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Inheritance:
In object-oriented programming, inheritance enables new objects to take on the proper�es of
exis�ng objects. A class that is used as the basis for inheritance is called a superclass or base
class. A class that inherits from a superclass is called a subclass or derived class.
The syntax for crea�ng a subclass is simple. At the beginning of your class declara�on, use the
“extends” keyword, followed by the name of the class to inherit from:
class MountainBike extends Bicycle
{
// new fields and methods de fi ning a mountain bike would go here }
Code Example public class A { A() { System.out.println("Inside A"); }
}
public class B extends A {
B() { System.out.println("Inside B"); }
}
public class C extends B { C() {
System.out.println("Inside C"); }
}
public class Lab { public static void main(String[] args) {
Overriding
C c=new C();//object of type C } }
Output Inside A Inside B Inside C
Member Access and Inheritance
Although a subclass includes all of the members of its superclass, it cannot access those
members of the superclass that have been declared as private. A class member that has been
declared as private will remain private to its class. It is not accessible by any code outside its
class, including subclasses. To avoid this we use protected access specifier. Protected enforces
the type or member can only be accessed by code in the same class, or in a derived class.
Using super
Whenever a subclass needs to refer to its immediate superclass, it can do so by use of the
keyword super.
super has two general forms.
- The fi rst calls the superclass constructor.
- The second is used to access members (protected or public data members and methods) of the superclass. Super use cases from an Inherited Class super(parameter-list); // constructor call super.member; // some member call of the super class super.method(); // some method call of the super class
Super is a special keyword which has two-fold purpose
- To call the constructors of base class. 1.a.In case of calling constructor of base a. Only a constructor of a child class can call constructor of the base class using “super(…)". b. Call to “super(…)” must be the very first line in the child’s constructor c. If you don't call super(...) explicitly as the first line in a constructor, Java will automa�cally insert a call to the default no-argument superclass constructor for you super();
- To access member variables and methods inherited from
Overriding
}
Output
**1. Inside A
- B's one arg Constructor
- A's 1 arg Constructor
- A's Implementation of callMe
- A's Implementation of callMe
- B's Implementation of callMe**
Coding Example public class A { protected int i;
public A() { System.out.println("Inside A"); }
public A(int i) { this.i=i; System.out.println("A's 1 arg Constructor"); }
void callMe() { System.out.println("A's implementation of callMe."); }
} public class B extends A{
B() { System.out.println("Inside B"); }
B(int input) { super(input); System.out.println("B's one arg Constructor"); }
@Override void callMe() { super.callMe(); System.out.println("B's implementation of callMe."); } } public class Lab {
public static void main(String[] args) {
Overriding
B b=new B(5); A a=new A(6); a.callMe(); b.callMe();
}
} Output
**1. A's 1 arg Constructor
- B's one arg Constructor
- A's 1 arg Constructor
- A's Implementation of callMe
- A's Implementation of callMe
- B's Implementation of callMe**
Ques � on 1: You are going to develop a reserva�on system for a train or flight related journeys. Reserva�on can be of two types; train reserva�on and flight reserva�on. Generally when we speak about reserva�on, one thing that’s common in all types of reserva�ons is Time (including day and �me of that day). We might need to specify some extra features for train or flight reserva�on like carriage number or flight number etc. In this exercise create a Reserva � on class and inherit this class in Train reserva � on and Flight reserva � on classes.
- Create a class named Reserva � on
- import java.util.Date;
- public class DateDemo {
- public static void main(String args[]) {
- // Instantiate a Date object
- Date date = new Date();
- // display time and date using toString()
- System.out.println(date.toString());
- }
- Default Constructor
- Overloaded constructer
- Ge � er and se � er.
Overriding
Person class fields are: Name and Gender.
- Default Constructor
- Constructor that takes 2 inputs, (Name, Gender);
- Ge � ers and se � ers.
- A toString() func�on.
Make a class Employee, derive it from Person class.
Employee has:
- Data Members: Employee ID and Designa � on, Grade (e.g 17 grade or 20 grades etc.) and Salary.
- Declare Grade as a protected data member.
- Default Constructor
- Overloaded Constructor that takes inputs: Name, Gender, Employee ID, Designa � on, Grade and call the Person constructor to set the values of Name and Gender using keyword “super”
- Ge � ers and Se � ers.
- ToString() method, use super.toString() and concatenate with Employee fi elds before returning.
- Implement a calculateSalary() func � on that calculates the salary based on the Grade. Use fixed salaries given below: Grade 16 – 50,000 PKR 17 – 75, 18 – 100,
Make a class Visi � ng Employee, derive it from Employee class.
Visi � ng Employee class:
- The Visi�ng Employee class has an addi�onal data member Working Hours.
- Default Constructor
- Overloaded constructer that takes input Name, Employee ID, Gender, Designa � on, Grade, Working Hours and calls the employee constructor to set the fi elds for Name, Employee ID, Gender, Designa � on and Grade using keyword “super”.
- Ge � ers and Se � ers.
Overriding
EmployeeVisiting Employee
- Overridden calculateSalary(). The CalculateSalary() will work on the basis of protected data member “Grade” and private data member “Working Hours”. E.g. working hours = 24 hrs/ month and he is 17 grade person. Salary = 24 *75000/
- ToString() method, use super.toString() and concatenate with Visi � ng Employee fi elds before returning.
Create Main Class and a main func � on:
- Create two objects, Employee and Visi � ngEmployee using overloaded constructors.
- (^) Call calculateSalary() for both objects.
- Display the objects.
Overriding
