A Simple Introduction To Java - Object Oriented Programming - Part 1 (Encapsulation)

In this session, we will discuss:

Shape class
Encapsulation
Package
Conclusion

We have already looked at how to create functions or methods and also how to create classes.You should refer to

A Simple Introduction To Java - Class and Method

Michael Otu ・ Mar 18 '22

#java #programming #beginners #codenewbie

Shape Class

The purpose of creating the Shape class is to demonstrate the use of the this keyword in java. The this keyword refers to the instance of the class and we can use it with the . operator to access the properties/fields and methods of the class anywhere in the said class.

Create a class, Shape.

Let the Shape class have the following attributes and methods:

length: double
breadth: double
area(): double
perimeter(): double

The methods have the ()

The above ADT tells us that the class has two fields and two methods, the types and return types. It doesn't tell us how the methods are implemented. Try an implement the Shape class.

public class Shape {
    double length;
    double breadth;

    public Shape(double length, double breadth) {
        length = length;
        breadth = breadth;
    }

    public double area() {
        return length * breadth;
    }

    public double perimeter() {
        return 2 * (length + breadth);
    }
}

What do you notice when you execute the Shape class?

// Main.java
public class Main {

    public static void main(String[] args) {
        Shape s = new Shape(2.5, 2.6);
        System.out.println("Area: " + s.area());
        System.out.println("Perimeter: " + s.perimeter());
    }
}

It will appear that the output will become:

Area: 0.0
Perimeter: 0.0

But why the 0.0s? This was because of our constructor. If you remember so far all our constructors have different names compared to the fields.

Replace the constructor with the snippet below:

public Shape(double l, double b) {
    length = l;
    breadth = b;
}

Now the output looks more correct and as expected just like the result below.

Area: 6.5
Perimeter: 10.2

Using different variable names was another way to solve the previous issue we had where all the outputs were 0.0. We could have used the this keyword.

Change the constructor to the snippet below:

public Shape(double length, double breadth) {
    this.length = length;
    this.breadth = breadth;
}

The output will be the same as the above. Using the same parameter names as the field names make our code, to some degree, self-documenting. What would l and b mean anyway?

We can use the this keyword on the rest of the fields called in the area and parameter methods too. We should add void methods to print the area and perimeter with some text, informing the user or displaying the length and breadth of the shape. Let's call this method print.

Our new Shape ADT will be:

length: double
breadth: double
area(): double
perimeter(): double
print(): void

Now our new class will look like the snippet below:

public class Shape {
    double length;
    double breadth;

    public Shape(double length, double breadth) {
        this.length = length;
        this.breadth = breadth;
    }

    public double area() {
        return this.length * this.breadth;
    }

    public double perimeter() {
        return 2 * (this.length + this.breadth);
    }

    public void print() {
        System.out.println("The shape has a length and a breadth of, "
                + this.length + " and " + this.breadth + ".");
        System.out.println("Shape has an Area of " + this.area()
                + " squared units.");
        System.out.println("Shape has a Perimeter of " + this.perimeter()
                + " units.");
    }
}

Our little output will be:

The shape has a length and a breadth of, 2.5 and 2.6.
Shape has an Area of 6.5 squared units.
Shape has a Perimeter of 10.2 units.

Encapsulation

In java, there are four main concepts that we have to understand about Object-Oriented-Programming. These are Encapsulation, Inheritance, Polymorphism and Abstraction.

We'd discuss Encapsulation here and discuss the other three in a separate article.

So, what is Encapsulation? Do you remember what we discussed about access modifiers? We said access modifiers regulate access to class methods and variables. There are public, protected, private and default. The private access modifier restricts the access to methods or variables declared as private outside the class. The concept of Encapsulation revolves around the private access modifier and how to resolve the problems that arise when the private access modifier is used.

We can prevent the user of our class from directly accessing and modifying the state of the objects they create. This is done to hide implementation details from the user. We provide the user of our class with some public methods to access the variables and or methods that we have declared private.

If we consider the Shape class, the length and breadth have no access modifiers. Well, they do, it's just that we didn't specify it. So it is by "default", default. What this means is that the fields can be accessed by another class in the same package as the Shape class.

Getters and Setters

I mentioned earlier that, we can hide implementation details by declaring some fields or methods private than allowing indirect access to these fields and methods via some public methods. These methods are known as getters and setters.

Let's experiment. Make the fields in the Shape class private and in the Main class, comment out s.print(); and do, System.out.println(s.length);. What happened?

Well, on my side, I am using vscode and before I even completed the whole print statement a red squiggly line appear below the print statement. Saying, The field Shape.length is not visible. In vscode, neither length nor breadth showed up on the IntelliSense.
Now we know that when we make a field private the field will be inaccessible outside the class.

Now let's talk about getters and setters, what they are and how to use them.

We made the length and breadth fields private which means we can not access them nor write them. We can not read their value nor can we reassign or update their value. With a public getter method, we can return the said private field. So if the return type of length is int, the getter will return an int value which is the private field. A setter is a public method with no return type. Its return type is void. We use a setter to set a value for a private field. Something like assigning a private field a new value via some method. So the setter method will take the value as an argument and reassign the private field with the passed argument.

The getter method looks like the snippet below:

public [type] getPrivateField() {
    return this.privateField;
}

[type] is just the data type. The getter method name starts with get (which indicates that it is a getter method). The remaining part is mostly the name of the private field.

The goes for the setter method but in place of "get" we use "set".

The setter method looks like the snippet below:

public void setPrivateField([type] someValue) {
    this.privateField = someValue;
}

Know that the type of someValue and the said private field must have the same type.

With this new information, update the Shape class using getters and setter.

This is what my Shape class now looks like after the new changes:

public class Shape {
    private double length;
    private double breadth;

    public Shape(double length, double breadth) {
        this.length = length;
        this.breadth = breadth;
    }

    public double getBreadth() {
        return breadth;
    }

    public void setBreadth(double breadth) {
        this.breadth = breadth;
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }

    public double area() {
        return this.length * this.breadth;
    }

    public double perimeter() {
        return 2 * (this.length + this.breadth);
    }

    public void print() {
        System.out.println("The shape has a length and a breadth of, "
                + this.length + " and " + this.breadth + ".");
        System.out.println("Shape has an Area of " + this.area()
                + " squared units.");
        System.out.println("Shape has a Perimeter of " + this.perimeter()
                + " units.");
    }
}

Now in the Main class, instead of System.out.println(s.length); we can now do, System.out.println(s.getLength());.

What do you think about getters and setters? Well, what I noticed is that, if I don't need direct access to the fields, I don't need getters and setters. In the Main class, if we have nothing to do with the fields then we don't need getters and setters. Apart from setting the fields using the constructors, there hasn't been anywhere the fields were set so we can do away with the setters. Know when to use the getters and setters. The basic knowledge here is restricting access.

Have a look at the print method in the Shape class.

 public void print() {
    System.out.println("The shape has a length and a breadth of, "
            + this.length + " and " + this.breadth + ".");
    System.out.println("Shape has an Area of " + this.area()
            + " squared units.");
    System.out.println("Shape has a Perimeter of " + this.perimeter()
            + " units.");
    }

We can comment out the print method and put its body in the Main class where we placed System.out.println(s.getLength());. We would have to change this to s and then use the getters in place of the fields. Mine Main class now looks like this:

public class Main {
    public static void main(String[] args) {
        Shape s = new Shape(2.5, 2.6);
        // s.print();
        System.out.println("The shape has a length and a breadth of, "
                + s.getLength() + " and " + s.getBreadth() + ".");
        System.out.println("Shape has an Area of " + s.area()
                + " squared units.");
        System.out.println("Shape has a Perimeter of " + s.perimeter()
                + " units.");
    }
}

And the output I got was,

The shape has a length and a breadth of, 2.5 and 2.6.
Shape has an Area of 6.5 squared units.
Shape has a Perimeter of 10.2 units.

Note that it is advised to let a class do one thing. This was why I said we should come out of the print method. Even though we create the getters and setter for external use, we can also use them internally.

In the constructor, instead of assigning a private field a value by assignment, use the setter. In the perimeter and area methods, use the getters.

Package

You may have heard of the term, Package at least once in your life as a human. I mean, as a java programmer you'd be dealing with packages a lot. So take what you know and translate that thought to java.

Do you know java has a Math class? What about we create our own Math class without the names of the classes conflicting. How do we do that? We use a package. A package is just a folder. For our case as starters, the package just sits right in our root directory. There are some benefits to using packages which include:

we can group related classes which will make it easier to debug and maintain the codebase.
this prevents the pollution of namespaces, no name will class with another.
by default, packages restrict the access of class to outsiders but are open to the classes in the said package.

Create MyMath Package

To create a package all we have to do is create a folder and then create our classes in that folder.

Let's create a folder, MyMath in the root of project files. Now inside MyMath folder, create a java class, Math

Now at the very top before the class header, put package MyMath;. This indicates that Math class is a file in the MyMath package.

My Math class looks like this:

package MyMath;

public class Math {

}

Let's add two methods, addOne(int num): int and incBy(int num, int val): int. So as the method name suggests about their implementation, the addOne method takes an int argument and adds one to it then returns the result. incBy takes two arguments, num and val. It adds val to num and returns the result. Since all the methods in the Math class static methods, let's make ours also static.

package MyMath;

public class Math {
    public static int addOne(int num) {
        return num + 1;
    }

    public static int incBy(int num, int val) {
        return num + val;
    }
}

In the Main class, let's clear the main method's body. Now to use the new package we have created, we have to import it in the Main class as import MyMath.Math;

So our Main class will now look like this:

import MyMath.Math;

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.addOne(1));
        System.out.println(Math.incBy(1, 3));
    }
}

There is a problem here. We can use our Math class freely now but what about the java Math class? Of course, we can use the java Math class anywhere but we can not do that in our Math class. It is for reasons like that we use packages.

I used the pascal case for my package name, the recommended approach is to have a root folder, then in the root folder, you'd have your package folder then your classes. so you'd import it as import root.package.ClassName;. Note the use of the lower case for the root and package names. This is similar to import java.util.Scanner;.

Conclusion

OOP is a programming paradigm where classes are used to model real-life objects.
There are four main concepts of OOP: Encapsulation, Inheritance, Polymorphism and Abstraction.
Encapsulation means data hiding, making use of the private keyword which restricts access to fields or and methods declared as private.
We make use of getters and setters to make private fields or and methods available to classes outside the said class.
We can use packages to group related classes and this prevents namespace pollution.

Projects

The projects here will be simple (not easier). We would implement the back account program using the knowledge we've acquired so far.

A bank account has an account name, account number and pin.
One can deposit and redraw from one's bank account.
One can check their balance.
After a withdrawal or a deposit, display a message saying how much the account owner previously had before the transaction was made, the amount used in the transaction and what their new balance is.
We can not have a balance less than zero and we can not deposit nor withdraw a negative amount from the balance.
Make the program interactive by asking the user to input their account name, number and pin when the program is first executed.
Assign the user an initial balance of 100.
Provide an interactive means for the user to deposit, withdraw or exit the program.

TIPS: Have a different class for the bank account and another for the interactivity (the main class will also do). Use integers for the amounts. If you think you can add some functionality that is not mentioned, do so.