🛡️ Sealed Classes in Java: Write Safer and Smarter Code

🚀 Why Sealed Classes?

In large applications, modeling a restricted hierarchy can be tricky — especially when every subclass is supposed to follow strict rules. Java’s sealed classes (stable since Java 17) solve this beautifully.

They let you explicitly declare which classes or interfaces can extend or implement a superclass — and nothing beyond that.

public sealed class Shape permits Circle, Square {}

public final class Circle extends Shape {}
public final class Square extends Shape {}
        

No other class can extend Shape. ✅ Clean. ✅ Controlled.

💡 Top Benefits

• Better control over your class hierarchy
• Works great with pattern matching (instanceof)
• Enables exhaustiveness checks in switch statements (Java 21+!)
• Leads to more maintainable and secure code

🧠 When to Use Sealed Classes

• Domain modeling — When types are finite and well-defined (like Status, Shape, Role, EventType)
• API design — Enforce which components can extend base classes
• Refactoring enums — Sealed classes can be more flexible than enums, especially when behavior differs

🔥 Bonus: Combine with Records & Pattern Matching!

Sealed classes shine when combined with modern Java features:

sealed interface Result permits Success, Failure {}

record Success(String message) implements Result {}
record Failure(String error) implements Result {}

void handle(Result result) {
    switch (result) {
        case Success s -> System.out.println("✅ " + s.message());
        case Failure f -> System.err.println("❌ " + f.error());
    }
}
        

No default needed — Java knows all the types!

⚠️ Things to Watch Out For

• All permitted classes must be in the same module or same package
• You must declare each subclass as final, sealed, or non-sealed

✅ Wrap-Up

Sealed Classes = Enum-like control + OOP flexibility If you want better maintainability, safety, and expressiveness, sealed classes are a no-brainer in your modern Java toolbox.

💬 Let’s Talk!

Are you using sealed classes in production? Any tips or lessons learned? Drop your insights 👇

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