Understanding Final Classes in Java: Definition and Usage

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質問：

What is the final class in Java?

回答：

In Java, the final keyword can be applied to classes, methods, and variables, and it serves different purposes in each context. When a class is declared as final, it means that the class cannot be subclassed or extended. This is useful when you want to prevent inheritance for security reasons, to maintain immutability, or to ensure that the behavior of the class remains unchanged.

Here’s an example of a final class in Java:

public final class FinalClass {
    // Class implementation
    public void display() {
        System.out.println("This is a final class.");
    }
}

// The following code would result in a compilation error
// class SubClass extends FinalClass {
//     // This will not compile
// }

In this example, FinalClass is declared as final, so any attempt to create a subclass (like SubClass) will result in a compilation error.

Using final for classes can help in designing a robust API and can also lead to performance optimizations by the Java compiler and JVM, as the final classes can be optimized more aggressively than non-final classes.

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