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Note: Original slides provided by www.apComputerScience.com and modified for Mr. Smith’s AP Computer Science A class

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(Ch3, pg 43)

What are the options for harvesting these beepers?How many robots could we use?

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(a.k.a. variables)

• Teams of Robots (e.g.)– Could have 1 robot harvest 6 rows (we’ve seen

that)– Could have 3 robots each harvest 2 rows like

this:Harvester botA = new Harvester(2,2,…,…);botA.move();botA.harvestTwoRows();Harvester botB = new Harvester(4,2,…,…);botB.move();botB.harvestTwoRows();Harvester botC = new Harvester(6,2,…,…);botC.move();botC.harvestTwoRows();

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• Could also intersperse the operations like this:

// same instantiations

Harvester botA = new Harvester(2,2,…,…);

Harvester botB = new Harvester(4,2,…,…);

Harvester botC = new Harvester(6,2,…,…);

botA.move();

botB.move();

botC.move();

botA.harvestTwoRows();

botB.harvestTwoRows();

botC.harvestTwoRows();

There are 3 separate robot references in this example:botAbotBbotC

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• Could just use one reference like this:

Harvester bot;

bot = new Harvester(2,2,…,…);

bot.move();

bot.harvestTwoRows();

bot = new Harvester(4,2,…,…);

bot.move();

bot.harvestTwoRows();

bot = new Harvester(6,2,…,…);

bot.move();

bot.harvestTwoRows();

bot is a reference

we use assignment to assign a specific object

to a reference

instantiating (i.e constructing)3 separate objects

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• Harvester bob;bob.harvestTwoRows();

• What’s wrong with the above?• This causes a NullPointerException

error– for now, an error in Java is called an

exception– NullPointerException means the

reference is not pointing to anything.

bob = new Harvester(2, 3, ..,..);

To correct the error, the object(bob) must be instantiated

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• References model what’s going on in the real world as well– There are lots of “Dave” references - but the

particular object (person) one is referring to depends on context and whom one is, in particular, referring to at the moment

– Well, these references are all neat and everything, but so what? Well, hold on a few more slides and you’ll see the power of using them - we’re headed toward an extremely important OO concept called Polymorphism.

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• Powerful example of Polymorphism:– pretend you are all objects - if I tell each of you to

“takeABreak()”, you all will hear the same message but will act in different ways (some of you will sleep, some will walk out the door and eat something, some will try to leave school!, some will do work, etc.) - that’s polymorphism

• sending the same message to different objects - each individual object has a particular way to interpret (implement) the message

• so, back to code and a Java/Karel example…

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• remember MileWalker? (Ch3, pg 34)– we named its one method moveMile()– we could have named the method move() and

then redefined what “move” means to a MileWalker. Again, we’re modeling the real world. The concept of “move” is different depending on what type of object is “moving” (think about how a dog, fish, bird, etc., “move”)

– so, since the general concept is the same, we often use the same name (it makes coding easy/logical) - why would you want to try to remember moveMile(), moveLegs(), moveWings(), etc. - why not just one identifier for that - move()

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Create three separate robot classes that extend BetterRobot. Each of these robots move differently when sent the move() message.

SquareRobot DiagonalLeftRobotDiagonalRightRobot

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• let’s have 3 different types of robots– MileWalker

• when move() is invoked, moves 8 miles

– DropBeeperAndWalker • when move() is invoked, always drops a beeper and

then moves one block forward

– BackwardWalker (a robot tribute to Michael Jackson!)

• when move() is invoked, moves one block backward

• for each of these new classes, we will only have to write one method, move() - each, however, will be implemented differently, and, in addition, override the original definition of move() inherited from UrRobot --- let’s see…

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As always, the Big Picture first

a.k.a. - Inheritance Hierarchy

UrRobot

MileWalker

BackwardWalker

DropBeeperAndWalker

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public class MileWalker extends UrRobot{

// constructor same as always

public void move() { super.move(); super.move();

super.move(); super.move();super.move(); super.move();super.move(); super.move();

}}

Method name needs to be

identical to the one in the API for UrRobot in order

for “overriding” to work

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public class DropBeeperAndWalker extends UrRobot

{

// constructor same as always

public void move()

{

putBeeper(); // inherited instruction still serves its purpose

super.move();

}

}Note: you would want to

check to make sure the object always has at least one

beeper for each time move() might be called

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• You write it! • In addition to writing this class, write

a sample Driver that would demonstrate using one robot each of type MileWalker, DropBeeperAndWalker, and BackwardWalker

– We’ll pick someone and put it up in 5 minutes…

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UrRobot bot;bot = new MileWalker(…);bot.move(); // polymorphic move()

bot = new DropBeeperAndWalker(…);bot.move(); // polymorphic move()

bot = new BackwardWalker(…);bot.move(); // polymorphic move()

a reference can refer to

any object as long as the object is of the same type or a

type of one of its subclasses somewhere down the

Inheritance tree!

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• at run-time, the correct implementation is chosen depending on what specific object is being referenced at that moment in time.

bot

instance of MileWalker

then yet even later…

instance of BackwardWalker

then later…

instance of DropBeeperAndWalker

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Create three separate robot classes that extend BetterRobot. Each of these robots move differently when sent the move() message.

SquareRobot DiagonalLeftRobotDiagonalRightRobot

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• polymorphism is ubiquitous (everywhere) in OOP

• there are many uses and examples of it

• let’s now build toward another example of polymorphism– but first, as last time, we need some

setup…

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• one object controlling others (overseeing the movement of others)

• we now want a MoveChoreographer class, which, when constructed, is passed 3 friends (robots)

• the MoveChoreographer has one method called, moveFriends() which, when invoked, “moves” each friend once

• this Choreographer model of problem solving, by the way, can been seen in the “general contractor” analogy we used in the ppt from Ch. 3 - the general contractor doesn’t really do the work, she just delegates it to other objects

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public class MoveChoreographer extends UrRobot{

private UrRobot friendA;private UrRobot friendB;private UrRobot friendC;

// constructor on next slide

// other methods

}

instance variables

objects not only do things (behavior), they can also remember things (state) using instance variables

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public MoveChoreographer ( int st, int av,

Direction dir, int numBeepers,UrRobot botA,UrRobot botB,UrRobot botC )

{super (st, av, dir, numBeepers); // must come first in method

friendA = botA;friendB = botB;friendC = botC;

}

instance variables being

assigned

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public void moveFriends(){friendA.move();friendB.move();friendC.move();

}

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public class MoveChoreographer extends UrRobot{

private UrRobot friendA;private UrRobot friendB;private UrRobot friendC;

public MoveChoreographer (int st, int av, Direction dir, int numBeepers, UrRobot botA, UrRobot botB, UrRobot botC )

{super (st, av, dir, numBeepers); // must come first in methodfriendA = botA;friendB = botB;friendC = botC;

}

public void moveFriends(){

friendA.move();friendB.move();friendC.move();

}}

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Can you now give some sample client code that uses a MoveChoreographer object? Pass a MileWalker, DropBeeperAnd Walker and a BackardWalker to the MoveChoreographer object (do so now for 5 minutes…)an example:UrRobot bot1 = new MileWalker(2, 4, North, 0) ;

UrRobot bot2 = new DropBeeperAndWalker(2, 5, North, infinity);

UrRobot bot3 = new BackwardWalker(2, 6, North, 0);

MoveChoreographer chor;

chor = new MoveChoreographer(1, 1, North, 0, bot1, bot2, bot3);

chor.moveFriends();

draw a picture and show the before and

after

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The statement from the previous slide,chor = new MoveChoreographer(1, 1, North, 0, bot1, bot2, bot3);

is kind of neat. When someone constructs a MoveChoreographer, he can pass any 3 robots in any order as long as each one is-A UrRobot (it extends from a UrRobot).

The MoveChoreographer only wants to be guaranteed that it can perform a move() on any object passed to it - since there is a move() in UrRobot, it chose to make its parameters of type UrRobot, guaranteeing (to itself and the compiler) that it will be able to call move() at run-time. The term that describes which particular move() will be called at run-time is ____________.

Polymorphism

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Initial Situation End Situation

Create a Gardener robot that will use GardenerHelper robots to help plant a garden. The Gardener class will have a constructor method similar to the MoveChoreographer robot that we created in class. Gardener’s specialty is to supervise 3 other GardenerHelper robots to plant beepers. You should program the GardenerHelper class to plant beepers around the plus-shaped wall. The GardenerHelper is a BetterRobot and the Gardener robot is a GardenerHelper. The Gardener robot should be programmed to use itself and the 3 other robots that are passed to it to plant each corner. The client program should pass the helper robots to the Gardener robot in the correct street/avenue that they should start on and facing in the correct direction.

gardenerWorld.txt

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• Sometimes we want to do several tasks, but the tasks are very similar. How can we build the classes to take advantage of the common parts of the task and yet distinguish the specific differences? Another way to say that is, how can we design the inheritance tree so that we don’t duplicate common code used among sub-classes, yet allow sub-classes to have some specific differences?

• The answer is: use an abstract class…

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Here is a task for a team of robots. We want to lay down beepers in a 5-by-4 field. The odd-numbered rows have 2 beepers per street corner, the even have 3.

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Here is how we’d organize that with what we currently know:

UrRobot

TwoRowLayer ThreeRowLayerlayBeepers() layBeepers()

putBeepers() putBeepers()

discuss problems with design

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On the previous slide, we saw that layBeepers() would have the exact same implementation for both types of beeper layers - namely:

public void layBeepers() {

move();putBeepers();move();putBeepers();move();putBeepers();move();putBeepers();move();

}

discuss why code duplication (a.k.a., copy/paste) and

lack of localization are poor/costly design patterns

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At the same time, we saw that putBeepers() would have a different implementation in each of the subclasses (one puts 2, the other puts 3). So here is the new design pattern:

We’ll extract out an abstract concept of what a general beeper layer would look like and put that into a class (in this case, an abstract class). Methods in the abstract class that have the exact same implementation regardless of the subclass will be implemented in the abstract class. Methods that would have different implementations in the subclasses will not be implemented in the abstract class, forcing each subclass to give its own unique implementation…

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TwoRowLayer

public void putBeepers() { … }

UrRobot

BeeperLayer public void layBeepers() { … }

public abstract void putBeepers();

ThreeRowLayer

public void putBeepers() { … }

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public abstract class BeeperLayer extends UrRobot {

public BeeperLayer(int street, int avenue, Direction direction, int beepers){

super(street, avenue, direction, beepers);}

// The following abstract method will be used by TwoRowLayer and ThreeRowLayerpublic abstract void putBeepers();

public void layBeepers(){

move();putBeepers();move();putBeepers();move();putBeepers();move();putBeepers();move();

} }

public class TwoRowLayer extends BeeperLayer{ public TwoRowLayer( int st, int av, Direction dir, int beeps) {

super(st, av, dir, beeps); }

public void putBeepers() {

putBeeper();putBeeper();

} }

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public class BeeperLayerTester implements Directions {

public static void main(String[] args) {

BeeperLayer lisa;lisa = new TwoRowLayer(1, 3 ,East, infinity);lisa.layBeepers();lisa = new ThreeRowLayer(2, 3, East, infinity);lisa.layBeepers();lisa = new TwoRowLayer(3, 3, East, infinity);lisa.layBeepers();lisa = new ThreeRowLayer(4, 3, East, infinity);lisa.layBeepers();lisa = new TwoRowLayer(5, 3, East, infinity);lisa.layBeepers();

}}

abstraction, abstract class, abstract method, polymorphism

making references to the code, the

inheritance tree, or whatever else we just

discussed in the BeeperLayer problem,

pick one of these terms and demonstrate that

you know what it means

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• Abstraction - Mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time (i.e. abstracting common code and putting it into a method)

• Abstract class – Special class used to provide common code used among sub-classes, yet allow sub-classes to have some specific differences. It is not possible to create an instance of an abstract class (it must be extended and then that class can be instantiated). An abstract class that only contains abstract methods is an interface (we’ll learn about that later).

• Abstract method – Method in an abstract class that must be redefined in a class that extends the abstract class.

• Polymorphism – Objects in different classes can behave differently when sent the same message. In practical terms, polymorphism means that if class B inherits from class A, it doesn’t have to inherit everything about class A; it can do some of the things that class A does differently.

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• Initialize – Set the value of a variable to a specific value For example: karel = new UrRobot(1, 2, North, 0);

• Assignment – Used to change the object to which an reference points. Use the equal sign (=) to denote assignment.

• Reference (or Variable) – name used to identify a field (i.e. the robot name “karel” is a reference or variable).

• Instance Variable (or Field) – The things that an object remembers. These are the variables defined in the object class.

• null – Special value to indicate that the reference to the variable does not refer to anything at all (i.e. UrRobot karel = null).

• Parameter – Information sent to a method. When we instantiate a new robot object, we are passing 4 different parameters which are used by the constructor method. Parameters can be of any type.