在Java中，使用final關鍵字修飾類意味著該類不能被繼承。具體原因包括：1. 強制保持特定行為不變；2. 保護敏感邏輯或安全相關代碼不被修改；3. 確保線程安全或不可變性（如String類）。適用場景主要有：1. 安全敏感類；2. 不可變類；3. 工具類或輔助類。若嘗試繼承final類，則會引發(fā)編譯錯誤，例如定義final類Animal後，Dog類嘗試繼承將導致編譯失敗。此外，final類的方法無法被重寫，因為不存在繼承關係。

In Java, using the final keyword for a class means that the class cannot be subclassed. That's it—no other class can extend a final class.

Why would you make a class final?

The main reason is to prevent inheritance. This might matter if:

• You want to enforce a specific behavior that shouldn't be changed.
• The class has sensitive logic or security-related code that shouldn't be altered.
• You're trying to ensure thread safety or immutability (like with String in Java).

For example, the String class in Java is final . That means nobody can create a subclass of String and override its behavior. This helps keep things consistent and secure across the language.

When should you use final on a class?

You don't need to mark every class as final, but there are good cases for doing so:

• Security-sensitive classes – like those handling encryption or user permissions.
• Immutable classes – such as value objects where changing behavior via inheritance could break assumptions.
• Utility or helper classes – sometimes these are not meant to be extended at all.

So if your class is designed to be a "leaf" in the class hierarchy and you never intend for anyone to extend it, marking it as final makes that clear and enforces it at compile time.

What happens if someone tries to extend a final class?

If a class is declared final , any attempt to extend it will result in a compilation error.

For example:

 final class Animal { }

class Dog extends Animal { } // Compilation error!

This won't compile because Dog tries to inherit from a class that explicitly forbids inheritance.

It's also worth noting: making a class final doesn't affect whether its methods can be overridden — because if the class can't be extended, there's no opportunity to override anything in the first place.

So yeah, final on a class is pretty straightforward — it blocks subclassing. It's not always needed, but when used appropriately, it can help protect design intent and improve robustness.

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