Final

The final keyword is used to declare constants or prevent modification.
It’s meaning depends on where its used.

Final Usage Summary

Use With	Effect
Variable	Value cant be changed (constant)
Method	Method cant be overridden
Class	Class can’t be extended/inherited

Example

Final Variable

final int x = 20;
x = 30; // Compilation error

Final Method

class Animal {
    final void sleep() {
        System.out.println("Sleeping...");
    }
}
 
class Dog extends Animal {
    // Cannot override sleep()
    // void sleep() { ... } → Error
}

Final Class

final class Utility {
    // no one can extend this class
}
 
class MyUtil extends Utility {} // Error

Final Object Reference

final List<String> list = new ArrayList<>();
list.add("Hello");        // Allowed
list = new ArrayList<>(); // Error

Final object reference = can’t reassign.
But the object itself can still be modified (unless immutable).

Tip

Be ready to explain:

• difference between final on primitive vs object reference,
• when to use final for immutability, security or clarity,
• why String is final in Java (thread-safety, caching, security).

---

Parent: _Core