Upcasting and downcasting in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Upcasting and Method Calls

What is the output of this Java code snippet?

Java

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}
class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
    void fetch() {
        System.out.println("Dog fetches ball");
    }
}
public class Test {
    public static void main(String[] args) {
        Animal a = new Dog(); // upcasting
        a.sound();
    }
}

A Animal sound

B Dog fetches ball

C Dog barks

D Compilation error

💡 Hint

Remember that overridden methods use the actual object's method, even if referenced by a parent type.

The variable 'a' is of type Animal but refers to a Dog object. The sound() method is overridden in Dog, so Dog's version runs.

❓ Predict Output

intermediate

Downcasting and Method Access

What will be the output of this Java code?

Java

class Vehicle {
    void start() {
        System.out.println("Vehicle started");
    }
}
class Car extends Vehicle {
    void openSunroof() {
        System.out.println("Sunroof opened");
    }
}
public class Test {
    public static void main(String[] args) {
        Vehicle v = new Car(); // upcasting
        ((Car) v).openSunroof(); // downcasting
    }
}

A Sunroof opened

B Vehicle started

C ClassCastException at runtime

D Compilation error

💡 Hint

Downcasting allows access to subclass-specific methods.

The object 'v' is actually a Car, so downcasting to Car is safe and allows calling openSunroof().

❓ Predict Output

advanced

Runtime Error from Invalid Downcasting

What error does this code produce when run?

Java

class Fruit {
    void eat() {
        System.out.println("Eating fruit");
    }
}
class Apple extends Fruit {
    void peel() {
        System.out.println("Peeling apple");
    }
}
class Orange extends Fruit {
    void peel() {
        System.out.println("Peeling orange");
    }
}
public class Test {
    public static void main(String[] args) {
        Fruit f = new Orange();
        Apple a = (Apple) f; // downcasting
        a.peel();
    }
}

A ClassCastException at runtime

B Peeling orange

C Peeling apple

D Compilation error

💡 Hint

Downcasting to a wrong subclass causes runtime errors.

The object is an Orange but cast to Apple, causing ClassCastException at runtime.

🧠 Conceptual

advanced

Understanding Upcasting Behavior

Given the code below, what will be the output when main runs?

Java

class Parent {
    void show() {
        System.out.println("Parent show");
    }
}
class Child extends Parent {
    void show() {
        System.out.println("Child show");
    }
    void play() {
        System.out.println("Child play");
    }
}
public class Test {
    public static void main(String[] args) {
        Parent p = new Child(); // upcasting
        p.show();
        // p.play(); // commented out
    }
}

A Parent show

B Child show

C Child play

D Compilation error due to p.play()

💡 Hint

Upcasting hides subclass-only methods but overridden methods run subclass versions.

The show() method is overridden, so Child's version runs. The play() method is not accessible via Parent reference.

❓ Predict Output

expert

Output and Behavior of Complex Casting

What is the output of this Java program?

Java

class A {
    void m() {
        System.out.println("A.m()");
    }
}
class B extends A {
    void m() {
        System.out.println("B.m()");
    }
    void n() {
        System.out.println("B.n()");
    }
}
class C extends B {
    void m() {
        System.out.println("C.m()");
    }
    void n() {
        System.out.println("C.n()");
    }
}
public class Test {
    public static void main(String[] args) {
        A obj = new C(); // upcasting
        obj.m();
        ((B) obj).n(); // downcasting
    }
}

A Compilation error

B C.m()\nB.n()

C A.m()\nB.n()

D C.m()\nC.n()

💡 Hint

Method calls use the actual object's version, even after casting.

obj refers to a C object. m() is overridden in C, so C.m() runs. Downcasting to B allows calling n(), but n() is overridden in C, so C.n() runs.

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