How can external code modify the state of a class?

External code can modify the state of a class in several ways:

1. Through public methods (setters) — the class provides methods to change its fields, for example setName(String name).
2. Through public fields — if the class fields are declared as public, external code can modify them directly (not recommended due to violation of encapsulation).
3. Using design patterns — for example, through the Builder pattern or methods that return mutable objects.

Example in Kotlin (Android):

class User {
    var name: String = ""
        private set // restricts external direct modification

    fun updateName(newName: String) {
        name = newName
    }
}

val user = User()
user.updateName("Ivan") // changing state via method

Thus, the best approach is to provide controlled access to modify the state through class methods, maintaining encapsulation.