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Practical 06: Classes and Objects

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.

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

   /* methods here */
}
TestHousehold.java
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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

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.

  1. 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
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    public class Household {
    /* attributes here */

    /* methods here */

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

  2. 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
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    public class Household {
    /* attributes here */

    /* methods here */

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

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

  3. 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
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    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
    }
}

  4. 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
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    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

  1. 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.

  2. 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

Task 3

Create a class based on the following class diagram:

Task 3 Class Diagram

Note:

  • Initialize the value of type to “Apartment”, zone to ‘A’, price to 68000.00, numberOfBedrooms to 3 and freehold to false for no-arg constructor.

  • Use the toString() method to print out all house details.

  • Create a driver program for the class you just created to test all the available constructors and methods.

  • Create three house objects.

  • Create an array to store the three house objects.

  • Modify the House class by using the this keyword to refer to the data member(s) and constructor (if possible).