What is a final keyword in Java? How is it used?

In Java, the final keyword is used to indicate that a variable, method, or class cannot be changed or overridden. Once a variable or reference is marked as final, its value cannot be modified. Similarly, once a method or class is marked as final, it cannot be overridden by any subclasses.

Here are some examples of how the final keyword can be used:

public class MyClass {
    public static final int MAX\_VALUE = 100;
    
    public void doSomething(final int x) {
        final int y = 10;
        // MAX\_VALUE = 200; // error: cannot modify final variable
        // x = 20; // error: cannot modify final parameter
        // y = 30; // error: cannot modify final local variable
    }
}

In this example, we declare a final variable MAX_VALUE that cannot be modified. We also declare a final parameter x and a final local variable y, both of which cannot be modified within the method.

public class MyClass {
    public final void doSomething() {
        // method implementation
    }
}
    
public class MySubclass extends MyClass {
        // error: cannot override final method
        // public void doSomething() {
        // // method implementation
    // }
}

In this example, we declare a final method doSomething() in the MyClass class. This method cannot be overridden by any subclasses, as attempting to do so will result in a compilation error.

public final class MyClass {
    // class implementation
}

public class MySubclass extends MyClass {
    // error: cannot inherit from final class
}

In this example, we declare a final class MyClass that cannot be subclassed. Any attempt to subclass MyClass will result in a compilation error.

In summary, the final keyword is used to indicate that a variable, method, or class cannot be changed or overridden. It is commonly used for constants, immutable objects, and to prevent unintended changes to method or class behavior.