Understanding the `final` Keyword in Java

What is the final variable in Java?

In Java, the final keyword is used to declare constants or to restrict the modification of variables, methods, and classes. When a variable is declared as final, it means that once it has been assigned a value, it cannot be changed (i.e., it becomes a constant). Here are the different contexts in which final can be used:

1. Final Variables: When a variable is declared as final, it can only be assigned once. After that, any attempt to change its value will result in a compilation error.

   final int x = 10;
   // x = 20; // This will cause a compilation error
   

2. Final Methods: A method declared as final cannot be overridden by subclasses. This is useful when you want to prevent modification of a method's behavior in derived classes.

   class Base {
       final void display() {
           System.out.println("This is a final method.");
       }
   }
   
   class Derived extends Base {
       // void display() { // This will cause a compilation error
       //     System.out.println("Trying to override.");
       // }
   }
   

3. Final Classes: A class declared as final cannot be subclassed. This is useful for creating immutable classes or to prevent inheritance.

   final class FinalClass {
       // Class implementation
   }
   
   // class SubClass extends FinalClass { // This will cause a compilation error
   // }
   

4. Final Parameters: You can also declare method parameters as final, which means that the parameter cannot be reassigned within the method.

   void myMethod(final int param) {
       // param = 5; // This will cause a compilation error
   }
   

In summary, the final keyword in Java is a way to enforce immutability and prevent changes to variables, methods, and classes, providing a level of safety and predictability in your code.