Why inheritance is used in Java - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class that inherits from another class.

Java

class Dog extends [1] {}

Drag options to blanks, or click blank then click option'

ACar

BHouse

CTree

DAnimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class unrelated to the child class as the parent.

Forgetting to use the 'extends' keyword.

In Java, a class inherits from another class using the extends keyword followed by the parent class name. Here, Dog inherits from Animal.

2fill in blank

medium

Complete the code to call the parent class constructor from the child class.

Java

class Dog extends Animal {
    Dog() {
        super[1];
    }
}

Drag options to blanks, or click blank then click option'

A()

B{}

C[]

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces instead of parentheses.

Omitting parentheses after super.

The super() call invokes the parent class constructor. Parentheses are needed to call it as a method.

3fill in blank

hard

Fix the error in the code to override a method from the parent class.

Java

class Dog extends Animal {
    @Override
    public void [1]() {
        System.out.println("Dog barks");
    }
}

Drag options to blanks, or click blank then click option'

Aspeak

Bbark

Crun

Djump

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the method name when overriding.

Forgetting the @Override annotation.

The method speak() is overridden from the parent class Animal. The method name must match exactly to override.

4fill in blank

hard

Fill both blanks to create a subclass that inherits and adds a new method.

Java

class [1] extends Animal {
    public void [2]() {
        System.out.println("Runs fast");
    }
}

Drag options to blanks, or click blank then click option'

ACheetah

Bfly

CrunFast

DBird

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated class names.

Using method names that don't describe the action.

The class Cheetah inherits from Animal and adds a new method runFast() to show its behavior.

5fill in blank

hard

Fill all three blanks to complete the code that demonstrates inheritance and method overriding.

Java

class [1] {
    public void sound() {
        System.out.println("Animal sound");
    }
}

class [2] extends [3] {
    @Override
    public void sound() {
        System.out.println("Meow");
    }
}

Drag options to blanks, or click blank then click option'

AAnimal

BCat

DDog

Attempts:

3 left

💡 Hint

Common Mistakes

Using different class names that don't match inheritance.

Not overriding the method correctly.

The Cat class inherits from Animal and overrides the sound() method to print "Meow" instead of the generic animal sound.

Practice

1. Why do we use inheritance in Java?

easy

A. To make programs run faster

B. To create unrelated classes

C. To reuse code from an existing class

D. To avoid using methods

Solution

1. Step 1: Understand inheritance purpose

   Inheritance allows a new class to use code from an existing class, avoiding repetition.

2. Step 2: Compare options

   Only To reuse code from an existing class correctly states code reuse. Others are incorrect because inheritance does not create unrelated classes, speed up programs directly, or avoid methods.

3. Final Answer:

   To reuse code from an existing class -> Option C

4. Quick Check:

   Inheritance = Code reuse [OK]

Hint: Inheritance means reusing code from another class [OK]

Common Mistakes:

• Thinking inheritance speeds up the program
• Believing inheritance creates unrelated classes
• Confusing inheritance with method removal

2. Which of the following is the correct syntax to inherit class Animal in Java?

easy

A. class Dog inherits Animal {}

B. class Dog extends Animal {}

C. class Dog implements Animal {}

D. class Dog uses Animal {}

Solution

1. Step 1: Recall Java inheritance syntax

   Java uses the keyword extends to inherit from a class.

2. Step 2: Check each option

   class Dog extends Animal {} uses extends, which is correct. Options A, C, and D use wrong keywords.

3. Final Answer:

   class Dog extends Animal {} -> Option B

4. Quick Check:

   Inheritance keyword = extends [OK]

Hint: Use 'extends' keyword to inherit a class in Java [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with class inheritance
• Using 'uses' keyword which doesn't exist

3. What will be the output of this Java code?

class Animal {
  void sound() { System.out.println("Animal sound"); }
}
class Dog extends Animal {
  void sound() { System.out.println("Bark"); }
}
public class Test {
  public static void main(String[] args) {
    Animal a = new Dog();
    a.sound();
  }
}

medium

A. Bark

B. Runtime error

C. Compilation error

D. Animal sound

Solution

1. Step 1: Understand method overriding and polymorphism

   Dog overrides the sound() method of Animal. The object is of type Dog but referenced as Animal.

2. Step 2: Determine which method runs

   At runtime, the Dog's sound() method runs due to polymorphism, printing "Bark".

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Overridden method runs = Bark [OK]

Hint: Overridden methods run from actual object type [OK]

Common Mistakes:

• Expecting base class method output
• Thinking it causes compile or runtime error
• Confusing reference type with object type

4. Find the error in this inheritance code:

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

medium

A. Class Vehicle should be abstract

B. Missing semicolon after class Vehicle

C. Method start() cannot be overridden

D. Incorrect inheritance syntax in class Car

Solution

1. Step 1: Check class inheritance syntax

   In Java, to inherit a class, use extends keyword. The code misses extends in class Car Vehicle.

2. Step 2: Verify other options

   No semicolon needed after class declaration, methods can be overridden, and Vehicle need not be abstract.

3. Final Answer:

   Incorrect inheritance syntax in class Car -> Option D

4. Quick Check:

   Use 'extends' keyword for inheritance [OK]

Hint: Inheritance needs 'extends' keyword in class declaration [OK]

Common Mistakes:

• Forgetting 'extends' keyword
• Adding semicolon after class header
• Thinking methods can't be overridden

5. You want to create a class SmartPhone that has all features of Phone plus new features like camera and GPS. Which is the best way to do this using inheritance?

hard

A. Make SmartPhone extend Phone and add new methods

B. Make Phone extend SmartPhone and add new methods

C. Create SmartPhone and Phone as separate classes with no relation

D. Copy all Phone code into SmartPhone without inheritance

Solution

1. Step 1: Identify real-world relationship

   A SmartPhone is a type of Phone with extra features, so it should inherit Phone.

2. Step 2: Apply inheritance correctly

   SmartPhone should extend Phone and add new methods for camera and GPS.

3. Final Answer:

   Make SmartPhone extend Phone and add new methods -> Option A

4. Quick Check:

   Subclass adds features to superclass [OK]

Hint: Subclass extends superclass to add features [OK]

Common Mistakes:

• Reversing inheritance direction
• Not using inheritance for related classes
• Copy-pasting code instead of extending