CS 40 - Lecture 8

David S. Taylor

Lecture 7 Recap

• variable: holds a value
• initializing the variable: setting the first value
• instance variable: defined in object
• Local variable: defined in method

Reminder: Object

• an entity that can be manipulated in the program
• current state (color, position, texture, etc.) stored in instance variables
• methods used to change state
• ex: toaster

Class Concept

• Class = set of objects with the same behavior
• Can have toaster, toaster2, toaster3 in the same program
  
• All belong to the same class
• Alice has hundreds of classes (Toaster, Hampster, Woman, etc. etc.)
• Fundamental operation: Make new instance of class (ex: add another toaster)

Object vs. Class

• Object = one instance, Class = set of all instances
• Alice doesn't always make this very clear (ex. Cleopatra)
• Isn't there just one Cleopatra
• Actually, you can populate a world with lots of them
• A better name might have been EgyptianQueen

Defining New Classes in Alice

• Alice makes this very simple
• Make an object
• Add instance variables
• Add methods
• Get new behavior
• Example: ComboLock

Defining New Classes - Cont'd

• Added instance variables, methods, to comboLock
• Have new behavior
• Do we have a new class?
• Can we make two instances?
• Not yet, need to save as new class

Recipe for Saving Class

1. Add instance variables, methods
2. Rename object (e.g. smartComboLock)
3. Right click, save object...
4. Accept file name SmartComboLock.a2c (note uppercase S)
5. To make another object of this class, use menu item File->Import

Lab

• Download the ComboLock solution lab07.a2w
• Rename the object to smartComboLock
• Make it look smarter too. Give it a different color or texture. (To get more texture choices, add some animals into the world.)
• Following the recipe for saving class, make two instances of SmartComboLock
• How do you know you got smart combo locks, not the regular ones? Answer in ActiveLecture.org

Reminders

• Submit any questions about the lab to GoogleGroups (no Alice world upload needed)
• Homework #3 is due Sunday
• Exam #1 is next Wednesday
• I will try to get you some Homework feedback for HW 1 and 2 on Thursday.
• Is your photo in GoogleGroups? If not, please put it there.