Object-Oriented Programming: Inheritance

Object-Oriented Programming

• Spring Semester 2024-25
• Inheritance and Polymorphism

Reusing Classes

• One class object construction.
• Using a class to define another: composition or aggregation.
• Example: creating an "engine" class, then a "car" class that "has" an "engine."

Inheritance

• Copies a class's structure and extends it with characteristics and functions.
• Changing the base class (parent or super) also changes the derived class (child).
• In Java each class can only inherit from one class.

Inheritance Examples

• Class Human includes name, surname, and age, with methods to set and get the name variable.
• Class Address is defined with street, number, state, and postcode attributes.
• An Employee class inherits from Human and includes position and Address fields.
• Employee can set position and address variables.

Visibility and Inheritance

• All members of Employee, including private, protected, and public, are visible within the class.
• Only public and protected members of Manager, PieceWorker, and HourlyWorker are visible to their objects. Bonus and all methods are included.
• e.g., In the Manager class, an attempt to directly assign a value to a private position field will result in an error, while using the setPosition method is correct.
• Only public members are visible in another class.

Inheritance and Constructors

• Constructors are not inherited.
• Constructors are closely tied to their respective classes.
• Suppose Employee had two constructors: one with name, wage, hours, and attitude, and another with name and wage.
• If Manager defines a constructor with name, wage, hours, attitude, and an Employee reference, Manager will not automatically inherit the two constructors from Employee.

Method Overloading

• getCompleteData method in Human class as an example of method overloading.
• The ability to call due to inheritance for an Employee, but it won't have all the information for them.

Overriding the Method

• Override a method by declaring it with the same name in the Employee class.
• Use super to call the employee's existing method.

Overriding toString

• In Java, every class inherits the Object class by default, which has a public toString method that returns a String.
• The public method can override in the three classes, the getCompleteData and getCompleteAddress can be renamed.
• Must be public because it is public in Object.

Final Methods

• "Final" variables become constants, assigned only once.
• "Final" methods cannot be overridden.
• The parent class defines them once and does not redefine them in subclasses.
• All private methods are implicitly final.
• This ensures that the behavior is maintained and cannot be changed in subclasses.

Final Classes

• "Final" classes cannot be inherited.
• All of its methods become implicitly final.
• This is used when one needs to be sure that no one will inherit them.
• final methods and classes are not often used.

Constructors for Object Management

• Class Human():
  • name and surname are set to Unknown.
  • Age is assigned the value of zero.
  • Prints “A new Human has been created”.
• Class Address():
  • street and city are set to Unknown.
  • Number is assigned the value zero.
  • Prints “A blank Address has been created”.
• Class Employee():
  • residence is set to new Address().
  • position is set to Unemployed.
  • Prints “A new Employee has been created”.
• Base constructor is called automatically.

Object constructors

• Address ad1 and Human h1 created.
• Constructor order matters.
• A new address, person and employee are created.
• The basic constructor is called automatically.

Human (No Default Constructor)

• If the Human class doesn't have a default constructor, you need to explicitly specify which Human constructor to call in the Employee constructor using super.
• Here, "Unknown" for name and surname, and 0 for age are used.

Example Usage

• Employee object is created.
• Outputs constructor messages.

Finalize Method Example

• In Human class, finalize() prints Human is being cleared.
• In Employee, super.finalize() is called followed by Employee being cleared being printed.

Finalize order

• Object created then labeled as useless when System.gc() is used
• The address is useless after Employee is cleared.

Polymorphism and Inheritance

• Upcasting: You can put Employee objects in a Human array.

Human[] group=new Human[];
group[0]=new Human();
group[1]=new Employee();

• It is safe to call the Human class's methods and properties on them.
• Downcasting: It is necessary to convert an Employee to employee before calling its members.

(Employee)group[1].getPosition();
(Employee)group[0].getPosition(); //Class Cast Exception

RTTI Solution

• The problem resolves automatically for toString because it is present in both types.
• Without the need for downcasting:

group[1].toString();
group[0].toString();

• Based on type, the function is invoked.
• Run Time Type Identification determines type.

Object structure

• ArrayList and Vector store objects deriving from same base.
• Base has methods derived types override.
• Extracting, converts to base, exhibiting polymorphism.

Polymorphism Advantages

• Focus on general behavior; specific behavior is determined at runtime.
• Eases expansion through uniform messaging.