What is final in Java?

The final keyword in Java is used to restrict the user.

Depending on where it's used, it means:

1. final Variable (Constant)

public class FinalVariableExample {
    public static void main(String[] args) {
        final int SPEED_LIMIT = 100;  // constant
        System.out.println("Speed Limit: " + SPEED_LIMIT);

        // SPEED_LIMIT = 120; //  Error: cannot assign a value to final variable
    }
}

Once a variable is declared final, its value cannot be changed.

2. final Method (Prevent Overriding)

class Animal {
    final void sound() {
        System.out.println("Animals make sound");
    }
}

class Dog extends Animal {
    //  Cannot override final method
    // void sound() {
    //     System.out.println("Dog barks");
    // }
}

public class FinalMethodExample {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();  // Output: Animals make sound
    }
}

Declaring a method final ensures that child classes cannot override it.

3. final Class (Prevent Inheritance)

final class Vehicle {
    void drive() {
        System.out.println("Vehicle is moving");
    }
}

//  Error: Cannot inherit from final class
// class Car extends Vehicle {}

public class FinalClassExample {
    public static void main(String[] args) {
        Vehicle v = new Vehicle();
        v.drive();
    }
}

A final class cannot be extended by any other class.

Bonus: Final Reference Variable (Object)

class Student {
    int id;
    Student(int id) {
        this.id = id;
    }
}

public class FinalReferenceExample {
    public static void main(String[] args) {
        final Student s = new Student(101);
        System.out.println(s.id);

        s.id = 202;        //  Allowed: modifying object content
        // s = new Student(303); //  Not allowed: reassigning final reference
    }
}

You can modify the fields of a final object, but cannot reassign it to a new object.

Summary