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();
There are 3
botB.move();
separate robot
botC.move();
references in
botA.harvestTwoRows();
this example:
botB.harvestTwoRows();
botA
botB
botC.harvestTwoRows();
botC
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• Could just use one
reference like this:
bot is a reference
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();
instantiating (i.e constructing)
3 separate objects
we use assignment to
assign a specific object to
a reference
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To correct the error, the object
(bob) must be instantiated
• Harvester bob;
bob
= new Harvester(2, 3, ..,..);
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.
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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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SquareRobot
DiagonalRightRobot
DiagonalLeftRobot
Create three separate robot classes that extend BetterRobot. Each
of these robots move differently when sent the move() message.
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