Lecture 13: Inheritance

Understanding Inheritance in Object-Oriented Programming

Introduction to Inheritance

• The lecture discusses a key object-oriented programming concept known as inheritance, which is essential for understanding encapsulation and polymorphism.

• Inheritance allows a child class to inherit properties and behaviors from a parent class, similar to how children inherit traits from their parents.

Types of Inheritance

• There are two main types of inheritance: single inheritance (where a class inherits from one superclass) and multiple inheritance (where a class can inherit from multiple superclasses).

• The concept of "IS-A" relationship is introduced, where subclasses represent specialized versions of their parent classes. For example, a lion is an animal, while a cow is also an animal.

Hierarchical Structure

• Classes can be organized hierarchically with various levels; this leads to multi-level inheritance where subclasses can further extend other subclasses.

• A superclass can have multiple subclasses. For instance, if 'Vehicle' is the superclass, then 'Car' and 'Truck' could be its subclasses.

Code Reusability through Inheritance

• Inheritance promotes code reusability by allowing subclasses to share methods and attributes defined in their superclasses.

• This sharing capability enhances code management and reduces redundancy in programming.

Implementation in Java

• The syntax for implementing inheritance in Java involves using the extends keyword followed by the name of the superclass.

• All members (methods and variables) of the superclass are accessible within the subclass unless they are private.

Practical Example: 2D Point vs. 3D Point

• An example illustrates how a 3D point class can inherit from a 2D point class, gaining its properties while adding new ones like z-coordinate.

• The 3D point extends functionality by including additional dimensions while maintaining access to existing methods from the 2D point.

Method Overriding

• Subclasses can override methods inherited from superclasses to provide specific implementations relevant to them.

• This allows for dynamic method resolution at runtime based on the object's actual type rather than its reference type.

Conclusion on Simple Inheritance

Understanding Inheritance in Java

Introduction to Points and Dimensions

• The discussion begins with the concept of 2D and 3D points, emphasizing that point 3D is fundamentally an extension of point 2D.

• It highlights that these are essentially initializations for understanding object-oriented programming concepts.

Types of Inheritance

Single Inheritance

• The speaker introduces the idea of single inheritance, where a subclass can inherit from a single superclass.

• An example is provided where subclass 1 inherits from superclass, while subclass 2 also derives from the same superclass.

Multiple Inheritance

• The concept of multiple inheritance is discussed, where one subclass can inherit from more than one superclass.

• This leads to multi-level inheritance examples, showcasing how subclasses can derive from multiple superclasses.

Hybrid Inheritance

• Hybrid inheritance combines both single and multiple inheritance but is not supported in Java.

• A practical example illustrates how a 'Person' class serves as a base class for 'Student' and 'Employee' subclasses.

Understanding Class Relationships

Attributes and Methods

• Each class (e.g., Student or Employee) contains specific attributes and methods inherited from the Person class.

• The importance of accessing methods through inheritance is emphasized, showing how subclasses utilize parent class functionalities.

Practical Example in Java Programming

• A code snippet demonstrates defining a Person class in Java, which serves as a foundation for other classes like Student.

• The relationship between classes is further clarified by discussing how both Student and Employee classes inherit properties from the Person class.

Advanced Concepts in Inheritance

Extending Classes Further

• The discussion touches on extending classes indefinitely without limits within the hierarchy.

• Examples illustrate how different types of employees (e.g., Permanent vs. Temporary Employees) can be derived through further specialization.

Geometric Object Hierarchy

• A metaphor using geometric objects explains how all objects belong to a general type but can have specific characteristics (e.g., 1D, 2D, or 3D).

• Specialization within geometric shapes (like triangles or rectangles being specific types of polygons), showcases practical applications of inheritance principles.

Method Overriding and Dynamic Binding in Object-Oriented Programming

Understanding Method Overriding

• The concept of method overriding allows for the implementation of various types of objects within a classification, enabling the writing of programs that utilize this principle.

• When a method is declared in a superclass and then redefined in a subclass, it is referred to as an overriding method. This means that the subclass can provide its own specific implementation.

• The overridden methods essentially replace the original methods from the superclass, allowing for customized behavior when invoked.

Dynamic Binding Explained

• Dynamic binding refers to runtime decision-making regarding which method to invoke based on the object type rather than reference type. This is crucial for achieving polymorphism.

• The concept emphasizes that if a subclass does not override certain variables or methods from its superclass, those elements remain accessible through dynamic binding.

Accessing Superclass Variables and Methods

• A subclass can access variables or methods defined in its superclass using the super keyword. However, direct access to these elements from outside the subclass context is not possible.

• The super keyword serves multiple purposes: referencing instance variables, invoking superclass methods, and calling constructors from parent classes.

Practical Example of Using Super Keyword

• An example illustrates how one can use super to refer to an instance variable defined in a parent class while also having an overridden variable in the child class.

• In this scenario, both classes (Animal and Dog) have their own definitions for color; thus demonstrating how scope affects variable accessibility.

Method Invocation with Super Keyword

• When invoking print statements within subclasses, if you want to reference a variable from the superclass instead of the overridden one, you must use super.color.

• This distinction highlights how different outputs are generated based on whether you call an instance variable directly or through super, showcasing polymorphic behavior.

Conclusion on Method Overriding Concepts

Understanding Method Overriding and Constructors in Object-Oriented Programming

Introduction to Method Overriding

• The concept of method overriding is introduced, highlighting that the super.eat() method signifies a new way of working within classes.

• It explains how the eat method can be referenced as part of a superclass or subclass, emphasizing its role in resolving namespace issues.

Example of Class Structure

• A simple example illustrates how an object of class Dog behaves when calling d.work(), demonstrating expected outputs and functionality.

• The discussion transitions to constructors, explaining that the Dog class inherits from the Animal superclass, which has its own constructor.

Importance of Super Keyword

• The significance of using the super keyword is emphasized; it allows access to parent class constructors and methods.

• Another example compares 2D and 3D point classes, discussing how constructors are called based on inheritance hierarchy.

Runtime Polymorphism Explained

• The concept of runtime polymorphism is introduced, where multiple objects can be created from different subclasses but treated as instances of their superclass.

• An example involving a Bike superclass and a Splendor subclass demonstrates how methods are resolved at runtime based on object type rather than reference type.

Practical Implications

• The discussion concludes with practical implications for using runtime polymorphism in Java programming, particularly regarding memory allocation and method resolution.

Understanding Abstract and Final Classes in Java

Key Concepts of Abstract Classes

• An abstract class is defined as a class that does not have any methods implemented. It serves as a conceptual framework for other classes to inherit from.

• Abstract classes can contain both abstract methods (without implementation) and concrete methods (with implementation). This allows subclasses to provide specific implementations while still adhering to the abstract class's structure.

• The purpose of an abstract class is to act as a template, enabling subclasses to implement their own versions of the defined methods.

Understanding Final Classes

• A final class in Java cannot be subclassed, meaning no other class can extend it. This ensures that the behavior of the final class remains unchanged.

• Declaring a method or variable as final prevents it from being overridden or modified in any subclass, ensuring consistency across instances.

Relationship Between Abstract and Final Classes

• An abstract class can have final methods, but if a class is declared as final, it cannot be abstract since it would not allow for further extension or instantiation.

• The discussion highlights how these concepts relate to inheritance in Java programming, emphasizing the importance of understanding when and how to use each type effectively.

Future Considerations