Object-Oriented Programming: Inheritance & Polymorphism

• Object-Oriented Programming is the topic
• Spring Semester 2024-25
• This course covers inheritance and polymorphism.

References

• Dr. Ioannis Violos is the reference
• Subject is Object-Oriented Programming II (Java)
• Department of Informatics and Telematics, Harokopio University

Topics Covered

• Covers inheritance
• Covers overloading
• Covers object management
• Covers polymorphism
• Covers abstract classes

Reusing Classes

• Creating an object of a class is one way to reuse classes
• Using a class in the definition of another class is composition or aggregation
• Composition example: A "Car" class includes a "Engine" class

Inheritance

• Inheritance copies the structure of a class and extends it with new characteristics and functions
• If the base class (super or parent class) changes, the derived class (inherited, sub, or child class) also changes
• In Java, each class can inherit from only one class

Example Classes

• The Human class has name, surname, and age. Includes methods to set and get the name
• The Address class has street, number, state, and postcode

Employee Class

• Employee class extends Human class through inheritance
• Has position and address attributes, which is composition
• Includes methods to set and get the position and address

Employee Class Diagram

• Employee (abstract): Includes residence, position, and bonus attributes and several methods.
• Manager: Inherits from Employee and includes weeklySalary attribute and Manager() and getSalary() methods.
• PieceWorker: Inherits from Employee and includes wagePerPiece and quantity attributes and PieceWorker() and getSalary() methods.
• HourlyWorker: Inherits from Employee and includes wage and hours attributes and HourlyWorker() and getSalary() methods.

Inheritance and Visibility

• Members of the Employee class: All (private, protected, and public) are visible
• Members of Manager, PieceWorker, HourlyWorker, and their objects
  • Only public and protected members are accessible
  • For example, bonus and the methods

Constructors and Inheritance

• Constructors are not inherited
• Constructors are linked to their class definitions
• If Employee defines two constructors, for example: public Employee(String nm, int wage, int hours, double attitude) and public Employee(String nm, int wage)
• If Manager defines one constructor: public Manager(String nm, int wage, int hours, double attitude, Employee under)
• Manager cannot inherit constructors with 2 and 4 arguments

Method Overloading

• It refers to defining a method more than once using same name
• Methods are defined in the Human class
• Inheritance allows to call methods for an Employee, but may not have all the required information

Overloading Solution

• Define the method in Employee with the same name
• Call the overloaded method from the employee the declaration super
• This adds employee-specific information

Overloading toString

• Every class in Java inherits the Object class, which has a public toString method that returns a String
• It can be overloaded in classes by renaming getCompleteData and getCompleteAddress
• Return method requires being public within the Object

Final Methods

• "Final" variables become constants, cannot be changed once assigned a value
• "Final" methods cannot be overridden
• Parent classes define them once and not redefined in subclasses
• Private methods are implicitly final
• Behavior is maintained and cannot be changed in subclasses

Final Classes

• "Final" classes cannot be inherited
• All their methods become implicitly final
• Used when be certain that no one will inherit them
• Example: public final class String{ .....}
• Final methods and classes are not used fequently

Constructors

• Human constructor: Sets name and surname to "Unknown", and age to 0. Prints "A new Human has been created"
• Address constructor: Sets street, number, and city to "Unknown". Prints "A blank Address has been created"
• Employee constructor: Creates a new Address, sets position to "Unemployed", and prints "A new Employee has been created".

Creating Objects

• Calling the Human constructor automatically creates the Human base constructor

Handling Missing Base Constructor

• Define a constructor that takes name, surname, and age as parameters

Super Keyword

• Use the super keyword to call the specific Human constructor: super("Unknown","Unknown",0);

Example Usage

• Code creates an Employee object, Unknown is created
• The blank Address and the new Employee is created also

Destructors - finalize

• Destructors are useful to clear memory when objects are deleted
• System calls it automatically, like Garbage collection

Order of calling destructors

• Employee is created which creates a human, which has unknown attributes
• A new address is created that is blank
• A new is created, Human cleared, Employee cleared, and address cleared.
• After the employee is destroyed the address is useless.

Polymorphism

• Structures and Inheritance: An array of Human objects, can hold also Employee objects through upcasting.

Calling Methods on Objects

• Can safely call the characteristics and methods of the Human class
• Employe characteristics and methods can be called via downcasting
• Run Time Type Identification – Type is determined at execution

Data Structures and polymorphism

• ArrayList and Vector store objects of different classes that inherit from the same base class
• Base class has methods that derived classes override
• Converting to the base class allow recall of class messages
• Behavior relies on object type.

Advantages of Polymorphism

• Can focus on the general behavior of objects and allow their specific behavior to be defined during execution
• Simplifies expansion due to message calls being identical to the base class
• New classes needs only to define their method of handling messages