1. Classes, Objects, and World-level Methods
Alice

2. Larger Programs Complex programs require many lines of code (LOC)
Games, movies: 1000s or millions of LOC
How to handle such large programs?
Break up into smaller, manageable pieces

3. Can’t see all the code even for our example from last class!

4. Classes, Objects, & Methods
Object-oriented programming (OOP):
program consists of objects acting on each other
each object is an “actor” with its own role
OOP helps to
organize programs into small modules
ease the design process
make code reusable
find and remove errors (bugs)
One object acts on another by calling the other objects methods. Another way to think of this is that one object is "sending a message" to the other object. For example, a "Driver" object might send a message to a "Car" object to turn right. Named messages correspond to named methods and functions in a class definition.

5. Class vs. Object Class: Object: a conceptual framework for an entity
describes properties, methods, functions
cannot be used directly in execution
Object:
a realization of a class in a program
instance of
used directly during execution
A class is like a blueprint for creating a specific kind of object. Even better, it is like a factory for creating a certain type of objects.
Objects are specific instances of classes that have been created for use in a particular program. The first object created in a program from a class has the same name as the Class, which can be a little confusing. But if you create multiple objects from the same class, they will automatically receive unique names, like Ambulance2, etc.

6. Class vs. Object class Person beth derek maria pete … objects
Don't confuse classes with the groupings Alice uses to display related objects. Since there is no "Person" class in Alice, a better example might be something like using the "RandomGirl" class to create instances named Beth, Maria, Susan, etc.
objects

7. More Specifics height weight eye color talk walk twiddle thumbs Person
properties
methods
height
weight
eye color
talk
walk
twiddle thumbs
Person
6’ 3”
285 lbs.
dreamy blue
6’ 0”
175 lbs.
exotic green
derek
prof. L

8. In our programs, we have been using…
Classes
In Alice, classes are predefined as 3D models
Objects
An object is an instance of a class.
Class: Frog (Uppercase name)
Objects: frog, frog1, frog2, frog3
(lowercase names)
Using lower case names for objects isn't consistent across all classes supplied with Alice, but it is a nice way of distinguishing between the Class that generates an object and the objects it generates.

9. We have also used… Built-in (predefined) methods World.my first method
Examples: move, turn to face, say
World.my first method
provided automatically by Alice
started when you press the “run” button
we have written all code in this method
What about writing other methods?

10. Solution to One Long Method
Organize instructions into separate smaller methods
Stepwise refinement:
break a problem into large tasks
then break each task into simpler steps

11. Chicken Baseball

12. Chicken Baseball Large tasks: Don’t worry about details at this level
chicken swings
chicken runs
chicken slides
chicken throws
chicken catches
Don’t worry about details at this level
abstraction

13. Chicken Runs This will be its own method Consider details:
turn to face a base
in a loop, do together:
move 0.5 meters forward
turn legs forward/backward

14. Why? Why write our own methods?
saves time: call the method again and again
reduces code size: no need to write the instructions again and again
allows us to "think at a higher level"
can think chicken run instead of
“The chicken turns to face the base, then loop doing… ”
easier to find bugs

15. Creating a new method
First, to associate the new method with the World
select the World tile in the Object Tree
select the methods tab in the details area
click on the "create new method" button