QMCS 230: Today in Class

1
QMCS 230 Today in Class

• A note on methods
• Objects
• Creating your own object
• Lab 13

2
Method calls

• A question about methods
• Its not enough to just write the method
• You have to call it from another method
• Otherwise it never, ever runs
• To call it
• Type its name and an open parenthesis
• Give the list of variables and literals to pass
  to it
• DO NOT include types (except as casts)
• Close parenthesis, semicolon

3
What do we mean by objects ??

• Its computer science jargon
• Its a way of combining some data with some
  methods to manipulate it
• You can customize the methods to the data being
  stored
• Its a controlled way of storing data
• You can save data in one method and pass it to
  another
• Its a metaphor (boy, is it!)
• Objects are the basis for all sorts of symbolic
  modeling of real world phenomena using computer
  based logic
• You can create models of anything you can talk
  about

4
Working with Objects

• Dot Notation
• Object name dot method (parentheses)
• Applies that method to that object
• If there are parameters in parentheses, theyre
  applied too
• Examples
• string1.equals(string2)
• outputfile.println(Writing this to the output
  file)
• inputfile.exists()

5
We use Classes to define Objects

• The class is the template for the object
• Think of a cookie cutter the class is the
  cutter, the object is the cookie
• Think of a printing press the class is the
  press and the object is the printed page
• Or something
• Weve been doing non-object classes
• Whats in a real class for an object?
• Fields variables that hold the objects data
• Methods for implementing the objects behavior
• Constructors create a new object of that class
  (new)
• Setters or mutators changes variables in the
  object
• Getters or accessors retrieve variables from
  the object

6
An example object

• A carpet for a room (Lab 8)
• What are the fields?
• What setters and getters would we have?
• Methods hinted at by the lab
• Setter with width, length, cost, room name
• Getters
• Retrieve the total price
• Print out the room description and carpet cost

7
Really creating an object

• Class name object name file name.java
• Then come variable declarations
• Outside of all those methods
• Can declare primitives or objects
• Then come methods
• Constructors have the objects name
• Overloading
• We can have different methods with the same name
• Each must have a different list of arguments
• Java sorts them out according to how they are
  called
• Trying it with the Carpet object

8
A Bank Account Object

• What the object does
• Keeps track of the balance
• Keeps track of the most recent transaction
• Fields
• balance a double with the current balance
• lastValue a double with amount of last
  transaction (/-)
• lastMemo string describing the last transaction
• Constructor start with zero balance
• Mutator do a transaction
• Accessors
• Retrieve/print recent transaction information
• Retrieve the balance

9
Lab 13 Bank Account

• Take a file containing transactions
• One line per transaction
• Line starts with a signed double the amount
• Rest of line is the memo explaining the
  transaction
• For each transaction
• Update the bank balance
• Write the balance and memo to System.out
• When end of file reached, write a Final Balance
  message
• Use a BankAccount object

10
How to do it

• Create two files
• Java file for lab 13 main
• Java file for BankAccount object
• Create a third file lab13data.txt
• Fill it with transactions.
• First should have a positive number
• Memo Starting Balance
• Rest start with positive or negative numbers,
  each with memo
• Goal make Main as short as possible
• Must still meet formatting and commenting
  requirements
• Main should have the Scanner and the input loop
• Everything else should be in the BankAccount
  methods

11
Thats it.

• Questions?
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