Intro to Computer Science Class

1
Intro to Computer Science Class 3Formalizing
Variables, Methods and Inheritance

• Instructor Ms. Catherine Stocker
• Teaching Assistants Alex, Katie, Siraaj,
  Isaiah, Allison, Thibault
• University of Pennsylvania
• 6 February 2008

2
Todays Agenda

• Discussion (Reaction paper and BotPlay)
• Javadocs
• Variables
• Methods
• Objects
• Intro to Inheritance
• BetterBot

3
Discussion

• Lets talk about the reaction paper and what you
  discovered in BotPlay.

4
Reminder What to do if youre stuck

• Try things out! Dont be scared of the computer.
  (Just make sure to have a backup copy of your
  work!)
• Check out the resources page
• Post to the bulletin board! (you can get to it
  now )
• Come talk to us (before/after class).
• Email me cstocker_at_seas.upenn.edu

5
Reminder What is Java?

• From someone who didnt know last week...
• A high-level, object-oriented, programming
  language
• Language
• A way for people to communicatewith computers
• Made of vocabulary and syntax (rules for how to
  arrange the vocabulary)which youll learn in
  this class
• High-level High-level of abstraction
• Abstraction Hiding unimportant detailslike what
  type of computer were communicating with.
• Object-Oriented Thinking about your program as a
  collection of objects that have a state and a
  behavior.

6
Reminder Other Objects

• Abstract away the unnecessary details only keep
  the important (relevant to the program) ones in
  the state.
• Program1 Keep track of baseball stats
• Object Baseball Player
• State name, G, AB, R, H, HR, on base, at bat
• Behavior run, walk, hit ball, slide, batting
• Program2 Simulate how athletes lifestyles affect
  their popularity with fans, salary and injury
  rate
• Object Athlete
• State name, sport, salary, days injured,
  popularity
• Behavior negotiate contract, party, complain,
  retire

Baseball Player
name
Ryan H
R
255
onBase
3
G
410
H
425
atBat
no
1461
128
353
AB
HR
RBI
Athlete
name
Ryan H
sport
baseball
salary
900,000
daysInjured
25
popularity
high
7
Reminder How Do We Translate What Weve Talked
About Into Java?

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• salary newSalary

class (object blueprint)
instance variables (hold the state)

• constructor.
• Creates the object
• Sets the starting state

• methods
• modify the instance variables (behaviors changing
  the state)
• return the instance variables (tell us about the
  state)

8
Summary of Last Week

• We learned about Java and objects and classes and
  methods
• But we didnt discuss the formal rulesso what?
• Theres no room for interpretation when we
  communicate with computers!
• I wnt to. class Yesterday, Ok for humans
• String yesterday Tuesday
• Yesterday Monday,
• whats wrong with this code?
• Computers are case-sensitive
• ? . ? ,
• Today were going to discuss the formal
  definition of methods

9
Summary of Last Week

• We learned about Java, objects, classes,
  variables and methods
• But we didnt discuss the formal rulesso what?
• Theres no room for interpretation when we
  communicate with computers!
• I wnt to. class Yesterday, Ok for humans
• String yesterday Tuesday
• Yesterday Monday,
• whats wrong with this code?
• Computers are case-sensitive
• ? . ? ,
• Computers do EXACTLY what you tell them. Even if
  youre wrong!
• Today were going to formalize all of that!

10
Variables

• Used to store and retrieve a value in/from memory
• ex. int x 5 int y x2
• Must be declared once before being used.
• Why? So the computer knows
• How much space in memory is needed to hold it.
• What operations are allowed to be performed on
  it.
• 12? .537.5? truefalse?
• Definitions of the different ways to use
  variables
• Instance variables Store the state of each
  object.
• Declared at top of class.
• Scope Accessible to the entire object.
• Local variables Stores the temporary state of a
  method.
• Declared at top of method.
• Scope Accessible to the entire method.
• Parameters Passed to methods.
• Scope Accessible to the entire method.
• Class variables Well talk about later in the
  course.

x
5
y
10
10
5
Hello
x
y
myStringVar
11
Snapshot of a modified Athlete class

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• int bargain 2
• salary newSalary/bargain

instance variables
parameters
local variable
12
Structure of a Variable Declaration

• type_of_variable variable_name
• type_of_variable
• Primitive types
• Defined by Java, part of its vocabulary already
  keywords
• Acted on with operators ,-,/,,,,lt,gt,etc
• Only 8
• int - whole number (-2,147,483,648,, -2, -1, 0,
  1, 2,2,147,483,647)
• double - fractional/floating-point number
  (-2.0,-1.9,-1.89,0.34123,12.3)
• char a single character enclosed by single
  quotes (a, , 1)
• boolean - truth value (true, false)
• Also byte, short, long, float, but we wont use
  those.
• Non-primitive types
• Any class defined by us (or Java) becomes a new
  type
• We can declare variables of that type
• String Anything enclosed by double quotes (a,
  , 1, Hello there friend!)
• Special case of non-primitive defined by java,
  keyword, acted on by an operator () but is
  actually an object.
• Acted on by methods variable_name.method_name(par
  ameters)

13
Structure of a Variable Declaration

• type_of_variable variable_name
• type_of_variable
• Primitive types
• Defined by Java, part of its vocabulary
  already keywords
• Acted on with operators ,-,/,,,,lt,gt,etc
• Only 8
• int - whole number (-2,147,483,648,, -2, -1, 0,
  1, 2,2,147,483,647)
• double - fractional/floating-point number
  (-2.0,-1.9,-1.89,0.34123,12.3)
• char a single character enclosed by single
  quotes (a, , 1)
• boolean - truth value (true, false)
• Also byte, short, long, float, but we wont use
  those.
• Non-primitive types
• Any class defined by us (or Java) becomes a new
  type
• We can declare variables of that type
• String Anything enclosed by double quotes (a,
  , 1, Hello there friend!)
• Special case of non-primitive defined by Java,
  keyword, acted on by an operator () but is
  actually an object.
• Acted on by methods variable_name.method_name(par
  ameters)

14
Structure of a Variable Declaration

• type_of_variable variable_name
• type_of_variable
• Primitive types
• Defined by Java, part of its vocabulary
  already keywords
• Acted on with operators ,-,/,,,,lt,gt,etc
• Only 8
• int - whole number (-2,147,483,648,, -2, -1, 0,
  1, 2,2,147,483,647)
• double - fractional/floating-point number
  (-2.0,-1.9,-1.89,0.34123,12.3)
• char a single character enclosed by single
  quotes (a, , 1)
• boolean - truth value (true, false)
• Also byte, short, long, float, but we wont use
  those.
• Non-primitive types
• Any class defined by us (or Java) becomes a new
  type
• We can declare variables of that type
• String Anything enclosed by double quotes (a,
  , 1, Hello there friend!)
• Special case of non-primitive defined by Java,
  keyword, acted on by an operator () but is
  actually an object.
• Acted on by methods variable_name.method_name(par
  ameters)

15
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

16
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

17
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

18
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

19
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

20
Structure of a Variable Declaration

• type_of_variable variable_name
• variable_name (Almost anything you want)
• Should be descriptive!
• ex. salary, name, etc
• Cant use keywords
• What keywords do we know already?
• General rule if it turns a different color when
  you write it in drJava, its a keyword
• Case-sensitive (hello ? Hello)
• Must begin with
• A letter (also or _ but dont use those)
• Followed by any combination of
• Letters
• Numbers
• _
• No spaces allowed write the 1st word lowercase,
  all other words capital
• myVariable, yourVariable, thisIsAReallyLongVariabl
  e

21
Structure of a Variable Assignment

• variable_name data_value
• Examples
• int myAge
• myAge24
• double pi
• pi 3.14
• Whats wrong with this
• int myAge
• myAge 24.5
• Will cause an errorwhy?
• A specific case variable_name new
  constructor(parameter_list)
• Where have we seen that?
• Well talk about this more in a minute
• Putting it all together type_of_variable
  variable_name data_value
• We can declare alone or assign alone or do both
  at once.

- char firstLetter firstLetter a -
boolean inClass inClass true
- String profession profession grad
student
22
Review

• What are
• Instance variables?
• Local variables?
• Parameters?
• Primitive types?
• How do you..
• Declare a variable?
• Whats wrong with these variables
• Hello
• xyz
• Myveryfirstvariable
• this is a variable?
• 1Variable
• variable1

23
Methods

• modifier return_type method_name(parameter_list)
  
• method_body
• modifier
• What can see it, well discuss more on this
  later.
• For now, assume all methods are public.
• return_type
• Remember types? Any of these void.
• The data type of the value returned by the
  method, void if returning nothing.
• method_name
• Same rules as variable names
• Methods represent behaviors, so generally named
  with a verb (remember the Athlete class
  negotiateSalary, retire, party, complain, etc)
• parameter_list
• Discussed before Just a special case of
  variables
• This section can be empty or contain a list of
  parameters preceded by their data type and
  separated by commas
• method_body
• May include Declaration of local variables,
  manipulation of variables (instance, local or
  parameters), return statement (returns data of
  type return_type).

public void party() daysInjured
daysInjured 10
24
Snapshot of a modified Athlete class

• public class Athlete
• int salary
• .
• public void negotiateContract ( int newSalary )
  
• int bargain 2
• salary newSalary/bargain
• public int getSalary ( )
• return salary
• ...//the rest of the methods

return_type (void so no return statement)
method_name
parameter_list
modifier
method_body
parameter_list (empty)
return_type
method_name
modifier

method_body (returns an int)
25
The Constructor

• modifier class_name(parameter_list)
• constructor_body
• Creates the object from the class blueprint.
• Looks like a method, with some differences
• No return_type.
• Its name is the same as the class.
• Sets up the initial state.
• Remember variable declaration and assignment?
• Remember any class you create becomes a
  non-primitive type?
• Variable declaration and assignment for
  non-primitive types type_of_variable
  variable_name new constructor(parameter_list)
• ex. Athlete a1 new Athlete(Ryan, Baseball
  Player) //call constructor
• int initialSalary a1.getSalary()
  //call method

public Athlete(String n, String s) name
n sport s salary 0 daysInjured
0 popularity average
26
Question

• We found some ways the Bots were deficient
• How do we make them better?
• Add to the code?
• What if we cant get to the code?
• What if we want to keep this Bot definition too?
• Copy and Paste?
• Waste of time and space (memory).
• Inheritance

27
Introduction to Inheritance

• modifier class subclass_name extends
  superclass_name
• class_body
• A subclass (class thats inheriting) inherits the
  methods and instance variables of the superclass.
• Constructor of superclass not inherited, so must
  call super(parameters) within the subclass
  constructor.
• Most common way this is done is Object class
• All classes extend the object class
• More about this next week.

public class BetterBot extends Bot public
BetterBot(BotWorld world) super(world)
//additional methods
28
Now

• Pennkeys!
• Memorize them!
• Go to the Bulletin Board to ask questions.
• Submit this weeks reaction paper
• BetterBot (http//www.seas.upenn.edu/cis1xx/proje
  cts/Botworld/betterbot/assignment.html)

29
Later

• Using inheritance, extend either the class you
  wrote last week or BetterBot. Include at least 2
  new instance variables and 2 new methods in your
  subclass.
• Write down something you were confused about from
  class and a short explanation about what confused
  you (1-3 sentences).
• Ask questions on the bulletin board.
• Submit using the link on the webpage