Aroma 3: Class Hierarchy, Inheritance and Polymorphism

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Aroma 3 Class Hierarchy, Inheritance and
Polymorphism

• Special Topics Java

Dr. Tim Margush Department of Mathematics and
Computer Science The University of Akron
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Counter Package

• package Counters
• The package statement indicates that this class
  will be part of a package named Counters.
• A package is a collection of closely related
  class files. All files in the Counters package
  must be stored in a directory named Counters.

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Counter Root Class

• Abstract class
• can not be used to instantiate objects
• contains methods that must be implemented in
  subclasses
• Public scope
• clients know about Counters

• Instance variables
• initialCount
• theCount
• Methods
• constructors
• count()
• reset()
• setCount(int)
• int getCount()

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Instance Variables

• abstract public class Counter
• protected int theCount
• private int initialCount
• Subclasses will contain these variables
• can directly access theCount
• can not directly access initialCount
• Clients have no knowledge of these

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Constructors

• Need to specify an initial counter value
• protected Counter(int initialCount)
• theCount this.initialCount initialCount
• Default constructor
• protected Counter()
• this(0) //indirecting call

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Methods

• public int getCount()
• return theCount
• protected void setCount(int setTo)
• theCount setTo
• public void reset()
• setCount(initialValue)
• abstract public void count()

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A Limited Counter Class

• A counter with built-in counting limit
• abstract class LimitedCounter
• extends Counter
• protected int limitValue
• protected LimitedCounter(int initialValue, int
  limit)
• super(initialValue) limitValuelimit
• public boolean isAtLimit()
• return theCountlimitValue

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LimitedCounter is Abstract

• There is still no implementation for count()
• should we count up or count down?
• what should we count by? 1, 2, ...
• what happens if count() called at limit?
• exception
• rollover
• ignore

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Real Simple Counters

• UpCounter, DownCounter
• public subclasses of Counter
• public class UpCounter extends Counter
• public UpCounter(int initialValue)
• super(initialValue)
• public void count()
• theCount

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Real Limited Counters

• LimitedUp, LimitedDown
• public class LimitedDown
• extends LimitedCounter
• public LimitedDown(int initial, int lowLim)
• super(initial, lowLimltinitial?lowLiminitial)
• public void count()
• if theCountgtlimitValue) theCount--

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Real Rollover Counters

• Rollover, RollUnder
• public class Rollover
• extends LimitedCounter
• public Rollover(int initial, int hiLim)
• super(initial, hiLimgtinitial?hiLiminitial)
• public void count()
• if theCountltlimitValue) theCount
• else reset()

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Warning Counter

• Public class WarningCounter extends LimitedUp
• Overrides LimitedUp count method to throw an
  exception if you try to count past the limit
• throw new CounterException("Attempt to count past
  limit!")

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CounterException Class

• package Counters
• class CounterException extends RuntimeException
• public CounterException( String reason )
• super( reason ) // end constructor
• //end class

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LimitedUpSkip Counter

• public class LimitedUpSkip extends LimitedUp
• private int skipValue
• public LimitedUpSkip ()
• this(0,20,5) // end default constructor
• public LimitedUpSkip ( int initial, int max, int
  skip )
• super( initial, max ) skipValueskipgt0?skip1
• public void count()
• for (int i1 iltskipValue i) super.count()

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UpSetCounter Class

• public class UpSetCounter extends UpCounter
• public void setCount(int newCount)
• super.setCount(newCount)
• This exposes setCount as public, making it
  available to clients
• setCount was defined in Counter
• access has been relaxed to public

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UpDownCounter Class

• Not part of Counters package
• public class UpDownCounter extends
  Counters.UpCounter
• public UpDownCounter(int initialValue)
• super(initialValue) //fails if package scope
  in UpCounter
• public void unCount()
• theCount-- //fails if package scope

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The Counter Hierarchy
Counter
UpCounter
UpDownCounter
UpSetCounter
LimitedUp
LimitedCounter
Abstract Classes
WarningCounter
LimitedUpSkip
Package Classes
RollOver