BIT 115
Lecture 10
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NOTE TO STUDENTS:
These notes are for the instructor's use. If you happen to find them to be useful,
that's great. If not, then please ignore these. Thanks!
—The Instructor
Extending A Class
Last lecture we saw how to extend a class (to create a new type of Robot)
We also saw how to create a constructor service
And how to use that to interact with instance variables
So that the robot can be created, and told to remember some number
It'll remember it throughout the life of the robot
This time, we want to look at a new topic: how to get the robot to behave differently when it's told to
perform some service.
Example: Let's say we create a new type of robot, called a SpinningRobot
when a dancing robot it told to turn left, we want to it turn around once, then turn left on top of that.
Why? Because it's cool. And a good demo. 
Normally, we'd create a new service, spinLeft or somesuch:
class SpinningRobot extends Robot
{
SpinningRobot ( City c, int ave, int st, int dir, int num)
{
super(c, ave, st, dir, num);
}
public void spinLeft ()
{
this.turnLeft();
this.turnLeft();
this.turnLeft();
this.turnLeft();
this.turnLeft();
}
}
However, this means that anybody else using this class must remember to call spinLeft, rather than just
calling turnLeft.
We'd like to not make other programmers learn new commands.
Instead, we'd like to just make turnLeft behave differently
We can do this by simply defining a new version of turnLeft.
public void turnLeft ()
{
// Your code here...
}
In order to be an exact replacement for the built-in turnLeft, it has to look exactly like the original – same
return value (void), same parameters (none)
BIT 115
SUMMER 2012
BIT 115
Lecture 10
Page 2 / 3
City DizyCity = new City();
SpinningRobot joe = new SpinningRobot(DizyCity,3,3,Directions.EAST, 0);
CityFrame frame = new CityFrame(DizyCity, 5, 3);
joe.turnLeft();
turnLeft is now said to be overridden
Go over how to resolve overridden methods: start at the class of the object that's making the call, if you
find the method, use that one, if not, go up to it's superclass, etc,etc.
Question: What'll happen when we run the following version of turnLeft:
public void turnLeft ()
{
this.turnLeft();
this.turnLeft();
this.turnLeft();
this.turnLeft();
this.turnLeft();
}
Answer: Infinite recursion
So, we need a way to forcibly call the superclass's version of turn left:
public void turnLeft ()
{
super.turnLeft();
super.turnLeft();
super.turnLeft();
super.turnLeft();
super.turnLeft();
}
<Demo #1>
ICE: Create a new type of robot called SafeRobot with both move, pickThing, and putThing overridden
to output an error message instead of breaking the robot.
ICE: D/l Demo #2, and explain trace the code.
Side Effects
Notice that methods can change the state of the robot, and last beyond the method call.
This is called a side effect –it's not the intended purpose of the method, but it still takes place
anyways.
Polymorphism
We saw last class one of the central concepts of Object Oriented Programming: You're not going to see
much of it in this class, but it's interesting to look at quickly, and good to know if you're planning on
continuing in programming.
BIT 115
SUMMER 2012
BIT 115
Lecture 10
Page 3 / 3
Inheritance:
A subclass gets a copy of all data members & services from it's superclass
We'll see, and use inheritance a lot.
Overriding methods is part of the second central concept of OOP: polymorphism
Polymorphism:
We can refer to any subclass using a reference that normally would refer to something in the superclass,
yet the subclass will still behave like a member of the subclass.
How would this be useful?
Going back to cars, a Car superclass, with a method named getWeight. Each subclass (Jetta, Echo,
Taurus) overrides this method, and returns a weight specific to that particular car.
We might then model a CargoShip, which can simply keep a collection of Car objects, and not
worry about exactly what's in the cargo hold.
<Demo #2> - demo this, then have them trace it
Nested Loops
So far you've seen single loops.
Could write a program to move the robot a variable number times
Notice : only 1 dimensional output.
What if you wanted 2D output?
What if you wanted 2D output, variable in both dimensions?
We call this nesting – you can nest loops, if statements, select cases, etc.
<Trace the code>
If we wanted to, we could nest 3 loops inside each other, etc
Make sure to trace a sample of this, in ICE_08_NestedWhile.java
ICE: Nested Loops, Nested If statement
BIT 115
SUMMER 2012