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.