Dependencies inside an object

Question

I have this code

class Duck {
  protected $strVocabulary;
  public function Learn() {
   $this->strVocabulary = 'quack';
  }

  public function Quack() {
   echo $this->strVocabulary;
  }
}

The code is in PHP but the question is not PHP dependent. Before it knows to Quack a duck has to Learn.

My question is: How do I make Quack() invokable only after Learn() has been called?

Answer 1

No, that does not violate any OOP principle.

A prominent example is an object who's behavior depends on whether a connection is established or not (e.g. function doNetworkStuff() depends on openConnection()).

In Java, there is even a typestate checker, which performs such checks (whether Duck can already Quack()) at compile time. I often have such dependencies as preconditions for interfaces, and use a forwarding class whose sole purpose is protocolling and checking the state of the object it forwards to, i.e. protocol which functions have been called on the object, and throw exceptions (e.g. InvalidStateException) when the preconditions are not met.

A design pattern that handles this is state: It allows an object to alter its behavior when its internal state changes. The object will appear to change its class. The design pattern book from the Gang of Four also uses the example above of a network connection either being established or not.