Ch 9

Chapter 9: Inheritance

Chapter Goals

  • Understand inheritance concepts.

  • Implement subclasses that inherit and override superclass methods.

  • Learn about polymorphism in Java.

  • Familiarize with the common superclass, Object, and its methods.

Inheritance Hierarchies

Definition of Inheritance

  • Inheritance describes the relationship between a general class (superclass) and a specialized class (subclass).

  • A subclass inherits data and behaviors from the superclass.

    • Example: Cars share common traits of all vehicles.

Vehicle Example Hierarchy

  • Superclass: Vehicle

    • Subclasses: Motorcycle, Car (which includes Truck, Sedan, SUV)

    • Shared trait: Transporting people.

Code Reusability via Inheritance

  • Inheritance allows code reuse, reducing duplication.

    • A subclass inherits methods from its superclass.

    • A subclass object can replace a superclass object without altering the program's logic (substitution principle).

Example of Substitution Principle

  • A method processing Vehicle objects can handle any specific vehicle type, e.g., Car.

Example: Question Hierarchy

Question Class Structure

  • Different question types (standard and computer-graded quiz questions) can be implemented with inheritance.

  • Common method functionalities include displaying text and checking answers.

Implementing Subclasses

  • A subclass is formed by modifying an existing class.

  • Example: ChoiceQuestion as a subclass of Question.

    • Inherits all methods from the superclass.

    • Only the new or different instance variables and methods are declared.

Choice Question Characteristics

  • Stores various answer choices.

  • Adds methods to handle these answer choices and to display them to the user.

Subclass Declaration

Syntax Overview
public class SubclassName extends SuperclassName {
    // instance variables
    // methods
}

  • extends indicates inheritance of the subclass from its superclass.

Method Inheritance

Access to Superclass Methods

  • A subclass method can utilize public methods of the superclass. Private instance variables, however, remain inaccessible.

  • Example of adding methods:

    • public void addChoice(String choice, boolean correct) in ChoiceQuestion.

Overriding Methods

  • If inherited behavior is unsuitable, it can be overridden by defining a new implementation in the subclass.

  • The super keyword is used to refer to superclass methods within the subclass.

public void display() {
    super.display(); // Calls the display method of the superclass
}

Polymorphism

Understanding Polymorphism

  • Allows the same method to behave differently based on the object it is called on.

  • A superclass reference can refer to subclass objects.

    • Example: public static void presentQuestion(Question q) can be used with both Question and ChoiceQuestion objects.

Common Errors in Inheritance

Mistakes to Avoid

  1. Replicating Instance Variables: Don't declare the same instance variable in the subclass.

    • Correct approach: Use inherited methods to access superclass variables.

  2. Overriding Needs: Ensure matching parameter types when overriding, or else a new method is created (overloading).

  3. Using super Keyword: Forgetting to use super when invoking a superclass method can lead to errors.

Object Class in Java

  • Every class in Java is a subclass of the Object class unless explicitly stated otherwise.

  • Common Object class methods: toString(), equals(), and hashCode().

Overriding Object Methods

toString() Method

  • Provides a string representation of the object, useful for debugging.

equals() Method

  • Compares content of two objects; must be overridden to provide meaningful equality checks.

Using the instanceof Operator

  • Allows checking if an object is of a certain type before casting to prevent exceptions.

Self-Check Questions

  • Practice questions allowing self-assessment of understanding about inheritance and polymorphism.