Introduction to Inheritance

• I saw one OO source that claimed this one of the foundations to OOP.
• This is chapter 10. Read it twice.
• Inheritance is the ability to extend one class to form a new class.
• He says "Without inheritance, classes would simply be data structures with associated behaviors."
• Some chapter 5 stuff
  • Read starting on page 175.
  • Composition or building one class with other classes as members can be expressed as a has a relationship.
  • Or an is a part of
    • There are several classes of this.
    • A pond has a frog.
      • Think vector of frogs as part of the pond class.
        • There might be no frogs in the pond when it is first dug.
      • The frog can exist outside of the pond.
      • The pond can exist without a frog.
      • This is aggregation
      • 
    • A frog has a heart
      • Think of a member component.
      • When the frog is created, the heart member is created.
      • When the frog is destroyed the heart member is also destroyed.
      • This is Composition
      • 
    • Of course, these do depend on the level of abstraction.
      • In a frog-pond simulation, you might not allow a frog to exist outside of a pond.
      • In a dissection simulation, you might allow a frog's heart to exist outside of a frog.
    • In general you can discuss composition without either of these.
      • But one will probably apply.
      • You can specify a count on one or both sides.
      • 
      • Here 0 to n frogs swim in 1 pond.
      • It could include a * for unlimited.
      • You will encounter diagrams like this in database as well.
  • Inheritance
    • This models an is-a
    • In this case, we are not building something up, we are extending something.
    • An Amphibian
      • Has four legs.
      • Have young through eggs
      • Have lungs, however most can breathe through their skin.
    • A frog is an amphibian
      • Without a tail.
      • Skin that secrete something (distasteful to toxic)
    • A bullfrog is a frog
    • 
    • As we move up this tree, we become more specialized.
    • As we move down, we become more generalized.
  • With inheritance we can
    • Add functionality
      • A frog adds the ability to ooze goo from its skin.
      • A bullfrog adds an extra strong Biomechanical tongue.
    • Replace functionality
    • Adding Properties
    • Replacing properties.
  • We can also employ polymorphous when we have inheritance.
• You need to be careful not to define a hierarchy based on your perception
  • He has a couple of examples on page 182.
  • These are hierarchies in our world.
    But probably not in the programming world.
    • A CEO really is not a base class.
    • He says to see if the different classes you design have different functionality.
      • If not, they are not hierarchies, just organizational structures.
    • This is where I have trouble making up examples, exercises, programs, ...
    • Sometimes you might violate "real world" hierarchies
      • You might organize things by how they move
      • IE a dolphin and a penguin might be in a swimming class.
      • They do share functionality.
  • Multiple Inheritance allows mixing classes
    • A penguin is derived from swimming animals ad from walking animals.
    • But this leads to coding problems.
    • We will discuss this later.