Classes and Objects

Classes & Objects

A class is the blueprint for an object. An object is a software bundle of related state and behavior. The software object is often used to model the real world objects that we see everyday such as desks, pets, and so on. Before an object is created, the programmer determine the fields and methods that are necessary, and then creates a class. Hence, a class is a blueprint of the objects that may be created from, so a class is not an object but it can be a description of an object. Each object created from a class is called an instance of the class. We have two types of classes, a private and public class. A private class, the member cannot be accessed by code outside the class. The member can be accessed only by methods that are members of the same class. A public class, the member can be accessed by code inside the class or outside. Now lets see a basic example on how class and object works.

Example

First, we create a class without the main method, then we create another class with the main method in it. The two class should be in the same directory.

Here is the first class:

/*
    Basic class & object ask for a name

*
*/

public class SetName
{
    private String name;

    // Assign newname to name

    public void setName(String newname)
    {
        name = newname;

    }

    // Gets the newname and return it

    public String getName()
    {
        return name;
    }

}

Here is the second class which contains the main method:

/*
    The program demonstrate the SetName class

*
*/

import java.util.*;

public class SetNameDemo
{
    public static void main(String[] args)
    {
        Scanner key = new Scanner(System.in);
        String getname;

        // Creates a SetName object

        SetName sname = new SetName();

        System.out.println("Enter a name: ");
        getname = key.nextLine();

        // Pass getname into the class method setName

        sname.setName(getname);

        /*
            Calls the getName method in SetName class using the object sname
            Then outputs the name entered by the user

        *
        */

        System.out.println("The name you enter is " + sname.getName());

    }
}
 

ClickHere to download firstclass.java 

ClickHere to download secondclass.java 

The above program we created a class without the main, inside the class contains two methods. One method is to set the name and the other method will returns the name. Hence, in other class which contains the main method, we created a object " SetName sname = new SetName()". By creating the object, we can access the SetName class using the object name that we created which is "sname". To access the SetName class, simply we use the object name "sname" followed by a period and the methods we want to access. Now lets see a bit more complicated example.

Here is the first class:

/*
    This is a Rectangle class
    It sets the length and width of the rectangle
    In addition it calculates the area

*
*/

public class Rectangle
{
    private double width;
    private double length;

    // Method that sets the length

    public void setLength(double newlength)
    {
        length = newlength;
    }

    // Method that sets the width

    public void setWidth(double newwidth)
    {
        width = newwidth;
    }

    // Method that returns a rectangle object's length

    public double getLength()
    {
        return length;
    }

    // Method that returns a rectangle object's width

    public double getWidth()
    {
        return width;
    }

    // Method that returns a rectangle object's area

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

}

Here is the second class:

/*
    This program contains the main method
    It demonstrates the Rectangle class

*
*/

import java.util.*;

public class RectangleDemo
{
    public static void main(String[] args)
    {
        Scanner key = new Scanner(System.in);

        // Creates the Rectangle Object

        Rectangle rec = new Rectangle();

        // Get the length from the user

        System.out.print("Enter the length: ");
        double len = key.nextDouble();

        // Get the width from the user

        System.out.print("Enter the width: ");
        double wid = key.nextDouble();

        /*
            Calls the setLength and setWidth methods in the Rectangle class
            Using the object name we created, "rec"
            Pass the len and wid into the methods

        *
        */

        rec.setLength(len);
        rec.setWidth(wid);

        /*
            Outputs the length and width entered by the user
            In addition to that the area will be output

        *
        */

        System.out.println("The length is " + rec.getLength());
        System.out.println("The width is " + rec.getWidth());
        System.out.println("The area is " + rec.getArea());

    }
}

ClickHere to download firstclass.java 

ClickHere to download secondclass.java 

More about Class

Often time two or more methods in a class may have the same name as long as their parameter lists are different. This also applies to constructors. We call this Overloading methods and constructors. Method overloading is an important part of object-oriented programming. When a method overloaded, it means that multiple methods in the same class have the same name, but different types of parameters. Method overloading is important because sometimes we need several different ways to perform the same operation.  Lets see an example on how it works. Recall, from our " Summary " chapter, we have an Atm program. For this example we will use the same Atm program.

Here is the first class:

/*
    This is an atm class, it demonstrates method overloading

*
*/

public class AtmOverload
{
    private double balance;


    // This constructor sets the starting balance at 0.0;

    public AtmOverload()
    {
        balance = 0.0;
    }

    /*
        This constructor sets the starting balance to the value
        passed as an argument

    *
    */

    public AtmOverload(double startBalance)
    {
        balance = startBalance;

    }

    // This is the deposit method

    public void deposit(double amount)
    {
        balance += amount;
    }

    // This is the withdraw method

    public void withdraw(double amount)
    {
        balance -= amount;
    }

    // This is the set balance method

    public void setBalance(double newbalance)
    {
        balance = newbalance;

    }

    // This is the get balance method

    public double getBalance()
    {
        return balance;
    }
}

Here is the second class:

/*
    This demonstrate overloading of Atm class

*
*/

import java.util.*;

public class AtmOverloadDemo
{
    public static void main(String[] args)
    {
        Scanner key = new Scanner(System.in);

        double money;

        // Gets the user account starting balance

        System.out.println("Enter your starting balance ");
        money = key.nextDouble();

        /*
            Creates an Atm object
            Pass the balance into AtmOverload
            Then assign money into balance

        *
        */

        AtmOverload atm = new AtmOverload(money);

        // Ask the user for deposit amount

        System.out.println("How much do you want to deposit");
        double damount = key.nextDouble();

        /*
            Calls the deposit method in AtmOverload class
            Passes damount into it

        *
        */

        atm.deposit(damount);

        // Prints out the new amount after deposit

        System.out.println("Your new amount after deposit is " + atm.getBalance());

        // Ask the user for withdraw amount

        System.out.println("\nHow much do you want to withdraw");
        double wamount = key.nextDouble();

        /*
            Calls the withdraw method in AtmOverload class
            Passes wamount into it

        *
        */


        atm.withdraw(wamount);

        // Prints out the new amount after withdraw

        System.out.println("Your new balance is " + atm.getBalance());

    }
}

ClickHere to download firstclass.java 

ClickHere to download secondclass.java