Upcasting and downcasting in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to upcast a Dog object to an Animal reference.

Java

Animal animal = new Dog(); // [1]

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

Aoverriding

Bdowncasting

Cupcasting

Doverloading

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing upcasting with downcasting.

Trying to cast explicitly when not needed.

Upcasting means treating a subclass object as an instance of its superclass. Here, Dog is upcast to Animal.

2fill in blank

medium

Complete the code to downcast an Animal reference back to a Dog object.

Java

Dog dog = (Dog) [1];

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

Aanimal

Bnew Animal()

Cnew Dog()

Danimal.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to cast a new Animal object instead of the reference.

Missing the explicit cast.

Downcasting requires an explicit cast. Here, the Animal reference named 'animal' is cast back to Dog.

3fill in blank

hard

Fix the error in the downcasting code to avoid ClassCastException.

Java

if ([1] instanceof Dog) {
    Dog dog = (Dog) animal;
}

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

Adog

Banimal

Cnew Animal()

Dnew Dog()

Attempts:

3 left

💡 Hint

Common Mistakes

Checking instanceof on the wrong variable.

Not using instanceof before downcasting.

Before downcasting, check if the object referenced by 'animal' is an instance of Dog to avoid errors.

4fill in blank

hard

Fill both blanks to create a list of Animals and add a Dog object using upcasting.

Java

List<[1]> animals = new ArrayList<>();
animals.add(new [2]());

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

AAnimal

BDog

CCat

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using Dog as the list type instead of Animal.

Adding an unrelated class object.

The list holds Animal references. Adding a Dog object is upcasting it to Animal.

5fill in blank

hard

Fill all three blanks to safely downcast and call a Dog-specific method.

Java

if ([1] instanceof [2]) {
    [2] dog = ([2]) [3];
    dog.bark();
}

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

Aanimal

BDog

CCat

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable for instanceof or casting.

Casting to the wrong class.

Check if 'animal' is a Dog, then downcast it to Dog and call the Dog-specific method bark().

Practice

1. What is upcasting in Java?
Upcasting means:

easy

A. Changing the value of a variable

B. Treating a specific object as a more general type

C. Creating a new object from a class

D. Converting a general type to a specific type

Solution

1. Step 1: Understand object type hierarchy

   In Java, classes can inherit from other classes, making some types more general (superclass) and others more specific (subclass).

2. Step 2: Define upcasting

   Upcasting means treating a subclass object as if it were an instance of its superclass, which is more general.

3. Final Answer:

   Treating a specific object as a more general type -> Option B

4. Quick Check:

   Upcasting = Treat specific as general [OK]

Hint: Upcasting = subclass object as superclass type [OK]

Common Mistakes:

• Confusing upcasting with downcasting
• Thinking upcasting creates a new object
• Believing upcasting changes the actual object type

2. Which of the following is the correct syntax for downcasting in Java?

Animal a = new Dog();
// Downcast here

easy

A. Dog d = (Dog) a;

B. Dog d = a;

C. Dog d = a.toDog();

D. Dog d = (Animal) a;

Solution

1. Step 1: Understand downcasting syntax

   Downcasting requires an explicit cast to convert a superclass reference back to a subclass type.

2. Step 2: Apply correct cast

   The correct syntax is: SubclassType var = (SubclassType) superClassVar; so here: Dog d = (Dog) a;

3. Final Answer:

   Dog d = (Dog) a; -> Option A

4. Quick Check:

   Downcasting needs explicit cast [OK]

Hint: Downcast with (Subclass) before variable [OK]

Common Mistakes:

• Omitting the cast operator
• Casting to wrong type
• Using methods like toDog() which don't exist

3. What will be the output of this code?

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

medium

A. Bark\nFetching

B. Bark\nAnimal sound

C. Animal sound\nFetching

D. Compilation error at line A

Solution

1. Step 1: Understand method overriding and upcasting

   Variable a is of type Animal but refers to a Dog object. Calling a.sound() calls Dog's overridden method, printing "Bark".

2. Step 2: Analyze method call fetch()

   Method fetch() is not in Animal, so a.fetch() is invalid (commented out). Downcasting (Dog) a allows calling fetch(), printing "Fetching".

3. Final Answer:

   Bark Fetching -> Option A

4. Quick Check:

   Upcast calls overridden method, downcast calls subclass method [OK]

Hint: Upcast calls overridden; downcast needed for subclass-only methods [OK]

Common Mistakes:

• Thinking a.sound() calls Animal's method
• Trying to call fetch() without downcasting
• Confusing compile vs runtime errors

4. Identify the error and fix it in this code:

class Animal {}
class Cat extends Animal { void meow() { System.out.println("Meow"); } }
public class Test {
  public static void main(String[] args) {
    Animal a = new Animal();
    Cat c = (Cat) a; // line X
    c.meow();
  }
}

medium

A. No error, code runs fine

B. Syntax error; fix by removing cast

C. ClassCastException at runtime; fix by checking instanceof before casting

D. Change Animal to Cat in line X

Solution

1. Step 1: Identify the casting problem

   Variable a refers to an Animal object, not a Cat. Casting Animal to Cat without checking causes ClassCastException at runtime.

2. Step 2: Fix with instanceof check

   Before casting, check if (a instanceof Cat) to ensure safe downcasting and avoid runtime error.

3. Final Answer:

   ClassCastException at runtime; fix by checking instanceof before casting -> Option C

4. Quick Check:

   Downcast only if instanceof true [OK]

Hint: Use instanceof before downcasting to avoid errors [OK]

Common Mistakes:

• Ignoring runtime ClassCastException
• Assuming cast always works
• Trying to fix with syntax changes only

5. Given these classes:

class Vehicle { void start() { System.out.println("Vehicle started"); } }
class Car extends Vehicle { void start() { System.out.println("Car started"); } void openTrunk() { System.out.println("Trunk opened"); } }
class Bike extends Vehicle { void start() { System.out.println("Bike started"); } void kickStart() { System.out.println("Kickstarted"); } }

Which code snippet correctly upcasts and downcasts to call openTrunk() safely?

hard

A. Vehicle v = new Vehicle();
if (v instanceof Car) {
((Car) v).openTrunk();
}

B. Vehicle v = new Bike();
((Car) v).openTrunk();

C. Car c = new Vehicle();
c.openTrunk();

D. Vehicle v = new Car();
if (v instanceof Car) {
((Car) v).openTrunk();
}

Solution

1. Step 1: Understand upcasting and downcasting here

   Variable v is declared as Vehicle but assigned a Car object (upcasting). To call Car-specific method openTrunk(), downcast is needed.

2. Step 2: Check safe downcasting

   Using instanceof ensures v is actually a Car before downcasting and calling openTrunk(). This avoids runtime errors.

3. Final Answer:

   Vehicle v = new Car(); if (v instanceof Car) { ((Car) v).openTrunk(); } -> Option D

4. Quick Check:

   Upcast then instanceof check before downcast [OK]

Hint: Always check instanceof before downcasting [OK]

Common Mistakes:

• Downcasting without instanceof check
• Assigning superclass object to subclass variable
• Calling subclass methods on superclass references without cast