Object-Oriented Concepts and Constructs

Four fundamental OOP concepts

• Encapsulation
• Inheritance
• Polymorphism
• Abstraction

Encapsulation

Encapsulation in Java is a mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit. In encapsulation the variables of a class will be hidden from other classes, and can be accessed only through the methods of their current class, therefore it is also known as data hiding.

To achieve encapsulation in Java:

• Declare the variables of a class as private.
• Provide public setter and getter methods to modify and view the variables values.

TestEncapsulation.java

public class TestEncapsulation{
    
    private String name;
    private String idNum;
    private int age;
   
    public int getAge(){
        return age;
    }
    
    public String getName(){
        return name;
    }
    
    public String getIdNum(){
        return idNum;
    }
    
    public void setAge(int newAge){
        age = newAge;
    }
    
    public void setName(String newName){
        name = newName;
    }
    
    public void setIdNum(String newId){
        idNum = newId;
    }
    
}

The public setXXX() and getXXX() methods are the access points of the instance variables of the TestEncapsulation class. Normally, these methods are referred as getters and setters. Therefore any class that wants to access the variables should access them through these getters and setters.

The variables of the EncapTest class can be accessed as below:

/* File name : RunEncapsulation.java */

public class RunEncapsulation{
    public static void main(String args[]){
        TestEncapsulation encap = new TestEncapsulation();
        encap.setName("James");
        encap.setAge(20);
        encap.setIdNum("12343ms");
        
        System.out.print("Name : " + encap.getName() + " Age : " + encap.getAge());
    }
}

This would produce the following result:

Name : James Age : 20

Benefits of Encapsulation:

• The fields of a class can be made read-only or write-only.
• A class can have total control over what is stored in its fields.
• The users of a class do not know how the class stores its data. A class can change the data type of a field and users of the class do not need to change any of their code.

Inheritance

Inheritance can be defined as the process where one class acquires the properties (methods and fields) of another. With the use of inheritance the information is made manageable in a hierarchical order.

The class which inherits the properties of other is known as subclass (derived class, child class) and the class whose properties are inherited is known as superclass (base class, parent class).

extends Keyword:

extends is the keyword used to inherit the properties of a class. Below given is the syntax of extends keyword:

class Super{
    ...
    ...
}


class Sub extends Super{
    ...
    ...
}

Calculation.java

package calculation;

class Calculation {
    int z;
    
    public void addition(int x, int y){
        z=x+y;
        System.out.println("The sum of the given numbers:"+z);
    }
    
    public void substraction(int x,int y){
        z=x-y;
        System.out.println("The difference between the given numbers:"+z);
    }
}

MyCalculation.java

package calculation;

public class MyCalculation extends Calculation{
    
    public void multiplication(int x, int y){
        z = x*y;
        System.out.println("The product of the given numbers is: "+z);
    }
    
    public static void main(String args[]){
        int a=20, b=10;
        MyCalculation demo = new MyCalculation();
        demo.addition(a,b);
        demo.substraction(a, b);
        demo.multiplication(a, b);
    }
}

Using extends keyword, My_Calculation inherits the methods addition() and Subtraction() from Calculation class.

You can instantiate the class as given below as well. But using the superclass reference variable ( cal in this case ) you cannot call the method multiplication(), which belongs to the subclass My_Calculation:

Calculation cal = new My_Calculation();
demo.addition(a, b);
demo.Subtraction(a, b);

A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

=The super keyword

The super keyword is similar to this keyword. The following are the scenarios where the super keyword is used.

• It is used to differentiate the members of superclass from the members of subclass, if they have same names.
• It is used to invoke the superclass constructor from subclass.

Differentiating the members

If a class is inheriting the properties of another class, and if the members of the superclass have the same names as the sub class, to differentiate these variables we use super keyword as shown below.

super.variable
super.method();

Sample Code

In the given program you have two classes namely Sub_class and Super_class, both have a method named display() with different implementations, and a variable named num with different values.

package btest2_the_super_keyword;

class Super_class{
    int num=20;

    //display method of superclass
    public void display(){
        System.out.println("This is the display method of superclass");
    }
}

package btest2_the_super_keyword;

public class Sub_class extends Super_class {
    int num=10;
    
    //display method of sub class
    public void display(){
        System.out.println("This is the display method of subclass");
    }
    
    public void my_method(){
        //Instantiating subclass
        Sub_class sub=new Sub_class();
        
        //Invoking the display() method of sub class
        sub.display();
        
        //Invoking the display() method of superclass
        super.display();
        
        //printing the value of variable num of subclass
        System.out.println("value of the variable named num in sub class:"+ sub.num);
        
        //printing the value of variable num of superclass
        System.out.println("value of the variable named num in super class:"+ super.num);
        
    }

    public static void main(String args[]){
        Sub_class obj = new Sub_class();
        obj.my_method();
    }
    
}

Polymorphism

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Any Java object that can pass more than one IS-A test is considered to be polymorphic. In Java, all Java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.

Abstraction