Java Programming Language

Chapter 6: Inheritance and Polymorphism

Advanced OOP Concepts

Course: 4343203 - Java Programming

What We'll Cover

Inheritance Fundamentals
Types of Inheritance
super Keyword
Method Overriding
Polymorphism Concepts
Method Overloading vs Overriding
Dynamic Method Dispatch
Final Classes and Methods

Inheritance Hierarchy

Inheritance Fundamentals

Inheritance allows a class to inherit properties and methods from another class

Inheritance Terminology

Key Terms:

Parent Class: Base class, Superclass
Child Class: Derived class, Subclass
extends: Keyword to inherit from a class
IS-A Relationship: Child IS-A type of Parent

Basic Inheritance:


// Parent class
class Animal {
    String name;
    int age;
    
    void eat() {
        System.out.println("Animal is eating");
    }
    
    void sleep() {
        System.out.println("Animal is sleeping");
    }
}

// Child class
class Dog extends Animal {
    String breed;
    
    void bark() {
        System.out.println("Dog is barking");
    }
}

Usage:


public class TestInheritance {
    public static void main(String[] args) {
        Dog dog = new Dog();
        
        // Inherited properties and methods
        dog.name = "Buddy";
        dog.age = 3;
        dog.eat();    // From Animal
        dog.sleep();  // From Animal
        
        // Own properties and methods
        dog.breed = "Labrador";
        dog.bark();   // Own method
    }
}

Benefits of Inheritance

Code Reusability

Reuse parent class code
Avoid duplication
Maintain consistency

Extensibility

Add new features
Modify behavior
Specialize functionality

Polymorphism

One interface, many forms
Runtime method resolution
Flexible code design

Types of Inheritance

Java supports single inheritance but achieves multiple inheritance through interfaces

Single Inheritance

One parent, one child:


class Vehicle {
    int speed;
    String fuel;
    
    void start() {
        System.out.println("Vehicle started");
    }
}

class Car extends Vehicle {
    int doors;
    
    void accelerate() {
        speed += 10;
        System.out.println("Car accelerating");
    }
}

Multilevel Inheritance:


class Vehicle {
    void start() {
        System.out.println("Vehicle started");
    }
}

class Car extends Vehicle {
    void accelerate() {
        System.out.println("Car accelerating");
    }
}

class SportsCar extends Car {
    void turboBoost() {
        System.out.println("Turbo activated!");
    }
}

Hierarchical Inheritance

One parent, multiple children:


class Animal {
    void eat() {
        System.out.println("Animal eating");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Dog barking");
    }
}

class Cat extends Animal {
    void meow() {
        System.out.println("Cat meowing");
    }
}

class Bird extends Animal {
    void fly() {
        System.out.println("Bird flying");
    }
}

Why No Multiple Inheritance?

Diamond Problem:

Ambiguity in method resolution
Conflicting implementations
Complexity in inheritance

Java's Solution:

Single class inheritance
Multiple interface implementation
Clear inheritance hierarchy

The super Keyword

super refers to the immediate parent class object

Uses of super Keyword

1. Access Parent Variables:


class Animal {
    String name = "Animal";
}

class Dog extends Animal {
    String name = "Dog";
    
    void printNames() {
        System.out.println(name);        // Dog
        System.out.println(super.name);  // Animal
    }
}

2. Call Parent Methods:


class Animal {
    void eat() {
        System.out.println("Animal eating");
    }
}

class Dog extends Animal {
    void eat() {
        super.eat(); // Call parent method
        System.out.println("Dog eating bones");
    }
}

3. Call Parent Constructor:


class Animal {
    String name;
    int age;
    
    Animal(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

class Dog extends Animal {
    String breed;
    
    Dog(String name, int age, String breed) {
        super(name, age); // Must be first statement
        this.breed = breed;
    }
    
    void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Breed: " + breed);
    }
}

Important: super() constructor call must be the first statement in child constructor

Method Overriding

Method Overriding allows child class to provide specific implementation of parent method

Method Overriding vs Overloading Comparison

Method Overriding Rules

Overriding Requirements:

Same method name, parameters, and return type
Child class method cannot have stricter access modifier
Cannot override static, final, or private methods
Use @Override annotation (recommended)

Example:


class Animal {
    void makeSound() {
        System.out.println("Animal makes sound");
    }
    
    void move() {
        System.out.println("Animal moves");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Dog barks: Woof!");
    }
    
    @Override
    void move() {
        System.out.println("Dog runs on four legs");
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Cat meows: Meow!");
    }
}

Usage:


public class TestOverriding {
    public static void main(String[] args) {
        Animal animal1 = new Dog();
        Animal animal2 = new Cat();
        
        animal1.makeSound(); // Dog barks: Woof!
        animal1.move();      // Dog runs on four legs
        
        animal2.makeSound(); // Cat meows: Meow!
        animal2.move();      // Animal moves
    }
}

@Override Benefits:

Compile-time checking
Code readability
Prevents typos

Polymorphism

Polymorphism means "many forms" - one interface, multiple implementations

Types of Polymorphism

Compile-time Polymorphism

Method Overloading:


class Calculator {
    int add(int a, int b) {
        return a + b;
    }
    
    double add(double a, double b) {
        return a + b;
    }
    
    int add(int a, int b, int c) {
        return a + b + c;
    }
}

Same method name
Different parameters
Resolved at compile time

Runtime Polymorphism

Method Overriding:


class Shape {
    void draw() {
        System.out.println("Drawing shape");
    }
}

class Circle extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing circle");
    }
}

class Rectangle extends Shape {
    @Override
    void draw() {
        System.out.println("Drawing rectangle");
    }
}

Same method signature
Different implementations
Resolved at runtime

Practical Polymorphism Example


class Employee {
    String name;
    double salary;
    
    Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }
    
    double calculateBonus() {
        return salary * 0.1; // 10% bonus
    }
    
    void displayInfo() {
        System.out.println("Name: " + name + ", Salary: " + salary);
        System.out.println("Bonus: " + calculateBonus());
    }
}

class Manager extends Employee {
    Manager(String name, double salary) {
        super(name, salary);
    }
    
    @Override
    double calculateBonus() {
        return salary * 0.2; // 20% bonus for managers
    }
}

class Developer extends Employee {
    Developer(String name, double salary) {
        super(name, salary);
    }
    
    @Override
    double calculateBonus() {
        return salary * 0.15; // 15% bonus for developers
    }
}

public class PolymorphismDemo {
    public static void main(String[] args) {
        Employee[] employees = {
            new Employee("John", 50000),
            new Manager("Alice", 80000),
            new Developer("Bob", 70000)
        };
        
        for (Employee emp : employees) {
            emp.displayInfo(); // Different bonus calculations
            System.out.println();
        }
    }
}

Dynamic Method Dispatch

Dynamic Method Dispatch is the mechanism by which a call to an overridden method is resolved at runtime

How Dynamic Dispatch Works

Example:


class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Woof!");
    }
}

class Cat extends Animal {
    @Override
    void sound() {
        System.out.println("Meow!");
    }
}

public class DynamicDispatch {
    public static void main(String[] args) {
        Animal animal;
        
        animal = new Dog();
        animal.sound(); // Woof! (Dog's method)
        
        animal = new Cat();
        animal.sound(); // Meow! (Cat's method)
        
        // Array of animals
        Animal[] animals = {
            new Dog(), new Cat(), new Dog()
        };
        
        for (Animal a : animals) {
            a.sound(); // Different sounds
        }
    }
}

Key Points:

Runtime Resolution:

Method determined by object type
Not by reference type
Happens at runtime

Benefits:

Flexible code design
Easy to extend
Loose coupling

Requirements:

Inheritance relationship
Method overriding
Upcasting (parent reference)

Method Overloading vs Overriding

Aspect	Method Overloading	Method Overriding
Definition	Multiple methods with same name, different parameters	Child class redefines parent class method
Inheritance	Not required (same class)	Required (parent-child relationship)
Method Signature	Different parameters	Same signature
Return Type	Can be different	Must be same or covariant
Access Modifier	Can be different	Cannot be more restrictive
Binding	Compile-time (Static)	Runtime (Dynamic)
Polymorphism	Compile-time polymorphism	Runtime polymorphism
Performance	Faster (compile-time)	Slower (runtime resolution)

Final Classes and Methods

final keyword prevents inheritance and method overriding

Final Methods and Classes

Final Methods:


class Animal {
    // Cannot be overridden
    final void breathe() {
        System.out.println("Animal breathing");
    }
    
    // Can be overridden
    void move() {
        System.out.println("Animal moving");
    }
}

class Dog extends Animal {
    // This would cause compilation error
    // void breathe() { } // Error!
    
    @Override
    void move() {
        System.out.println("Dog running");
    }
}

Final Classes:


// Cannot be extended
final class String {
    // String class implementation
}

// This would cause compilation error
// class MyString extends String { } // Error!

final class Math {
    public static final double PI = 3.14159;
    
    public static double sqrt(double a) {
        // Implementation
        return 0;
    }
}

Use Cases:

Security-sensitive classes
Immutable classes
Utility classes
Performance optimization

Chapter Summary

Inheritance Concepts:

Code reusability through extends
IS-A relationship modeling
Single inheritance in Java
super keyword usage
Constructor chaining

Polymorphism Features:

Method overloading (compile-time)
Method overriding (runtime)
Dynamic method dispatch
@Override annotation
Final methods and classes

Next: Interfaces and Abstract Classes

Thank You!

Questions?

Ready to explore Interfaces and Abstract Classes!