1. Object References 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,…,…);
Harvester botB = new Harvester(4,2,…,…);
Harvester botC = new Harvester(6,2,…,…);
botA.move(); botA.harvestTwoRows();
botB.move(); botB.harvestTwoRows();
botC.move(); botC.harvestTwoRows(); Ch

2. Object References Could also intersperse the operations like this:
// same instantiations (making the objects)
botA.move();
botB.move();
botC.move();
botA.harvestTwoRows();
botB.harvestTwoRows();
botC.harvestTwoRows(); Ch

3. Object References Could just use one reference like this:
Harvester bot;
bot = new Harvester(2,2,…,…);
bot.move();
bot.harvestTwoRows();
bot = new Harvester(4,2,…,…);
bot = new Harvester(6,2,…,…);
a reference
instantiating 3 objects
we use assignment to assign a specific object to a reference Ch

4. Object References - Common Error
Harvester bob; bob.harvestTwoRows();
What’s wrong with the above?
NullPointerException
for now, an error in Java is called an exception
do you think this error happens at run-time or compile-time? why?
Binky Pointer Video Ch

5. Object References
References model what’s going on in the real world as well
There are lots of “n00b” 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. Ch

6. Polymorphism Powerful example:
you are all objects - if I tell all 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… Ch

7. Overriding move() remember MileWalker?
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() Ch

8. Example let’s have 3 different types of bots
MileWalker
when move() is invoked, moves 1 mile
DropBeeperAndWalker
when move() is invoked, always drops a beeper and then moves one block forward
BackwardWalker (sort of the Michael Jackson of robots!)
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… Ch

9. As always, the Big Picture first a.k.a. - Inheritance Hierarchy
UrRobot
MileWalker
BackwardWalker
DropBeeperAndWalker Ch

10. MileWalker public class MileWalker extends UrRobot {
// constructor same as always
public void move()
super.move(); super.move(); }
heading needs to be identical to the one in the API for UrRobot in order for “overriding” to work Ch

11. DropBeeperAndWalker public class DropBeeperAndWalker extends UrRobot {
// constructor same as always
public void move()
putBeeper(); // inherited instruction still serves its purpose
super.move(); } Ch

12. BackwardWalker 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… Ch

13. Your sample Driver vs. mine
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!
UrRobot bot;
bot = new MileWalker(…);
bot.move(); // polymorphic move()
bot = new DropBeeperAndWalker(…);
bot.move(); // polymorphic move()
bot = new BackwardWalker(…); Ch

14. Polymorphism
at run-time, the correct implementation is chosen depending on what specific object is being referenced at that moment in time.
bot
then later…
instance of DropBeeperAndWalker
then yet even later…
instance of BackwardWalker
instance of MileWalker Ch

15. Polymorphism - cont’d 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… Ch

16. Choreographers one object controlling 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 another object(s) Ch

17. MoveChoreographer public class MoveChoreographer extends UrRobot {
private UrRobot myBotA;
private UrRobot myBotB;
private UrRobot myBotC;
// constructor on next slide
// other methods }
instance variables
objects can not only do(behavior) things, they can also remember(state) things Ch

18. MoveChoreographer’s constructor
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
myBotA = botA;
myBotB = botB;
myBotC = botC; }
instance variables being assigned Ch

19. MoveChoreographer’s moveFriends()
public void moveFriends() {
myBotA.move();
myBotB.move();
myBotC.move(); } Ch

20. Sample Client code using a MoveChoreographer
can you now give some sample client code that uses a MoveChoreographer object? (do so now for 5 minutes…)
draw a picture and show the before and after
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(); Ch

21. examining the constructor’s reference types
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, she can pass any 3 robots in any order as long as each one is-A UrRobot or 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 _________? Ch

22. Abstract classes
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 = use an abstract class… Ch

23. contrived/simple task to demo the need for an abstract class
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 corner, the even have 3. Here is how we’d organize that with what we currently know:
UrRobot
TwoRowLayer
ThreeRowLayer
layBeepers()
putBeepers()
discuss problems with design Ch

24. BeeperLayers
On the previous slide, we saw that layBeepers() would have the exact same implementation for both types of beeper layers - namely: {
move(); putBeepers();
move(); }
discuss why code duplication (a.k.a., copy/paste) and lack of localization are poor/costly design patterns Ch

25. BeeperLayers
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 would 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… Ch

26. Inheritance Hierarchy
UrRobot
BeeperLayer
public void layBeepers() { … }
public abstract void putBeepers();
TwoRowLayer
public void putBeepers() { … }
ThreeRowLayer
public void putBeepers() { … } Ch

27. Terminology & Concepts
BeeperLayer lisa = null;
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();
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
abstraction, abstract class, abstract method, polymorphism Ch