Ch.3 Classes

1
Ch.3 Classes Stepwise Refinement

• STEP 1 Define a new class of robot (see next
  slide)
• When designing a new class (whether thats
  robots, cars, bank accounts, etc.), the first
  question we are asking ourselves is What can I
  steal? !! In other words, one big benefit of
  OOD is the concept of code-reuse. There is no
  need to reinvent the wheel. (by the way, that
  doesnt mean to copy your friends lab!! ?)

2

• import kareltherobot.
• public class MileWalker extends UrRobot
• public MileWalker (int st, int av, Direction
  dir, int beeps)
• super(st, av, dir, beeps)
• public void moveMile( )
• move() move() move() move()
• move() move() move() move()

subclass
superclass
constructor
invokes superclass constructor
note no object names preceding methods (why
not?) - lets look at client code to see why
INHERITANCE
3
Inserting MileWalker into the Inheritance
Hierarchy
UrRobot
If you have an object (say, bob) of type
MileWalker, what methods are available to
bob? What if bob were of type UrRobot?
move()
turnLeft()
pickBeeper()
putBeeper()
turnOff()
MileWalker
the is-A relationship a MileWalker is-A UrRobot
moveMile()
4
STEP 2write application(client) to use new
class(server)(a.k.a. a driver)

• import kareltherobot.
• public class MileWalkerDriver implements
  Directions
• public static void main(String args)
• MileWalker bob new MileWalker(2, 1, East,
  0)
• bob.moveMile() // new instruction
• bob. move() // inherited instruction
• bob.turnOff() // inherited instruction

5
Misc. Note(dont sweat this its not computer
science)

• These 4 method invokations may be necessary and
  may be placed first within the main() of the
  driver
• World.reset()
• World.readWorld(c\\first.kwld")
• World.setDelay(50)
• World.setVisible(true)
• Alternatively you can place them in a static
  block (no need to understand what a static
  block is) within your driver class

6
STEP 3

• Put each class in its own file making sure that
  the file name matches the class name Exactly
  (convention class names begin with a capital
  letter, method names begin with a lowercase
  letter if an identifier combines several words,
  use the interCap technique. We will follow
  convention.)
• Well now demo the whole process in BlueJ

7
Now You Try!

• we want a BetterTurnerRobot class
• turnRight, turnAround, stepBackward

8
Now, try something a bit more!

• Design an HBot class on paper right now (yes, it
  should do the same thing as our non-class version
  in Ch.2)
• In 10 minutes, well pass the wireless keyboard
  mouse and well build/test the class together
• In addition, where does it go in the Inheritance
  Hierarchy?
• Why is putting all the same code into one method
  within a class (encapsulation) better than just
  leaving it in the driver i.e., what do we
  gain?(lets informally discuss before I give you
  the fancy cs language)

9
Benefits of Encapsulation

• So, we just learned that encapsulation promotes
  Code Reuse. By putting the code into a method, we
  no longer need to write that code again. In
  addition, from the clients point of view, she is
  no longer concerned with how to draw the H
  shes the beneficiary of a concept called
  Abstraction (can focus on WHAT, not HOW).
• In general, if you find yourself doing a
  cut-and-paste, then there is a better way to do
  things i.e., use procedural abstraction and
  abstract out the common code, putting it into a
  method
• In that light, how should we now modify our
  current 1-method(if thats how you wrote it)
  HBot?

10
Improving HBot

• Yep, find the common code and create other
  methods.
• drawLine() and turnRight() might be what you
  choose you might choose others depending on how
  you see the problem being decomposed (stepwise
  refined)
• Should they have public or private visibility?
• That depends on whether we believe a client would
  be calling drawLine() and turnRight() lets
  discuss public/private, then you guess/justify
• Ill argue, No. Ill argue like this the name of
  the class is HBot so I (the client) am trying
  to have some object draw Hs for me how that
  object gets it done is of no concern of
  mine(abstraction) so I dont need (and
  shouldnt be allowed) to see the other
  helper/auxiliary methods. Therefore, they should
  be private, helper-like methods for the class
  use only.

11
Stepwise Refinement

• technique for writing modules which are concise,
  correct, easy to read/modify/understand
• Would a general contractor just start building a
  house or would she break up the task into
  foundation, frame, electrical, plumbing, etc.?
  Makes sense, doesnt it. Explain why from the
  contractors view use our cs terms weve been
  learning.
• write main task first, breaking up the BIG task
  into smaller tasks (using methods) then take
  each method one at a time and also break it up
  --- continue until each method is compact and
  singular in focus (cohesion)
• Look back at what we just did do you see this
  re-factoring?

12
Practicing Stepwise Refinement

• Lets write a class called DiamondPlanter
  together using stepwise refinement. Its like
  Harvester except the field is diamond shaped.
  There are always 4 beepers on a diagonal. Assume
  the robot is facing North to begin, has 16
  beepers, and is standing on the corner where the
  bottom of the diamond is to be.
• What are we asking ourselves first?
• Now, using some sort of pseudocode, write the top
  level method(call it, plantDiamond() ). While
  writing it, pretend any helper methods youd like
  to use already exist (abstraction) and work
  already (automagically, if you will). After we
  pseudocode plantDiamond(), well take each helper
  method in turn and repeat this stepwise-refinement
  process until we have cohesion.

13
Debriefing DiamondPlanter

• So, we wrote DiamondPlanter. More than likely we
  wrote a turnRight() and maybe even a turnAround()
  to help us plant. Anyone want to make any
  comments about that?
• I dont know about you, but I found it ANNOYING
  to write the same thing again! Lets look at the
  Inheritance Hierarchy and see if we can come up
  with a solution. Lets discuss possible solutions
  before going on

14
Improving overall object design
Discuss what our modifications would be to the
HBot and DiamondPlanter classes in terms of
syntax - in terms of concepts weve been
discussing
UrRobot
BetterTurnerBot
turnRight()
turnAround()
HBot
DiamondPlanter
15
Now YOU try! But be efficient!
design a robot class that would be conducive to
solving the following diagrammed situation (robot
should climb and pick up all beepers always
3/stair) also, different clients need to be able
to climb different numbers of stairs
starts off facing East
When designing, keep in mind everything weve
been discussing.
16
Why create a Class?

• Why reinvent the wheel? Youre allowed to be lazy
  in my class but you have to be smart to be
  lazy!
• Code Reuse
• Abstraction free your mind from the irrelevant
  and work on the relevant!
• Ex. If Im going to write a system to have a bot
  climb stairs in several buildings, Im going to
  use the StairClimber class so I can call
  climbStair() I can work on a bigger/better/harde
  r problem and free my mind from the irrelevant
  details of taking a step and picking beepers

17
Why use Inheritance?

• You get stuff for free! You can be lazy! (cs
  term?)
• Use things without knowing/caring how they work!
  (cs term?)
• Why reinvent the wheel! (cs term?)
• Localize changes to one class/method
  (localization)

18
Sample Inheritance QuestionsHow many times does
each Bot below move?

• public class MysteryBot1 extends UrRobot /
  constructor not shown /public void step1()
  move() public void move() super.move()
  super.move()

• public class MysteryBot2 extends MysteryBot1
  / constructor not shown /public void
  step1() move() public void move()
  super.move() super.move()

MysteryBot1 john new RobotStuff(10, 10 , North,
0)john.step1() // where is the bot now? john
new MoreRobotStuff(10, 10 , North,
0)john.step1() // where are both bots now?
19
Another Inheritance QuestionGive the state
(location and Direction) of each Bot below?

• public class ABetterBot extends UrRobot /
  constructor not shown /public void step1()
  move() public void step2() turnLeft()
  

• public class AnEvenBetterBot extends ABetterBot
  / constructor not shown /public void
  step1() move()
• super.step1() step2() public
  void step2() turnLeft() super.step2()

ABetterBot ucla new ABetterBot(10, 10 , North,
0)ucla.step1()AnEvenBetterBot usc new
AnEvenBetterBot(10, 10 , North, 0)usc.step1()
20
A final Inheritance QuestionGive the state
(location and Direction) of each Bot below?

• public class TestBot extends UrRobot /
  constructor not shown /public void step1()
  move()
• step2() public void step2()
  turnLeft()

• public class HarderTestBot extends TestBot /
  constructor not shown /public void step1()
  move()
• super.step1() public void step2()
  turnLeft() super.step2()

TestBot ucla new TestBot(10, 10 , North,
0)ucla.step1()HarderTestBot usc new
HarderTestBot(10, 10 , North, 0)usc.step1()
21
Is-A questions

• A is-a Letter
• Letter is-a Symbol
• Semicolon is-a PunctuationMark
• _at_ is-a Symbol
• PunctuationMark is-a Symbol

• The classes that are inherited (either directly
  or indirectly) by
• PunctuationMark
• A
• Semicolon
• Symbol
• _at_
• Letter