Unit 9 Notes

Unit 9: Inheritance – AP Computer Science A

🔑 Key Concepts

• extends keyword for inheritance
• Superclass and subclass relationships
• Method overriding
• Using super to call parent constructors/methods
• Polymorphism basics

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🧬 Inheritance Hierarchy

Inheritance allows a subclass to inherit fields and methods from a superclass.

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

public class Dog extends Animal {
    public void speak() {
        System.out.println("Woof");
    }
}

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📌 Overriding Methods

A subclass can override methods from its superclass.

@Override
public void speak() {
    System.out.println("Bark!");
}

• Must have same name, return type, and parameters
• Optional @Override annotation helps catch errors

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🧩 Calling Superclass Methods

Use super.methodName() to call the parent’s version:

public class Puppy extends Dog {
    public void speak() {
        super.speak(); // calls Dog's speak
        System.out.println("Yip!");
    }
}

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🧱 Calling Superclass Constructors

Use super() to call parent constructor (must be first line):

public class Dog extends Animal {
    public Dog() {
        super(); // calls Animal constructor
    }
}

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🌀 Polymorphism (Intro)

A superclass reference can point to a subclass object:

Animal a = new Dog();
a.speak(); // calls Dog's speak method

• Java uses the object type (not reference type) to decide which method to run

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🧪 Practice Multiple Choice (5 Questions)

1. What is inheritance?
(A) Repeating code in every class
(B) Creating methods in a loop
(C) Using one class to base another on
(D) Sharing variables between methods
✅ Answer: (C)

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2. What does the super keyword do?
(A) Creates a new object
(B) Refers to the parent class
(C) Imports a library
(D) Stops inheritance
✅ Answer: (B)

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3. What will this print?

class A {
    public void hello() {
        System.out.println("A");
    }
}
class B extends A {
    public void hello() {
        System.out.println("B");
    }
}
A obj = new B();
obj.hello();

(A) A
(B) B
(C) AB
(D) Error
✅ Answer: (B)

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4. What must be true to override a method?
(A) Same name, different return type
(B) Same name, same parameters
(C) New parameters
(D) Private visibility
✅ Answer: (B)

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5. Why use @Override?
(A) It’s required
(B) Adds security
(C) Helps catch mistakes when overriding
(D) Makes code run faster
✅ Answer: (C)

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