7.5. Casting¶

When one class extends another, as HouseCat extends Cat, a field or local variable of the type of the base class may hold an object that is of the type of the child class.

In other words, this is allowed:

Cat suki = new HouseCat("Suki", 8);

This is acceptable because a HouseCat is a Cat. Furthermore, when we call methods on such an object, the compiler is smart enough to determine which method it should call. For example, the following call to noise() will call the version defined in HouseCat:

// Calls HouseCat's noise() method
suki.noise(); // Hello, my name is Suki!

This only works for methods that are declared in the base class, however. If we have a HouseCat object stored in a Cat variable or field, then it is not allowed to call methods that are only part HouseCat.

// Results in a compiler error, since Cat
// doesn't have such a method
suki.isSatisfied();

Here, isSatistfied() is defined in HouseCat, and there is not a corresponding overridden method in Cat. If we were really, really sure that we had a Cat that was actually a HouseCat, we could call such a method by first casting:

// As long as suki really is a HouseCat, this works
((HouseCat) suki).isSatisfied();

The danger here is that if suki is in fact not a HouseCat (it was declared only as a Cat, after all) then we’ll experience a runtime exception. A runtime exception is an error that occurs upon running the program, and is not found by the compiler beforehand. These are dangerous, and situations where they might come up should be avoided. So you should only cast an object to another type when you are very sure that it’s safe to do so.

Storing objects of one type (e.g. HouseCat) in a variable or field of another “compatible” type (e.g. Cat) is an example of polymorphism. Polymorphism is another one of the pillars of OOP and we’ll have more to say about it in a future lesson.

public class Message {
   private boolean friendly = true;
   private String language;
   private String text;

   public Message(String aLanguage, String aText) {
      language = aLanguage;
      text = aText;
   }

   public boolean getFriendly() {
      return friendly;
   }

   public String getLanguage() {
      return language;
   }

   public String getText() {
      return text;
   }
}

public class Greeting extends Message {
   private boolean waving;

   public Greeting(String aLanguage, String aText) {
      super(aLanguage, aText);
   }

   public void wave() {
      waving = true;
   }
}