Click Finish.

Observe that IDE generated Cartoon.java gets displayed in the editor window.
Modify the IDE generated Cartoon.java as shown in Code-3.23 below.

package myonlineshopusinginterface;

public class Cartoon extends Book {

    String characterName;

    /** Creates a new instance of Cartoon */
    public Cartoon(double regularPrice,
            String publisher,
            int yearPublished,
            String characterName) {
        super(150, "Sun press", 1978);
        this.characterName = characterName;
    }

    // Override this method
    public double computeSalePrice(){
        return super.getRegularPrice() * 0.4;
    }

}

Code-3.23: Cartoon.java

4. Modify Main.java as shown in Code-3.24 below.

package myonlineshop;

public class Main {

    public static void main(String[] args) {

        // Declare and create Product array of size 5
        Product[] pa = new Product[7];

        // Create object instances and assign them to
        // the type of Product.
        pa[0] = new TV(1000, "Samsung", 30);
        pa[1] = new TV(2000, "Sony", 50);
        pa[2] = new MP3Player(250, "Apple", "blue");
        pa[3] = new Book(34, "Sun press", 1992);
        pa[4] = new Book(15, "Korea press", 1986);
        pa[5] = new ChildrenBook(15, "Pee Wee press", 1987, 8);
        pa[6] = new Cartoon(14, "Pee Wee press", 1924, "Batman");

        // Compute total regular price and total
        // sale price.
        double totalRegularPrice = 0;
        double totalSalePrice = 0;

        for (int i=0; i<pa.length; i++){

            // Call a method of the super class to get
            // the regular price.
            totalRegularPrice += pa[i].getRegularPrice();

            // Since the sale price is computed differently
            // depending on the product type, overriding (implementation)
            // method of the object instance of the sub-class
            // gets invoked. This is runtime polymorphic
            // behavior.
            totalSalePrice += pa[i].computeSalePrice();

            System.out.println("Item number " + i +
                    ": Type = " + pa[i].getClass().getName() +
                    ", Regular price = " + pa[i].getRegularPrice() +
                    ", Sale price = " + pa[i].computeSalePrice());
        }
        System.out.println("totalRegularPrice = " + totalRegularPrice);
        System.out.println("totalSalePrice = " + totalSalePrice);
    }

}

Code-3.24: Main.java

5. Build and run MyOnlineShopUsingInterface project

Right click MyOnlineShopUsingInterface project and select Run. (Figure-3.25 below)
Observe the result in the Output window of the IDE.

Figure-3.25: Result

return to top of the exercise

Summary

In this exercise, you built MyOnlineShopUsingInterface project and exercised the polymorphic behavior through implementing (or overriding) methods of an interface.

return to the top

Homework

1. The homework is to modify the MyOwnAutoShopProject you did before: Java Inheritance homework to use Java interface. Here I am going to call the new project MyOwnAutoShopUsingInterface.

The task you will have to do is something similar to the MyOnlineShopUsingInterface project above.