Upcasting and downcasting in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is upcasting in Java?

Upcasting is when you convert a subclass object to a superclass type. It is safe and done automatically by Java.

beginner

What is downcasting in Java?

Downcasting is when you convert a superclass reference back to a subclass type. It requires explicit casting and can cause errors if done incorrectly.

intermediate

Why is upcasting considered safe?

Because every subclass object is also an instance of its superclass, so no data or behavior is lost when upcasting.

intermediate

What happens if downcasting is done incorrectly?

Java throws a <code>ClassCastException</code> at runtime if the object is not actually an instance of the subclass you are casting to.

beginner

Show a simple Java example of upcasting and downcasting.

class Animal {}<br>class Dog extends Animal {}<br><br>Animal a = new Dog(); // upcasting<br>Dog d = (Dog) a; // downcasting

Which of the following is true about upcasting?

A It is automatic and safe.

B It requires explicit casting.

C It can cause ClassCastException.

D It converts superclass to subclass.

Upcasting is automatic and safe because a subclass object is always an instance of its superclass.

What must you do to perform downcasting in Java?

A Use the instanceof keyword only.

B Nothing, it is automatic.

C Use explicit casting syntax.

D Declare a new superclass object.

Downcasting requires explicit casting syntax like (Subclass) object.

What exception can occur if downcasting is done incorrectly?

A IllegalArgumentException

B ArrayIndexOutOfBoundsException

C NullPointerException

D ClassCastException

ClassCastException is thrown when you try to cast an object to a type it is not an instance of.

Given: Animal a = new Dog(); What is this an example of?

A Upcasting

B Encapsulation

C Polymorphism

D Downcasting

Assigning a subclass object to a superclass reference is upcasting.

Which keyword helps check type before downcasting?

A cast

B instanceof

C typeof

D check

The instanceof keyword checks if an object is an instance of a class before downcasting.

Explain the difference between upcasting and downcasting in Java.

Think about which direction the object reference is converted.

Describe a real-life example that helps you understand upcasting and downcasting.

Consider a general category and a specific item within it.

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