Programming - 5 - Inheritance

Question 1

Definition of public, private or protected class members

Answer 1

• Public members can be accessed from outside the class, i.e. by objects of other classes that have a reference to an object of this class
• Private members can only be accessed from methods of this class
• Protected members can only be accessed from methods of this class or of any derived classes through inheritance.

Question 2

Get and Set Methods

Answer 2

• Private or protected instance variables that need to be accessed externally typically have associated and methods
• Difference with maing the methods public is that:
  
  • using get and set methods controls how these variables are accessed

Question 3

What is at the top of the Inheritance hierarchy?

Answer 3

• Top of the hierarchy always the Java class Object
• Every class that we define without inheritance inherits from Object, with the compiler setting implicitly the superclass to Object
• So effectively there is a class inheritance tree with Object the root
• A key method supported by Object is toString which subclasses can redefine (override)

Question 4

What is Data Encapsulation

Answer 4

Implementation details of a class are kept hidden fro the user (getters and setters are used)

Question 5

What is an Abstract Class?

Answer 5

• Superclass that will never be instantiated only subclassed (e.g. class Shape)
• Incomplete, only providing methods to be redfined in subclasses.
• Key: If at least one method is “abstract”, class should be “abstract” !

Question 6

What is an interface?

Answer 6

• Abstract type used to specify behaviours that classes must implement
• variables are final, all methods have to be defined (for abstract class, they don’t)
• classes can extend multiple interfaces
• essentially an abstract class but with all the methods having to be defined by subclasses.

Question 7

What is polymorphism?

Answer 7

• Object passes multiple is-a tests
• Example
  
  • public interface vegetarian {};
  • public class animal {};
  • public class Deer extends Animal implements vegetarian{};

Question 8

What sort of relationship does inheritance represent?

Answer 8

is-a

Question 9

What sort of relationship does composition represent?

Answer 9

has-a