密封類和接口通過限制哪些類或接口可以繼承或?qū)崿F(xiàn)它們，增強(qiáng)了Java中對繼承的控制。問題在於，在引入密封類之前，任何類都可以繼承非final類，任何類都可以實(shí)現(xiàn)接口，導(dǎo)致潛在的安全性和維護(hù)性問題。使用密封類時，開發(fā)者必須顯式聲明允許繼承或?qū)崿F(xiàn)的子類列表，例如public sealed class Shape permits Circle, Rectangle, Triangle {}，確保只有指定的類能夠擴(kuò)展或?qū)崿F(xiàn)。關(guān)鍵規(guī)則包括：所有允許的子類必須在同一模塊或包中，並且必須顯式聲明。密封類適用於需要限制繼承、構(gòu)建領(lǐng)域模型、設(shè)計(jì)API以及使用模式匹配等場景，為類型層次結(jié)構(gòu)提供更安全、更清晰的設(shè)計(jì)。

Sealed classes and interfaces in Java are features introduced to give developers more control over inheritance . In short, they let you specify exactly which classes or interfaces can extend or implement them. This is useful when you want to limit who can inherit from a class or implement an interface, making your code safer and more predictable.

What problem do sealed classes solve?

Before sealed classes, any class could extend another class (unless it was marked final ), and any class could implement an interface. That openness can sometimes lead to issues:

• Unexpected subclasses breaking assumptions
• Harder-to-maintain hierarchies because anyone can add new implementations
• Difficulty reasoning about all possible subtypes

With sealed classes and interfaces, you can explicitly declare which classes are allowed to extend or implement them, no guessing involved.

How do you define a sealed class or interface?

You use the sealed keyword after the class or interface declaration, followed by the permits clause listing the allowed subclasses or implementations.

 public sealed class Shape permits Circle, Rectangle, Triangle {
    // class body
}

In this example, only Circle , Rectangle , and Triangle can extend Shape . No other class can.

You can also spread the subclasses across different files, but they must be in the same module or package, depending on how your project is structured.

If the subclasses are in the same file, they can be declared as non-sealed or final , depending on whether you want them to allow further extension.

Example:

 public sealed interface Animal permits Dog, Cat {}

public final class Dog implements Animal { ... }
public non-sealed class Cat implements Animal { ... }

Here, Dog can't be extended further because it's final , while Cat can be if needed.

Key rules and restrictions

There are a few important things to keep in mind:

• The permitted subclasses must be in the same module (if using modules) or package.
• All permitted subclasses must explicitly extend or implement the sealed class or interface.
• You can't have a sealed class without specifying the permits clause unless all subclasses are in the same file.

Also, sealed classes work well with pattern matching for switch and instanceof , especially when you know all the possible subtypes — that makes exhaustive checks easier.

When should you use sealed classes?

Use sealed classes when:

• You want to restrict which classes can extend or implement something
• You're building domain models where only certain types are valid
• You're designing APIs and want stricter control over extensibility
• You're using pattern matching and want the compiler to help ensure completeness

They're particularly helpful in modeling algebraic data types (like enums but with more flexibility).

So, sealed classes and interfaces are a neat way to tighten up inheritance and make your type hierarchies more intentional and safe. They don't replace final or regular inheritance, but they offer a middle ground where you can say, "Only these specific types are allowed."

基本上就這些。

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