Final keyword (final variables, methods, classes)

The final keyword restricts modification. Its meaning depends on where it is applied.

final variables

Cannot be reassigned after initialization. They are constants.

final double PI = 3.14159;
PI = 3.14; // compilation error

// Blank final (initialized in constructor)
final int MAX_SIZE;
MyClass() {
    MAX_SIZE = 100;
}

final methods

Cannot be overridden by subclasses.

class Parent {
    final void display() {
        System.out.println("Cannot override");
    }
}
class Child extends Parent {
    // void display() { } // error: cannot override
}

final classes

Cannot be extended (subclassed).

final class ImmutableClass {
    // ...
}
// class SubClass extends ImmutableClass { } // error

Use cases:

• final variables: for true constants (often static final)
• final methods: to prevent breaking internal logic
• final classes: for immutability (e.g., String, Integer)

Note: final does not make objects immutable; only the reference is fixed.