The final keyword in Java is a non-access modifier used for variables, methods, and classes. It means "cannot be changed".

🔹 1. final Variable

  • Once assigned, its value cannot be changed.
  • Used for constants. 
final int MAX_USERS = 100;
// MAX_USERS = 200; ❌ Error: can't assign a new value

✅ Use it when you want a value to stay constant (like configuration values, limits, etc.)

🔹 2. final Method

  • A method declared as final cannot be overridden by subclasses. 
class Parent {
    final void display() {
        System.out.println("Parent display");
    }
}

class Child extends Parent {
    // void display() ❌ This will cause an error
}

✅ Use it when you want to protect method behavior from being changed in child classes.

🔹 3. final Class

  • A class marked as final cannot be inherited. Eg: 
final class Vehicle {
    // ...
}

// class Car extends Vehicle ❌ Error: can't extend final class

✅ Use it when you want to prevent inheritance, usually for security or design reasons (e.g., String class is final).

🔹 Bonus: final with reference types

If you declare an object as final, the reference cannot be changed, but the object’s internal state can still be changed.
EX:

final List list = new ArrayList<>();
list.add("Hello"); // ✅ okay
list = new ArrayList<>(); // ❌ error