Inheritence, Terminology, Inheritance in java, The class called Object, Super keyword, Example, Method Overriding, Method Overriding example, Abstract Class, Abstract Class Number and the Java Wrapper Classes, Final Method and Classes, Multiple Inheritance

1. Inheritence

2. Inheritance is a fundamental Object Oriented
concept
A class can be defined as a "subclass" of
another class.
The subclass inherits all data attributes of its superclass
The subclass inherits all methods of its superclass
The subclass inherits all associations of its superclass
The subclass can:
Add new functionality
Use inherited functionality
Override inherited functionality
Person
- name: String
- dob: Date
Employee
- employeeID: int
- salary: int
- startDate: Date
superclass:
subclass:
Terminology

3. Inheritance is declared using the "extends" keyword
If inheritance is not defined, the class extends a class called Object
Person
- name: String
- dob: Date
Employee
- employeeID: int
- salary: int
- startDate: Date
public class Person
{
private String name;
private Date dob;
[...]
public class Employee extends Person
{
private int employeID;
private int salary;
private Date startDate;
[...]
Employee anEmployee = new Employee();
Inheritance in java

4. At the very top of the inheritance tree is a class
called Object
All Java classes inherit from Object.
All objects have a common ancestor
This is different from C++
The Object class is defined in the java.lang
package
Examine it in the Java API Specification
The class called Object

5. Superclass constructors can be called using the
"super" keyword.
It must be the first line of code in the constructor
If a call to super is not made, the system will
automatically attempt to invoke the no-argument
constructor of the superclass.
Super keyword

6. public class BankAccount
{
private String ownersName;
private int accountNumber;
private float balance;
public BankAccount(int anAccountNumber, String aName)
{
accountNumber = anAccountNumber;
ownersName = aName;
}
}
public class OverdraftAccount extends BankAccount
{
private float overdraftLimit;
public OverdraftAccount(int anAccountNumber, String aName, float aLimit)
{
super(anAccountNumber, aName);
overdraftLimit = aLimit;}}
Example

7. Method Overriding
• If a derived or sub class has a method found
within its base class or super class, that method
will override the base class’s method
• The keyword super can be used to gain access to
superclass methods overridden by the base class
• A subclass method must have the same return
type as the corresponding superclass method

8. public class Abc
{
void display()
{
System.out.println(“Hello”);
}
}
public class Xyz extends Abc Output
{
void display() World
{
System.out.println(“World”);
}
public static void main(string str[])
{
Xyz obj = new Xyz();
obj.display();
} }
Example

9. public class Abc
{
void display()
{
System.out.println(“Hello”);
}
}
public class Xyz extends Abc Output
{
void display() Hello
{ World
super.display(); //this call Abc display()
System.out.println(“World”);
}
public static void main(string str[])
{
Xyz obj = new Xyz();
obj.display();
} }
Continue...

10. Abstract Class
• An abstract class can have abstract methods, data
fields, and concrete methods
• Abstract class differs from a concrete class in that
• An abstract class cannot be instantiated
• An abstract class can declare abstract methods,
which must be implemented in its subclasses

11. Abstract Class Number and the
Java Wrapper Classes
Chapter 3: Inheritance and Class Hierarchies 11

12. Final Method and Classes
Methods can be qualified with the final
modifier
Final methods cannot be overridden.
This can be useful for security purposes.
Classes can be qualified with the final modifier
The class cannot be extended.

13. Multiple Inheritance
• Multiple inheritance: the ability to extend more
than one class
• Multiple inheritance is a language feature that is
difficult to implement and can lead to ambiguity
• Therefore, Java does not allow a class to extend more
than one class