ch.3 classes

1Ch. 3

Ch.3 Classes

STEP 1 Define a new class of robot (see next slide)

When designing a new class (whether that’s 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 (object oriented design) is the concept of code-reuse. There is no need to reinvent the wheel. (by the way, that doesn’t mean to copy your friend’s lab!! )

2Ch. 3

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();

}}

note: no object names preceding methods (why not?) - let’s look at client code to see

why

superclass subclass

constructor

INHERITANCE

invokes superclass’ constructor

3Ch. 3

Inserting MileWalker into the Inheritance Hierarchy

UrRobot

MileWalker

move()

moveMile()

turnLeft()

pickBeeper()

putBeeper()

turnOff()

If you have an object (say, bob) of type MileWalker, what methods are available to bob?

What if bob were of type UrRobot?

the is-A relationship

a MileWalker is-A UrRobot

4Ch. 3

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( ) { MileWalker bob = new MileWalker(2, 1, East, 0);

bob.moveMile(); // new instructionbob. move(); // inherited instructionbob.turnOff(); // inherited instruction

}}

5Ch. 3

Misc. Note(don’t sweat this – it’s 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

6Ch. 3

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.)

• We’ll now demo the whole process in BlueJ

7Ch. 3

Now You Try!

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

8Ch. 3

Now, try something a bit more!1. 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, we’ll pass the wireless keyboard & mouse and

we’ll build/test the class together

– In addition, where does it go in the Inheritance Hierarchy?

2. 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?(let’s informally discuss before I give you the fancy cs language)

9Ch. 3

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 client’s point of view, she is no longer concerned with how to draw the H – she’s 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 that’s how you wrote it) HBot?

10Ch. 3

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() – let’s discuss public/private, then you guess/justify

• I’ll argue, No. I’ll argue like this: the name of the class is HBot – so I (the client) am trying to have some object draw H’s for me – how that object gets it done is of no concern of mine(abstraction) – so I don’t need (and shouldn’t be allowed) to see the other helper/auxiliary methods. Therefore, they should be private, helper-like methods for the class’ use only.

11Ch. 3

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, doesn’t it. Explain why from the contractor’s view – use our cs terms we’ve 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?

12Ch. 3

Practicing Stepwise Refinement• Let’s write a class called DiamondPlanter together using

stepwise refinement. It’s 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 you’d like to use already exist (abstraction) and work already (automagically, if you will). After we pseudocode plantDiamond(), we’ll take each helper method in turn and repeat this stepwise-refinement process until we have cohesion.

13Ch. 3

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 don’t know about you, but I found it ANNOYING to write the same thing again! Let’s look at the Inheritance Hierarchy and see if we can come up with a solution. Let’s discuss possible solutions before going on…

14Ch. 3

Improving overall object design

UrRobot

BetterTurnerBot

HBot DiamondPlanter

turnRight()

turnAround()

Discuss what our modifications would be to the HBot and DiamondPlanter

classes

– in terms of syntax

- in terms of concepts we’ve been

discussing

15Ch. 3

Now YOU try! But be efficient!

starts off

facing East

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:

When designing, keep in mind everything

we’ve been discussing.

16Ch. 3

Why create a Class?• Why reinvent the wheel? You’re 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 I’m going to write a system to have a bot climb

stairs in several buildings, I’m going to use the StairClimber class so I can call climbStair() – I can work on a bigger/better/harder problem and free my mind from the irrelevant details of taking a step and picking beepers

17Ch. 3

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)

StairClimber MileWalker Harvester DiamondPlanter

BetterTurner

UrRobot

18Ch. 3

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 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?

19Ch. 3

Another Inheritance QuestionGive the state (location and Direction) of each Bot below?

public class ABetterBot extends UrRobot { /* constructor not shown */


public void step2(){ turnLeft(); }

}

public class AnEvenBetterBot extends ABetterBot {


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();

20Ch. 3

A final Inheritance QuestionGive the state (location and Direction) of each Bot below?

public class TestBot extends UrRobot { /* constructor not shown */


step2(); }

public void step2(){ turnLeft();}

}

public class HarderTestBot extends TestBot {



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();

21Ch. 3

Is-A questions

• “A” is-a Letter• Letter is-a Symbol• Semicolon is-a

PunctuationMark• “@’ is-a Symbol• PunctuationMark is-a

Symbol

• The classes that are inherited (either directly or indirectly) by:

– PunctuationMark

– “A”

– Semicolon

– Symbol

– “@”

– Letter

ch.3 classes

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