Session 17

1
Session 17

• CountedAccumulator
• (Introducing Inheritance)

2
Test 1

• Monday, March 2nd
• Chapters 1-5 in the book
• Test format closed book, closed notes

3
Study Hints

• 1. Go through each chapter and make a list of
  the vocabulary terms that we have discussed.
  Could you explain what each means and how they
  relate to each other?
• 2. Review the slides I used during class and
  consider the points I have emphasized either
  directly on the slides or in addition to the
  slides.

4
Study Hints

• 3. Review the variants of the programs we have
  worked with (Accumulator, DOME, etc). I will
  provide you with any code I expect you to
  use/modify, but you dont want to waste test time
  reviewing how the code actually works.
• 4. Read through the questions at the end of
  the chapters. Play the "second guess the
  instructor game. That is, think about what you
  reviewed in the previous steps and think about
  how well each question would fit into these
  topics. Some of my questions will come from
  these questions. Others will be modifications to
  these questions based more on what I have chosen
  to emphasize.

5
Activity

• We need a new kind of object, an Accumulator that
  counts how many operations it executes. Lets
  call this class CountedAccumulator.
  
• Write the code for the CountedAccumulator class.
• It responds to all the same messages as a regular
  Accumulator and also responds to an
  operationsExecuted() message, by returning its
  count.

6
Adding Behavior to a Class

• Any time that we need to add behavior to a class
  we have at least three options
  
• Add code to the class itself, keeping the
  original class.
  
• Copy all the old code into a new class and add
  code to this new class.
  
• Create a subclass that extends the original
  class' behavior.

7
Pros and Cons

• Add code to the class itself, keeping the
  original class.
  
• Pros Quick. Convenient. Simple.
• Cons May change the behavior of the class.
  Thus, it isnt always an option.
  

8
Pros and Cons

• Copy all the old code into a new class and add
  code to this new class.
  
• Pros Quick. Convenient. Simple.
• Cons Duplicated code. Error trap! Error trap!

9
Pros and Cons

• Create a subclass that extends the original
  class' behavior.
  
• Pros Doesnt break existing code. Virtually
  eliminates duplicate code. Provides the most
  flexibility.
  
• Cons Slightly more time consuming.

10
Developing an Extended Class

• There are typically four steps in developing an
  extended class.
  
• declare the class
• declare the new data
• create the constructors  
• adjust the methods

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Developing an Extended Class

• declare the class
• public class CountedAccumulator extends
  Accumulator

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Developing an Extended Class

• declare the new data
• private int numberOfOperations

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Developing an Extended Class

• create the constructor        
• public CountedAccumulator ()
• super()
• numberOfOperations0

14
A solution

• Technically, at this point we can compile this
  and use CountedAccumulator (although at this
  point it still behaves like an Accumulator).
  
• We need to set up the AddingFrame so that it
  works with a Counted Accumulator rather than a
  regular Accumulator.  We do this in the AdderApp
  class for simplicity.
• Accumulator a new CountedAccumulator()
• AddingFrame f new AddingFrame(a)

15
Developing an Extended Class

•  adjust the methods
• Leave inherited methods alone
• Modify/Override inherited methods
• Add completely new methods

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Developing an Extended Class

•   Leave inherited methods alone
• clear() and prepareForNextNumber() are both
  inherited from Accumulator and there is no need
  to change them.

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Developing an Extended Class

• Modify/Override inherited methods
• plus() and minus() are inherited, but they don't
  do what we want them to. 
  
• We can make them do more without completely
  replacing the code however.
  
• public void plus()
• super.plus()
• numberOfOperations

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Developing an Extended Class

• Add completely new methods
• We need an accessor method for numberOfOperations
  
• public void operationsExecuted()
•         return numberOfOperations

19
A Solution

• After putting it all together we develop a
  version of CountedAccumulator
  
• CODE 04_CountedAccumulator.java