6.21     Exercises

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1. To make a variable shared across all objects of a given class, what keyword do you use?

2. Objects are always passed by reference: true or false?

3. Given that dog is a class, poodle is a class inheriting from dog, and $poppy is an object of class poodle, would $poppy be considered an instance of the class dog when using the instanceof keyword?

4. To list all classes currently available to your script, what function should be used:

   • a) list_classes()

   • b) get_classes()

   • c) list_declared_classes()

   • d) get_declared_classes()

5. Instanceof and is_subclass_of() are identical: true or false?

6. Which of the following statements are true about interfaces:

   • a) They allow you to force classes to implement a set of functions

   • b) They can be based on other interfaces

   • c) They can be based on other classes

   • d) They are faster than using class inheritance

   • e) All of the above

7. When must the "override" keyword be used:

   • a) Whenever a child class overrides a function inherited from its parent

   • b) Whenever a child class inherits from an interface

   • c) Whenever a child class implements an interface and inherits from a parent class

   • d) Always

   • e) Never

8. Objects of an abstract class can only be created when the class is declared final: true or false?