In-depth Notes on Inheritance in Java Programming

Objectives

• Understand overriding methods
• Grasp class hierarchies
• Learn about visibility in inheritance
• Design for inheritance effectively

Overriding Methods

• Definition: A child class can override an inherited method with its own implementation.
  • Must match the parent's method signature (same name, parameters).
  • The execution context determines which version of the method runs (parent or child).
• Example in Code:
  • Messages Class: Demonstrates overriding methods
    java public class Messages { public static void main(String[] args) { Thought parked = new Thought(); Advice dates = new Advice(); parked.message(); dates.message(); // overridden } }
• Output:
  • I feel like I'm diagonally parked in a parallel universe.
  • Warning: Dates in calendar are closer than they appear.

Messages.java

• Thought Class: Base for the overridden method; prints a message.
  java public class Thought { public void message() { System.out.println("I feel like I'm diagonally parked in a parallel universe."); } }
• Advice Class: Overrides the message method of Thought.
  java public class Advice extends Thought { public void message() { System.out.println("Warning: Dates in calendar are closer than they appear."); super.message(); } }
• Invoking Parent's Method: Uses super.message().
• Overriding Restrictions:
  • A final method cannot be overridden.
  • Shadowing variables occurs when child classes redefine variables from parent classes (can lead to confusion).

Overloading vs Overriding

• Overloading: Same method name with different parameters in the same class.
• Overriding: Same method name and parameters, one in parent class and one in child class.
  • Usage: Overloading allows flexibility in methods while overriding allows specialization of methods in child classes.

Class Hierarchies

• Definition: Structure where child classes derive from parent classes, forming a tree-like structure.
• Siblings: Two child classes from the same parent.
• Design Principle: Common features should reside higher up in the hierarchy to ensure all child classes inherit them.

The Object Class

• Base Class: All classes in Java inherit from the Object class if not explicitly defined otherwise.
• Methods: Includes useful methods such as toString() and equals(Object obj).
  • toString: Default returns class name and hash code, commonly overridden.
  • equals: Checks if two references point to the same object but can be overridden for meaningful equality checks.

Abstract Classes

• Definition: Acts as a placeholder for a general concept in a hierarchy; cannot be instantiated directly.
  • Declaration: public abstract class Product { }
• Characteristics:
  • May have abstract methods (no bodies) and concrete methods (fully defined).
  • Child classes must implement abstract methods or also be declared abstract.

Interface Hierarchies

• Inheritance for Interfaces: One interface can inherit from another; implementing class must define all inherited methods.

Visibility Revisited

• Access Control: All variables and methods are inherited, even private members (accessible via parent methods).
• Indirect Access: Ensure encapsulation while allowing derived classes to access parent methods indirectly.

Designing for Inheritance

• Focus on creating maintainable and reusable class designs through careful inheritance relationships.
  • Ensure that every derivation represents an is-a relationship.
  • Keep generic functionality in higher hierarchy levels.
  • Override methods where necessary for functionality.
• Design Principles:
  • Use abstract classes for general concepts.
  • Apply visibility modifiers judiciously to maintain encapsulation.

Restricting Inheritance

• Final Modifier: If applied to a method, it cannot be overridden. If applied to a class, no subclasses can be created.