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Sealed Classes and Interfaces

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🛡️ Sealed Classes and Interfaces in Java 15 – Now With 100% More Sass ☕​

Welcome, brave Java warrior! Today, we embark on a journey through the mysterious and powerful realm of sealed classes and interfaces—a realm where only the chosen few are allowed to inherit.

Before Java 15, your public interfaces were wide open like a coffee shop with no closing hours. Now? We’re slapping on some velvet ropes and exclusive guest lists. 🚫👨‍💻

1. 🔐 Sealed Classes​

1.1 🎭 sealed Modifier and permits Keyword​

By default, Java classes are like open mic nights—anyone can walk up and implement your public interface. But now with Java 15’s preview feature, you can seal the deal.

How? Just toss in the magical sealed modifier, and declare your A-list subclasses using permits. 💫

sealed class Account
    permits CurrentAccount, SavingAccount, LoanAccount {
}

final class CurrentAccount extends Account {}
non-sealed class SavingAccount extends Account {}
sealed class LoanAccount extends Account permits HomeloanAccount, AutoloanAccount {}

final class HomeloanAccount extends LoanAccount {}
final class AutoloanAccount extends LoanAccount {}

🎉 Result? Only these 3 subclasses are allowed to extend Account. No party crashers allowed! 🎯 And for LoanAccount, only HomeloanAccount and AutoloanAccount get VIP access.

1.2 🧬 final, sealed, and non-sealed Subclasses​

Once you're on the guest list, how you behave matters:

  • final: “I’m the last in my line. No one inherits from me. Period.” 🔒
  • sealed: “You can inherit, but only if you’re invited.” 🔐
  • non-sealed: “I’m chill. Anyone can extend me. Live your life.” 🎈

💡 Important: A sealed class cannot force its subclasses to keep the sealing tradition. Rebels gonna rebel.

1.3 📦 Packaging of Sealed Classes​

There are some location rules to keep your sealed family together:

  • All permitted subclasses must be in the same module.
  • If you're in the unnamed module, then they must be in the same package.
  • Bonus hack: If you put all the subclasses in the same .java file, you can omit the permits keyword!

Example:

public sealed class Account {}    // No 'permits' keyword needed

final class CurrentAccount extends Account {}
non-sealed class SavingAccount extends Account {}
sealed class LoanAccount extends Account {}

final class HomeloanAccount extends LoanAccount {}
final class AutoloanAccount extends LoanAccount {}

🧙 Magical, isn’t it?

1.4 📇 Records as Sealed Subclasses​

Guess what? You can totally add a record class to the permits list! Because records are:

  • Automatically final
  • Immutable
  • Made for modeling data with ✨ style ✨

So go ahead and drop a record into your sealed class family. They won’t ruin the party.

1.5 🧪 API Support​

Java's reflection API decided to get in on the action too. Say hello to:

  • Class::permittedSubclasses() → Returns a nice little array of all the permitted subclasses.
  • Class::isSealed() → Returns true if the class is sealed. 🔍

Now you can seal and reflect at the same time. 💅

2. 🤝 Sealed Interfaces​

Interfaces wanted in on the fun—and Java said, “Sure, why not?”

Same rules, just a different vibe. Add sealed to your interface and use permits to name the lucky few who can implement or extend it.

public sealed interface IReport
        permits Printable, Formattable, ExcelReport, PdfReport {
}

non-sealed interface Printable extends IReport {}
non-sealed interface Formattable extends IReport {}
non-sealed class ExcelReport implements IReport {}
non-sealed class PdfReport implements IReport {}

🧾 Sealed interfaces follow the exact same declaration rules as sealed classes. No extra drama.

3. 🧠 When Should You Use Sealed Classes?​

You know that one time you wrote 10 instanceof checks and STILL missed a subclass? Yeah... sealed classes are here to save you.

Why use them?

  • Lock down class hierarchies like a boss.
  • Prevent surprise subclassing from rogue developers.
  • Help the compiler ensure you've handled all the cases in your instanceof checks.

Future versions of Java + pattern matching = no more default or else blocks. 🙌

As soon as a new subclass joins the sealed class squad, the compiler taps you on the shoulder and says, “Yo, you forgot to handle this one.”

** 🎉 Happy Learning!!

That’s the sealed class circus, folks! Use it to tame your type hierarchies, control subclassing chaos, and become the master of your Java destiny. 🧙‍♂️☕

Until next time—keep your classes sealed and your bugs revealed!