Week 02 a

1
Week 02 - a

• Anatomy of a Program
• The Math Class
• Debugging

2
Sample Programs Referenced(code located in
course folder)

• java floating.FloatFrame
• java strings.StringLab
• round vs. rint
• MowTheLawn

• BuggyPayroll
• Base26 Arithmetic

3
Review from Lect01-b (AnimatedIllustrations)

• java floating.FloatFrame
• java strings.StringLab

4
Writing Java Code

• Java syntax imposes some rules
• Java is case-sensitive (camelNotation not same as
  CamelNotation)
• Must put a semicolon at the end of each statement
  
• Must balance parentheses and braces

5
Formatting Java Code

• Programmer conventions make the code more
  readable but are not imposed by the compiler
• White space (blank lines and spaces) is used to
  improve readability
• Spaces between operands and operators
• Leading spaces for indentation to show blocks
• Blank lines between methods
• Line breaks
• are not required, but should be used, at the end
  of each statement
• may occur in the middle of a statement but not in
  the middle of an identifier

6
Formatting Blocks of Java Code

• Placement of opening and closing braces for class
  and method definitions
• Comments at end of classes and methods
• public class Sample
• private int y
• public int doubleIt()
• return 2y
• // end of doubleIt method
• // end of Sample class

7
Identifiers

• Name of a data field, method, class or object
• Specified in declaration statements and headers
• Rules
• Should begin with a letter
• Can contain letters, digits, underscores
• Must not be a reserved word (e.g. class)
• see Table 2.8, page 81 of text
• Can be any length
• Some examples of valid and invalid identifiers
• Valid xrayVision, p28, TAX_RATE
• Invalid TAX-RATE, p.28, File5

8
Naming Conventions

• Class names start with capital letter
• e.g. Washer
• Other identifier names start with lowercase
  letter
• e.g. inner
• Each following word in the name starts with a
  capital letter (soCalledCamelNotation)
• e.g. inRadius
• Identifiers for constants (final) use all caps
  with underscores to separate words
• e.g. FEET_PER_MILE

9
Comments

• Comments are for the human reader
• Program Maintenance
• Single line comments start with //
• // Create a Washer object
• Multi-line comments are surrounded by / ... /
• /
• WasherApp.java Authors Koffman Wolz
• A class that finds the weight of a batch of
• flat washers
• /

10
CSc-037 HW Comments(from Grading Sheet)

• Class Comments / CSc-037 HW Assignment __
  ltone-line description of classgt   _at_author  Joe
  Schmoe _at_version ___/___/2005
• Method Comments/ ltone-line description of
  purpose of methodgt  _at_param ltone-line
  description of each parametergt _at_return
  ltone-line description of returned valuegt
• Prompts for All Input (use InputDialogs for now)
• Headings/Labels on All Output
• Comment after each    // end of
  ltdescription of blockgt
• Comment line for each 10-15 line block of code
• Meaningful CONSTANT, variable NamesMeaningful
  method Class Names
• Indent 2 spaces for sub-blocks, 4 spaces for
  continuation of statement onto a second line

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The main Method(in Client Class--"Application")

• public static void main(String args)
• // end of main method
• public --
• static --
• void --
• main --
• String --
• args --

12
Problem AnalysisReview

• Input value(s)?
• Output value(s)?
• Known Constants Needed? (e.g. PI)
• Classes (Methods)?
• Existing
• New
• Algorithm?

13
Finding Existing MethodsThe Math Class

• Java includes the Math class with methods for
  performing common functions
• E.g. sqrt, sin, cos, round, random
• No objects of class Math can be instantiated
• Technical term is "static"
• Methods are called using the class name
• Examples
• double diagonalSquare Math.sqrt( 2 side
  side )
• int wholeNum Math.round(diagonalSquare)
• int randNum (int) (Math.random()10) 1

14
BlueJ "Tools"gtUse Library Class

• See list of available methods
• Try Math
• min
• sqrt

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Methods of the Math Class

• See Table 2.11 of the text, page 87
• Check the data types expected as arguments
• Check the data types of the return values
• Note Methods round and rint both round a double
  argument. round returns an int while rint
  returns a double
• Why is this important? What is the value of y?
• // Math.PI 3.14159
• y Math.round(Math.PI 100) / 100
• ---- or ----
• y Math.rint(Math.PI 100) / 100
• Try this in BlueJ Code Pad

16
Sample Programming Process

• How long will it take to Mow the Lawn?
• Inputs?
• house dimensions, yard dimensions
• Outputs?
• time to mow lawn
• Classes (Methods)?
• Rectangle (area)
• Lawn (setSize, getMowingTime)
• Constants? (rate of mowing2.3 sq. meters/sec)
• Algorithm?
• ASS U ME yard and house are rectangular!
• Try on paper/board
• Write and test together using BlueJ

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Client (Main Program)

• import javax.swing.
• public class MowTheLawn
• public static void main(String args)
• Rectangle yard, house
• Lawn grass new Lawn()
• String yardDimensions JOptionPane.showInputD
  ialog(
• "Yard length, width (in
  meters)?")
• yard new Rectangle(yardDimensions)
• String houseDimensions JOptionPane.showInput
  Dialog(
• "House length, width (in
  meters)?")
• house new Rectangle(houseDimensions)
• grass.setSize(yard.area() - house.area())
• JOptionPane.showMessageDialog(null,grass.getMo
  wingTime()
• " seconds to
  mow the lawn")
• // end of main method
• // end of MowTheLawn class
• Question What attributes does a Rectangle need?
• Question How will Rectangle get numeric length
  and width from the String?

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Rectangle Class

• public class Rectangle
• private double length, width // dimensions of
  the Rectangle
• public Rectangle(String dimensions) //
  Constructor
• int len dimensions.length()
• int comma dimensions.indexOf(",")
• length Double.parseDouble(dimensions.substri
  ng(0,comma))
• width Double.parseDouble(dimensions.su
  bstring(comma1,len))
• // end of Rectangle constructor
• public double area()
• return length width
• // end of area method
• // end of Rectangle class

19
Lawn Class

• public class Lawn
• private double size
• private final double MOWING_SPEED 2.3
• public void setSize(double newSize)
• size newSize
• // end of setSize method
• public double getMowingTime()
• return size / MOWING_SPEED
• // end of getMowingTime method
• // end of Lawn class

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How Do We KnowThat It Worked???

• Program Testing
• Test Data
• Compare with results computed by hand

21
To the Computers...

• View BlueJ Code Pad
• (need to create/open a project to activate Code
  Pad)
• Math.round(4.4)
• Math.round(4.6)
• Math.rint(4.4)
• Math.rint(4.6)
• Try BlueJ access to Library Methods
• Look at Documentation for String and Math

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Java BREAK
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Common Errors and Debugging

• Syntax errors
• What is a syntax error?
• When do we find out that our program has a syntax
  error or errors?
• What common syntax errors did we explore in lab?
• Others not explored?
• Run-Time errors
• Detected during the execution of a program
• Examples division by 0, Double.parseDouble(non-nu
  mber)
• Logic errors
• Occur when the program is written from an
  incorrect algorithm
• Example printing the result of a computation
  before the computation is performed.

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Trace the Executionof the Program

• Follow the statements in the order they would be
  executed.
• What happens when a call to a method is reached?
• By Hand Draw pictures of the contents of memory
  as the execution proceeds.
• Primitives and objects
• Parameters and local variables
• Use System.out.println statements
• Use the BlueJ debugger to step through the
  program execution.

25
Buggy Payroll Program

• The term bug was coined by Admiral Grace Hopper
  in the early days of computers (1950s)
• Nanosecond Story
• Program has at least three logic errors
• Usually you wont know a priori that there are
  errors
• We need to first test the program to find the
  errors!

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Testing

• Standard Rate of Pay is 10/hour
• Standard Tax Rate is 30
• Check to be printed using
  FirstName LastName
• Test Data
• Einstein, Al (stored like this so sort works)
• 40 hours
• Output should be Al Einstein and 280

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Payroll Client

• import javax.swing.
• public class BuggyPayroll
• public static void main(String args)
• String name JOptionPane.showInputDialo
  g( "Employee Name?")
• double hoursWorked Double.parseDouble(
  JOptionPane.showInputDialog(
  "Hours?"))
• Employee worker new Employee(name)
• worker.printPayCheck(hoursWorked)
• // end of main method
• // end of BuggyPayroll class

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Debugging

• Use BlueJ debug tracing
• Project View menu -- show Debugger
• Click line number to add breakpoint
• Step thru program

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Printing Dollars Cents

• We will discuss Formatting of numbers next time

30
Base-26 Arithmetic

• Need 26 distinct digits
• Just happen to be 26 letters in alphabet
• Use A for zero, B for one, etc.
• Client program to create a couple of base-26
  numbers, print out their sum
• Our example will deal with 3-digit base-26 numbers

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Base26Client

• public class Base26Client
• public static void main(String args)
• Base26 x, y, z
• x new Base26("BBC")
• y new Base26("CBS")
• z x.add(y)
• System.out.print("Sum of " x.toString())
• System.out.print(" and " y.toString())
• System.out.println(" " z.toString())
• // end main method
• // end of Base26Client
• Note Need to decide how to store attributes of
  object of type Base26

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Base26 Class

• public class Base26
• private char ch26_2, ch26_1, ch26_0 //
  attributes of objects of type Base26
• public Base26(String str3)
• ch26_2 (char)(str3.charAt(0) - 'A')
• ch26_1 (char)(str3.charAt(1) - 'A')
• ch26_0 (char)(str3.charAt(2) - 'A')
• // end of Base26 constructor
• public Base26 add(Base26 arg)
• int first, second
• first 2626 ch26_2 26 ch26_1
  ch26_0
• second 2626arg.ch26_2 26arg.ch26_1
  arg.ch26_0
• int sum first second
• String result "" (char)(sum/26/26'A')
• (char)(sum/2626'A')
• (char)(sum 26'A')
• return new Base26(result)
• // end of add method
• public String toString()
• String result "" (char)(ch26_2'A')

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What Might Go Wrongwith Base26 Class?

• Construct Invalid Number
• fewer or more than 3-letter number
• invalid characters
• Overflow (more than 3-letter sum)
• ZZZ AAB BAAA
• More on "throwing" exceptions later

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HW Assignment 2

• Two parts
• Problem 9, page 103
• Problem 5, page 178
• Turn in Together
• staple all .java files to same Grading Sheet
• Grading Sheet is Available on Course Web Site
  (Homework Page)

35
To the Computers...

• Base26 Arithmetic
• Drag from course folder to "My Documents"
• Try other values
• Run with BlueJ Debugger turned ON
• Step through program to be sure you understand
  how it works

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Java BREAK