Kotlin Sealed Classes vs. Java Enums (and Sealed Interfaces!): A Tale of Limited Hierarchies

Kotlin vs Java (21 Part Series)

1 Kotlin Null Safety vs. Java: A Comedy of Errors (But Mostly in Java)
2 Kotlin Data Classes vs Java: A Tale of Two Cities (But One Has Way Less Boilerplate)
… 17 more parts…
3 Kotlin Properties vs. Java Fields: A Tale of Two Variables (Where Kotlin Has More Tricks Up Its Sleeve!)
4 Kotlin Coroutines vs. Java Threads: A Concurrency Conundrum (Solved with a Sprinkle of Kotlin Magic!)
5 Kotlin Smart Casts vs. Java Casts: A Type-Safe Tale (with Fewer Runtime Surprises!)
6 Kotlin Type Inference vs. Java: A Deductive Dance (Where Kotlin Takes the Lead!)
7 Kotlin Primary Constructors vs. Java Constructors: A Construction Conundrum (Solved with Kotlin’s Elegance!)
8 Kotlin Operator Overloading vs. Java: A Mathematical Magic Show (Where Kotlin Bends the Rules!)
9 Kotlin String Templates vs. Java String Concatenation: A Tale of Two Strings (Where Kotlin Sings!)
10 Kotlin Sealed Classes vs. Java Enums (and Sealed Interfaces!): A Tale of Limited Hierarchies
11 Kotlin Companion Objects vs. Java Static Members: A Tale of Two Companions (Where Kotlin Offers More Than Just Friendship!)
12 Kotlin Infix Functions vs. Java: A Grammatical Twist (Where Kotlin Breaks the Rules!)
13 Kotlin Delegated Properties vs. Java: A Tale of Shared Responsibilities (Where Kotlin Lightens the Load!)
14 Kotlin Range Expressions vs. Java: A Journey Through the Numerical Landscape (Where Kotlin Takes a Scenic Route!)
15 Kotlin Destructuring Declarations vs. Java: Unpacking the Joy of Multiple Assignments (Where Kotlin Delivers the Goods!)
16 Kotlin Lambdas with Receivers vs. Java: A Code Symphony (Where Kotlin Plays a Different Tune!)
17 Kotlin’s “No Checked Exceptions” Policy vs. Java: A Tale of Two Error-Handling Philosophies
18 Kotlin Tail Recursion Optimization vs. Java: A Deep Dive into Efficient Recursion
19 Kotlin Extension Functions vs. Java: Adding a Touch of Magic to Existing Classes
20 Kotlin Object Declarations vs. Java: Summoning Singletons with Ease
21 Kotlin vs. Java: A Grand Finale and Farewell (But Not Goodbye!)

Imagine you’re a king with a royal decree. You want to declare that only certain individuals can inherit the throne, and no one else. In the programming world, that’s kind of like a sealed class! It restricts the hierarchy, ensuring that only a specific set of subclasses can exist.

Java: The Enum Kingdom

Java has enums, which are a special type of class that represents a fixed set of constants. They’re like the royal court, with each member having a specific role and no possibility of adding new members.

// Java
public enum RoyalTitle {
    KING,
    QUEEN,
    PRINCE,
    PRINCESS
}

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Enums are great for representing a limited set of options, but they lack the flexibility of classes. You can’t add new members or extend them with additional behavior. It’s like having a rigid court structure with no room for new titles or roles.

Kotlin: The Sealed Class Dynasty

Kotlin introduces sealed classes, which allow you to define a restricted hierarchy of subclasses. Only the subclasses defined in the same file as the sealed class can inherit from it. It’s like the king declaring the specific individuals who are eligible for the throne.

// Kotlin
sealed class RoyalTitle {
    object King : RoyalTitle()
    object Queen : RoyalTitle()
    data class Prince(val name: String) : RoyalTitle()
    data class Princess(val name: String) : RoyalTitle()
}

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Sealed classes offer several advantages:

Controlled inheritance: They prevent unintended subclasses from being created, ensuring type safety.
Exhaustive when statements: When using when expressions with sealed classes, the compiler can check if you’ve covered all possible subclasses, preventing errors.
Flexibility: Subclasses can have different properties and behaviors, providing more expressiveness than enums.

Java’s Counterpart: Sealed Interfaces (Java 17+)

Java, recognizing the limitations of enums, introduced sealed interfaces in Java 17. These allow you to define interfaces with a restricted set of permitted subclasses, similar to Kotlin’s sealed classes.

// Java
public sealed interface RoyalTitle permits King, Queen, Prince, Princess {}

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This provides more flexibility than enums while maintaining control over the inheritance hierarchy. However, it’s still a relatively new feature in Java, and Kotlin’s sealed classes offer more concise syntax and integration with when expressions.

In Conclusion (The Royal Proclamation)

Kotlin sealed classes provide a powerful way to define restricted class hierarchies, offering more flexibility and type safety than Java enums. With the addition of sealed interfaces, Java is catching up, but Kotlin’s sealed classes remain a more mature and expressive solution. So, if you’re ready to rule your code with a royal decree, embrace the power of sealed classes!

P.S. If you’re a Java developer still relying on enums, don’t worry. You can always explore sealed interfaces for more flexibility. It might not be as regal as Kotlin’s sealed classes, but it’s a step towards a more controlled inheritance kingdom!

Kotlin vs Java (21 Part Series)

原文链接：Kotlin Sealed Classes vs. Java Enums (and Sealed Interfaces!): A Tale of Limited Hierarchies

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