[VV121] The Java 21 Newsletter: 🔼 Upcasting and 🔽 Downcasting (Subcasting)

🔼 Upcasting

• Definition: Casting a subclass object to a superclass type.
• Safe: Always safe, no explicit cast required.
• What is accessible: Only the declared type’s methods/fields (superclass), unless overridden.

Key Point:

• ✅ Methods are determined at runtime → actual object's type (Dog) decides what method is executed (polymorphism).
• ❌ Attributes/fields (if any) are not polymorphic → they are resolved at compile-time using the declared type.

🔽 Downcasting (Subcasting)

• Definition: Casting a superclass object to a subclass type.
• Risky: Must be done explicitly, and it can throw a ClassCastException if not valid.
• What is accessible: After downcasting, you can access the subclass-specific methods/fields.

Wrong downcast example:

🔧 Summary Table

🔍 Real Example with Field

🎢 Mental Model: Think of a Class Hierarchy as a Tree

Object

• The top of the tree is more general (Object, Animal)
• The bottom of the tree is more specific (Dog, Poodle, etc.)

🔼 Upcasting = Going up the hierarchy

• From specific to general (e.g., Dog → Animal)
• Safe, because a Dog is always an Animal
• Loses access to specific features

🔽 Downcasting = Going down the hierarchy

• From general to specific (e.g., Animal → Dog)
• Risky, because not all Animals are Dogs
• Gains access to subclass features, but only if valid

🧠 How to Remember

Try one of these mnemonics:

1. "Upcasting climbs the tree, downcasting dives deep."

• Imagine the class hierarchy as a vertical structure (like a tree or org chart).
• "Up" means more general (e.g., to Animal or Object)
• "Down" means more specific (e.g., to Dog)

2. Think in terms of "zoom level":

• Upcasting = zooming out → you see less detail (fewer features)
• Downcasting = zooming in → you see more detail (more features, but may not always be safe)

3. Java's automatic vs. manual:

• Upcasting = automatic (Java does it for you)
• Downcasting = manual (you have to tell Java: Dog d = (Dog) a;)

🧪 Example Recap