CS 303E Class 7 Part 1: Designing Complex Programs

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CS 303E Class 7 Part 1Designing Complex
Programs
The sooner you start to code, the longer your
program will take.-Roy Carlson, U Wisconsin
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Tally Grades(pp. 96110)

• Request A program to tally grades on a test.
• Analysis (details of input and output)
• Input scores on a test.
• Output Number of As, Bs, Cs, Ds, Fs (tallies)
  and running average.
• Interface Fields, buttons, etc. needed for I/O.
• Fields and labels for input (score) and outputs
  (tallies and average).
• Buttons to tally another score and to reset all.

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Design Variables

• Window components.
• Global variables
• Needed? Yes
• Click Tally button for each score user loop.
• Tallies, average, other variables must retain
  their values between button clicks.
• Global variables tallyA, tallyB, , tallyF,
  numberOfTests, totalOfTests, average.

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Method buttonClicked

• Determines which button was clicked and calls the
  appropriate method
• buttonClicked
• if (reset button clicked)
• resetGlobalVariables ()
• else
• processScore ()
• set focus to score input field

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Method resetGlobalVariables

• resetGlobalVariables
• set tallies to 0 // tallyA,
• set totals to 0 // totalOfTests,
• set average to 0
• display 0 in score field
• displayTalliesAndAverage ()
• Method call for last action because it will be
  needed again when processing scores.

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Method displayTalliesAndAverage

• displayTalliesAndAverage
• display the tallies and average on the screen
• Computation done in global variables, but display
  done in fields or other window components.

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Method processScore

• processScore
• if (score is invalid)
• display error message
• else
• get score value from field
• if (score lt 0 or score gt 100)
• display error message
• else
• updateTallies (score)
• updateAverage (score)
• displayTalliesAndAverage ()
• Separate methods for computations of a few lines
  or more.

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Update Methods

• updateTallies (int score)
• update the proper tally for score
• updateAverage (score)
• update the totals for score
• compute the average
• Simple enough that pseudocode isnt needed.

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Coding Step 1 (pp. 99101)

• Code
• the import, class, and method main.
• all the window objects labels, fields, and
  buttons declared.
• method buttonClicked.
• Omit global variables not needed yet.
• Stub out the methods buttonClicked calls.
• Run and debug.
• What should you see?

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Coding Step 2( pp. 100-101)

• Code the methods called by buttonClicked.
• Add global variables needed now.
• Stub out the methods called by methods added
  above.
• Run and debug.
• What should you see?

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Coding Step 3(pp. 101-104)

• Code the rest of the methods, removing all stubs.
• Run and debug.
• What should you see?

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Preconditions and Postconditions

• Preconditions
• Statement of what must be true before a method
  can be invoked inputs required in parameters or
  globals.
• Postconditions
• Statement of what the method will guarantee to be
  true after it is executed if the preconditions
  are met values computed and returned or
  changed, errors detected.

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Example

• Text pp. 94-96
• // Preconditions number is an integer gt 0.
• // Postconditions The number of divisors in
• // number is returned.
• private int computeCount (int number)
• . . .
• return count

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Call Stack Run-time Errors

• Exception occurred during event dispatching
• java.lang.ArithmeticException / by zero
• at TallyGrades.updateAverage(TallyGrades.java)
• at TallyGrades.processScore(TallyGrades.java)
• at TallyGrades.buttonClicked(TallyGrades.java)
• at BreezyGUI.GBFrameButtonListener.actionPerform
  ed
• (GBFrame.java241)
• at java.awt.Button.processEvent(Button.java281)
  
• at . . .
• Press Enter to continue

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CS 303E Class 7 Part 2Characters, Strings, and
the Math class
"The time has come, "the Walrus said, "To talk
of many things of shoes - and ships - and
sealing wax - of cabbages - and kings - And why
the sea is boiling hot - And whether pigs have
wings." - Lewis Carroll, 1871, in Through the
Lookinglass.
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Primitive Data Types

• Numeric types
• int, double, and others
• Character type
• char
• Boolean type.

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Characters type char

• Type char represents the 64,768 characters (2
  bytes) in the Unicode system.
• The ASCII character set (representing English
  keyboard characters) is shown in Appendix D.
• Char constants use single quotes 'A'.

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Wrapper Classes and Objects

• pages 371 - 373 in text
• All primitive data types have a class associated
  with various methods.
• These wrapper objects can also store one
  primitive data type value for each object. The
  usefulness of this is described later.
• The wrapper class for a char is the Character
  class.

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Character Operations
char letter 'a', digit '4' System.out.printl
n (Character.isLetter (letter)) System.out.print
ln (Character.digit (digit, 10)) System.out.prin
tln (Character.digit (letter, 16)) int i
letter System.out.println(i) Note Character
? char
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Type Conversions

• Each character value maps to an integer.
• For the ASCII character set, these numbers range
  from 0 to 127.
• Use (char) i to get a character value from an
  int.
• Use (int) ch to get the ASCII value from a char.
• examples of casting.

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Casting for ints

• Java will not assign a value of a more inclusive
  type to a variable of a less inclusive type
  unless the code explicitly converts the type.
• Each primitive type can be cast to any other
  primitive type, but information may be lost.
• int i 5
• double d 3.5
• i (int) d // Cast operation
• System.out.println (i) // Displays 3
• (int) truncates by dropping the fractional part.

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Strings (page 126)
String str "Hey Joe!" System.out.println
(str.length()) System.out.println
(str.charAt(4)) System.out.println
(str.indexOf('J')) System.out.println
(str.toUpperCase()) A string is an array of
chars
'H'
'e'
'y'
' '
'J'
'o'
'e'
'!'
0 1 2 3 4 5
6 7
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Standard String Processing Loop
for (int i 0 i lt str.length() i) char ch
str.charAt(i) ltprocess chgt
'H'
'e'
'y'
' '
'J'
'o'
'e'
'!'
0 1 2 3 4 5
6 7
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Defining a String Method
Write a method that tests a String to see whether
or not it represents an integer. A String
represents an integer if it is not empty and
contains just decimal digits. // Input
parameter a String // Returns true if the
String is not empty and // contains just digits
or false otherwise boolean validInt (String
str) . . .
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Defining a String Method
// Input parameter a String // Returns true if
the String is not empty and // contains just
digits or false otherwise. public boolean
validInt (String str) if (str.equals(""))
return false // Empty for (int i 0 i lt
str.length() i) char ch
str.charAt(i) if (! Character.isDigit(ch))
// Not digit return false
return true // Got through
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Equality

• Use and ! with primitive types and window
  objects.
• Use equals and ! equals with all other types,
  such as String.
• String a "xyz", b "xyz"
• if (a b) ... // Always false
• if (a.equals(b)) ... // Use this instead

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Lexicographical Order
"Ann" lt "Bill" lt "bill" // Good idea, bad
Java String str1 "Ann", str2
"Bill" str1.compareTo("Ann") // Returns
0 str1.compareTo(str2) // Returns int lt
0 str2.compareTo(str1) // Returns int gt 0 if
(str1.compareTo(str2) lt 0) // str1 comes
before str2
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Palindrome, pp. 129130 Example of String
processing public void buttonClicked (Button
buttonObj) String aString stringField.getText
() aString aString.toUpperCase() if
(isPalindrome (aString)) messageBox ("Yes,
you entered a palindrome.") else
messageBox ("No, you did not enter a
palindrome.") private boolean isPalindrome
(String s) int lastPosition s.length() - 1
int middlePosition lastPosition / 2 int
forward 0 int backward lastPosition
while (forward lt middlePosition) if
(s.charAt (forward) ! s.charAt (backward))
return false forward
backward-- return true
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The Math Class(pp. 134135)

• Contains several useful methods and constants
• abs (number) Returns the absolute value of
  number.
• sqrt (number) Returns the square root of number.
• pow (x, y) Returns xy (all double).
• PI Constant double value closest to ?.
• double side . . .
• double area Math.sqrt (side)
• Use the class name Math before the method name.

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Rounding
Math.round rounds to the nearest whole number, as
a long. int i 5 double d 3.5 i (int)
Math.round (d) // Round operation System.out.pri
ntln (i) // Displays 4
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Random Numbers
Problem Generate a random integer between 1 and
6 for rolling dice. Math.random() // Returns a
double, d, where 0 lt d lt 1 Math.random() 6
// Returns a double, d, where 0 lt d lt 6 (int)
(Math.random() 6) // Returns an int, i, where
0 lt i lt 5 (int) (Math.random() 6) 1 //
Returns an int, i, where 1 lt i lt 6
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Floating point arithmetic

• Limited precision
• float a 1
• float b 100000000 // 100,000,000
• float c, d, e
• c b 5 // Yields 100,000,008
• d b (aaaaa) // Yields 100,000,008
• d b aaaaa // Yields 100,000,000
• Less problem with double than float.

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CS 303E Class 7 Part 3 Data Structures
IObjects and Classes
"I have a cat named Trash. In the current
political climate it would seem that if I were
trying to sell him (at least to a Computer
Scientist), I would not stress that he is gentle
to humans and is self-sufficient, living mostly
on field mice. Rather, I would argue that he is
object-oriented."- Roger King
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What Is a Data Structure?

• A data structure is a construct that collects
  several data items together to be treated as a
  unit.
• Examples
• a string (a collection of characters)
• a bank account (a name, ID, and balance)

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What is an Object?

• An object is a data structure that collects
  information describing some thing so that a
  program can manipulate it a collection of data
  that can be treated as a unit.
• Examples
• a person (name, ID, address, phone number, etc.)
• a bank account (name, ID, balance, etc.)
• a window (labels, fields, text areas, buttons,
  etc.)

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An Object Contains its own Data

• Each object has a separate area in memory and has
  space for a value for each datum.
• E.g., each object of type Person might have space
  for the following data

name ID address phoneNumber

int Strings
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What Is a Class?

• A class is a specifcation of
• the data needed in objects of the class type, and
• methods for manipulating those objects.
• That is, a class is a set of related methods and
  data.
• Examples of classes
• BreezyGUI classes Label, IntegerField,
  DoubleField, Button, etc.
• String and Math classes

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Example Class DoubleField

• Specifies
• all the data needed for an object (window
  component) of type DoubleField such as the
  components location and extent, its initial
  value, the characters it contains, etc., and
• the methods for manipulating objects of type
  DoubleField, such as addDoubleField, getNumber,
  setNumber, setPrecision, etc.

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Classes and Objects

• A class defines the methods and a template for
  the data for a set of objects.
• An object is an instance of a class.
• E.g.

DoubleField
Class
Objects
balance
transaction
interestRate
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Software Design with Classes

• Like methods, classes are a convenient tool for
  structuring code collecting of data (objects)
  and related methods.
• Classes are often useful when we need data
  structures in a program.
• A large system consists of several interacting
  classes.

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Clients and Servers

• Code that uses a class is also called a client.
• Code that implements a class is also called a
  server.
• To the client, the class provides an abstract
  data type (ADT), which is a black box that hides
  information about the details of the class from
  the client and provides only an interface to
  objects of the class type.

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One File per Class

• Java requires a separate file for each class.
• Your code will have
• A file with code that defines a class.
• A separate file for code that uses the class. It
  will create objects and manipulate them using
  methods in the class.

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Example Class Student(pp. 159180)

• Analysis (design)
• What attributes (data) and behavior (methods)
  are needed by users for each object in the class
  (each student)?
• Attributes (data) (p. 159)
• student name (type String) and
• three test scores (each of type int).

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Declaring class Student and data

• public class Student extends Object
• // Instance variables
• // Each Student object will have a
• // name and three test scores
• private String name
• private int test1
• private int test2
• private int test3

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Declaring the class and data

• public class ClassName extends Object
• // Declare Fields or Instance variables
• private type name1
• private type name2, name3
• // etc.

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An Object of type Student

• Each object of type Student occupies a separate
  area in memory and has space for a value for each
  instance variable

a String 3 ints
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Instantiation Creating an object of a class type
Code like the following goes in a program to use
a class to create objects of the class
type String sentence "A sentence
fragment." Frame frm new AccountManager() ge
orge new Student()
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Class Student - Behavior

• What can be done with a Student object from the
  clients perspective?
• instantiation -- create an object of type Student
• set the name and the test scores
• get the name and the test scores
• get the students average, highest score, lowest
  score, etc.

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Methods - Constructors

• Constructor methods create or instantiate
  objects
• // Constructor method
• // Initialize a new student's name to the empty
  // string and his test scores to zero
• public Student()
• name ""
• test1 0
• test2 0
• test3 0

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Design the methods interface.

• E.g.
• setName (aString) -- returns void
• getName () -- returns String
• These are object methods -- the object is implied
  and supplied before the method name with a dot.
  Calls
• Student stu
• stu.setName ("Bill Jones")
• String stuName stu.getName()

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Implement the Method

• // Set a student's name
• public void setName (String nm)
• name nm
• // Get a student's name
• public String getName ()
• return name