Practical 06: Classes and Objects (Part 1)

We will now begin to implement classes and declare them as objects in this practical session.

In procedural programming, the idea is to break down problems into required actions to be taken. Here, one may associate solutions as verbs (i.e., to-do actions). Previously, we only dabbled in one class file per task solution. In those classes, we still approached our solutions in a procedural programming manner.

However, in object-oriented programming, one has to decompose problems into objects (i.e., nouns). One must consider what objects are needed in order for the program to function. These objects will contain attributes and methods (i.e., their own functions) to play roles in a program.

Note

Up to this point, you may have ensured that each task is completed within one Java class file. However, going forward for the next few practical exercises, your tasks will necessitate the need to create more than one Java class file per task. If you wish to keep each task work separate rather but not need to create new Java projects each time, consider keeping them in separate packages instead.

Activity: Object-Oriented Thinking

Activity 1: The `Rectangle` Class

Design a class named Rectangle to represent a rectangle. The class should contain:

Two double data fields named width and height which specify the width and height of the rectangle respectively. The default values are 1 for both width and height.
A no-arg constructor that creates a default rectangle.
A constructor that creates a rectangle with the specified width and height.
A method named getArea() that returns the area of the rectangle.
A method named getPerimeter() that returns the perimeter of the rectangle.

Draw the UML diagram for the class and then implement the class. Write a test program that creates two Rectangle objects — one with a width of 4 units and height of 40 units, and the other with a width of 3.5 units and height of 35.9 units. Display the width, height, area, and perimeter of each rectangle in this order.

Rectangle Class (Solution)

Rectangle.java
public class Rectangle {
    double width;
    double height;

    public Rectangle() {
        width = 1;
        height = 1;
    }

    public Rectangle(double newWidth, double newHeight) {
        width = newWidth;
        height = newHeight;
    }

    public double getArea() {
        return width * height;
    }

    public double getPerimeter() {
        return 2 * (width + height);
    }

}

Test Class (Solution)

TestRectangle.java
public class TestRectangle {
    public static void main(String[] args) {
        Rectangle r1 = new Rectangle(4, 40);
        Rectangle r2 = new Rectangle(3.5, 35.9);

        System.out.println("RECTANGLE 1");
        System.out.println("Width: " + r1.width);
        System.out.println("Height: " + r1.height);
        System.out.println("Area: " + r1.getArea());
        System.out.println("Perimeter: " + r1.getPerimeter());

        System.out.println("\nRECTANGLE 2");
        System.out.println("Width: " + r2.width);
        System.out.println("Height: " + r2.height);
        System.out.println("Area: " + r2.getArea());
        System.out.println("Perimeter: " + r2.getPerimeter());
    }
}

OUTPUT:

RECTANGLE 1
Width: 4.0
Height: 40.0
Area: 160.0
Perimeter: 88.0

RECTANGLE 2
Width: 3.5
Height: 35.9
Area: 125.64999999999999
Perimeter: 78.8

Activity 2: The `Stock` Class

Design a class named Stock that contains:

A String data field named symbol for the stock's symbol.
A String data field named name for the stock's name.
A double data field named previousClosingPrice that stores the stock price for the previous day.
A double data field named currentPrice that stores the stock price for the current time.
A constructor that creates a Stock object with the specified symbol and name.
A method named getChangePercent() that returns the percentage changed from previousClosingPrice to currentPrice.

Draw the UML diagram for the class and then implement the class. Write a test program that creates a Stock object with the stock symbol ORCL, the name Oracle Corporation, and the previous closing price of 34.5. Set the new current price to 34.35 and display the price-change percentage.

Tasks

Task 1

Create a class named Household (save as Household.java) that includes a default constructor and two data fields (number of occupants and annual income). The first data field has been created for you in the following code snippet. And then, create a driver program (save as TestHousehold.java) to test the class.

Household.java
public class Household {
   /* attributes here */
   public int numberOfOccupants;
   // declare the annual income attribute here

   /* methods here */
}

TestHousehold.java
public class TestHousehold {
    public static void main(String[] args) {
        // enter stuff here to test your program for this task
    }
}

Sample output for the driver program:

Household 1
***********
Number of occupants: 0
Annual Income: .00

Values have been modified
Number of occupants: 6
Annual Income: 25,000.00

Modify the default constructor for the Household class to set the occupants field to 1 and annual income field to 0. Then, run the driver program again.

Household.java
public class Household {
    /* attributes here */

    /* methods here */

    // default constructor (no-arg constructor)
    public HouseHold() {
        // work here
    }
}

Create an additional overloaded constructor for the Household class. This constructor receives an integer argument and assigns the value to the occupant field. Change any needed statements to the driver program to ensure that the overloaded constructor works correctly. Save and test the changes made.

Household.java
public class Household {
    /* attributes here */

    /* methods here */

    // default constructor (no-arg constructor)
    public HouseHold() {
        /* ... */
    }

    // overloaded constructor
    public Household(int numberOfOccupants) {
        // work here
    }
}

Create another overloaded constructor for the Household class. This constructor receives two arguments, the values of which are assigned to the occupant and income fields respectively. Add any needed statements to the driver program to ensure that the overloaded constructor works correctly. Save and test the changes made.

Household.java
public class Household {
    /* attributes here */

    /* methods here */

    // default constructor (no-arg constructor)
    public HouseHold() {
        /* ... */
    }

    // overloaded constructor #1
    public Household(int numberOfOccupants) {
        /* ... */
    }

    // overloaded constructor #2
    public HouseHold(int numberOfOccupants, double annualIncome) {
        // work here
    }
}

Create a method named calcAverageIncome() (add it to the Household class) to calculate the average income for each household. The formula is as follows:

Average income = annual income / number of occupants

Add any needed statements to the driver program to ensure that the method works correctly. Save and test the changes made.

Household.java
public class Household {
    /* attributes here */

    /* methods here */

    // default constructor (no-arg constructor)
    public HouseHold() {
        /* ... */
    }

    // overloaded constructor #1
    public Household(int numberOfOccupants) {
        /* ... */
    }

    // overloaded constructor #2
    public HouseHold(int numberOfOccupants, double annualIncome) {
        /* ... */
    }

    // calculates average income of household
    public double calcAverageIncome() {
        // work here
    }
}

Task 2

Define a class named Planet, which contains two instance variables that store the name of the planet (name) and the number of days the planet takes to travel around the Sun (travelDays). Write a method printPlanet() which displays the planet's details. In addition, write a test class to test your Planet class.

Sample output for the driver program:

Enter the planet name: Earth
Enter the travel days: 365
-----------------------------
Planet Name: Earth
Travel Days: 365

Add the following constructors to your Planet class:
- A no-arg constructor to initialize the value of name to "Earth" and travelDays to 365.
- A constructor with two parameters.
Modify your test class to test the constructors you have defined.
Add a calculateAge() method to your Planet class which calculates the person's age on that planet. Use the following formula:
```
Age on the planet = (age * 365) / travelDays
```
The calculateAge() method should receive an argument that represents the age on Earth. Modify your test class to obtain user input on their age on Earth and test your calculateAge() method.

Planet Travel Days

Venus 255

Mercury 88

Jupiter 4380

Saturn 10767

Sample output for driver program:
```
Enter your age on Earth: 20
Enter the planet name: Venus
Enter the travel days for Venus: 255
-------------------------------------
Planet Name: Venus
Travel Days: 255
Age on Venus: 28
```

Planet	Travel Days
Venus	255
Mercury	88
Jupiter	4380
Saturn	10767