Alice in Action with Java

1
Alice in Action with Java

• Chapter 7
• From Alice to Java

2
The Algorithm

• Algorithm sequence of steps that solve a problem
• Algorithm for converting dollars to euros
• 1. Display "How many dollars do you want to
  convert?"
• 2. Read dollars
• 3. Display "What is the euros-per-dollar exchange
  rate?"
• 4. Read eurosPerDollar
• 5. Compute euros dollars eurosPerDollar
• 6. Display dollars and euros, plus descriptive
  labels

3
Writing a First Java Program

• Integrated development environment (IDE)
• Program editor
• Compiler
• Run-time environment
• Debugger
• Eclipse
• Free IDE for various languages developed by IBM
• Java programs in this text are built using
  Eclipse

4
Starting Eclipse

• Double-click eclipse.exe or eclipse shortcut
• Select a folder to hold your Java programs
• Items in the first time welcome screen
• An overview of Eclipse
• Tutorials for using Eclipse
• Some code samples
• A summary of whats new in the current edition
• The Eclipse IDE is divided into several regions

5
Starting Eclipse (continued)
6
Writing a Java Program Using Eclipse

• Creating a project
• Select File-gtNew-gtProject from the menu bar
• New Project dialog box opens
• Make sure that Java Project is selected
• Click the Next button
• New Java Project dialog box opens
• Enter a descriptive name
• Click the Finish button
• After clicking the Finish, the main window
  reappears
• The new project appears in the package explorer
  area

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Writing a Java Program Using Eclipse (continued)
8
Writing a Java Program Using Eclipse (continued)
9
Writing a Java Program Using Eclipse (continued)

• All Java programs are classes
• Creating the DollarsToEurosConverter class
• Select the project in the package explorer area
• Select File-gtNew-gtClass
• The New Java Class dialog box opens
• Type DollarsToEurosConverter as the name
• Ensure Public static void main()is checked
• Click the Finish button
• After the class has been created, enter program
  code
• Lines in main() correspond to steps in the
  algorithm

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Writing a Java Program Using Eclipse (continued)
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Writing a Java Program Using Eclipse (continued)
12
Compiling a Java Program Using Eclipse

• Java compiler checks a program for syntax errors
• Invoke the compiler by saving your program
• How the Java compiler reports errors
• Error symbol displayed next to problem statement
• Error is listed in the problems output area
• Example of an error omit semicolon after
  statement
• The compiler marks the line as an error
• Error details are shown in the problems output
  area
• All errors must be fixed before you can run a
  program

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Compiling a Java Program Using Eclipse (continued)
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Running a Java Program Using Eclipse

• Running your program the first time
• Choose the Run-gtRun... menu choice
• The Run dialog box opens
• Ensure Java Application is selected
• Click New button below Configurations column
• A new Java Application run configuration appears
• Ensure project is listed in the box below Project
• Use Search to identify class containing main()
• Click the Apply button and then click Run
• Initiate future executions by simply clicking Run

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Running a Java Program Using Eclipse (continued)
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Running a Java Program Using Eclipse (continued)
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Running a Java Program Using Eclipse (continued)
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Testing a Java Program Using Eclipse

• Functional testing
• Running a program multiple times, using various
  values
• Example use various dollar values and exchange
  rates
• Sanity checking testing with easily verified
  values
• Logic error problem with the program structure
• User testing
• Utilizing another person to uncover hidden flaws
• Example roommate reveals euros formatting error
• Solution use printf()to round off values of
  euros

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Testing a Java Program Using Eclipse (continued)
20
Java Basics

• Strategy compare the first program to the
  algorithm
• References
• Algorithm developed in Section 7.1
• DollarsToEurosConverter.java in Figure 7-20

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Java Basics (continued)
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Comments

• Comments explanatory remarks
• Comments are ignored by the Java compiler
• Three different kinds of comments
• Inline begins comment with // and ends at lines
  end
• Block (C-style) begins with / and ends with /
• Javadoc begins with / and ends with /
• Opening comments of converter are Javadoc type
• Every file should have an opening comment
• In general, add comments to improve readability

23
Import Statements and Packages

• Package group of related, predefined classes
• Example Scanner class is in java.util package
• Access packaged classes with an import statement
• Place import statements before class declaration
• Example import java.util.Scanner
• Wild-card import statement
• Used to import all of the classes in a package
• Example import java.util.
• java.lang contains commonly used classes
• Automatically imported into a program

24
The Simplest Java Program

• Consists of a class and an empty main()method
• General pattern used to define a class
• public class NameOfTheClass
• The word public makes the class visible to users
• The word class indicates a new type name follows
• Every Java program needs a main()method
• The main()method is defined within the class body
• main()is like myFirstMethod()in Alice

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The Simplest Java Program (continued)
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The Simplest Java Program (continued)

• Understanding the structure of the main()method
• public allows method to be invoked outside class
• static indicates that main()belongs to the class
• void indicates that main()does not return a value
• main is the name of the method
• Stringargs provides a way to pass values
• Statements placed within body are delimited by
  and
• Adding Java statements using Eclipse
• Type or paste statements into the editing area
• Differs from drag-and-drop style used in Alice

27
Some Java Statements

• Technique for writing a program
• Go through an algorithm step by step
• Translate each step into an equivalent Java
  statement
• Goal apply technique to dollars-to-euros
  algorithm
• Step 1
• Display "How many dollars do you want to
  convert?
• Use System.out.print(String query)
• Step 2
• Read dollars
• Use a Scanner object to retrieve value from
  keyboard

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Some Java Statements (continued)

• Step 3
• Display "What is the euros-per-dollar exchange
  rate?
• Use System.out.print(String query)
• Step 4
• Read eurosPerDollar
• Reuse the Scanner object from Step 2
• Step 5
• Compute euros dollars eurosPerDollar
• Assign the value in the expression to euros
  variable
• double euros dollars eurosPerDollar

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Some Java Statements (continued)

• Step 6
• Display dollars and euros, plus descriptive
  labels
• Use System.out.println(String output)
• Concatenation operator () combines String values
• The printf()statement
• Controls the format of printed values
• Must have at least one argument (format-string)
• Arguments after the format-string need a
  placeholder
• Example ".2f dollars gt .2f euros
• Placeholder .2f provides precision and type
  information

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Some Java Statements (continued)
31
A Second Java Program

• Problem how to determine relative value
• Scenario 1
• Regular size cereal costs 2.90 per 12 ounces
• Economy size cereal costs 4.00 per 15 ounces
• Scenario 2
• 60-gigabyte MP3 player costs 150
• 80-gigabyte model costs 190
• Solution compare items using unit prices
• Goal program should find the unit price of an
  item

32
Designing the UnitPricer Program

• Step 1 elements of user story built around unit
  price
• Query What is the price of the first item?
• Read the first price from the keyboard
• Query How many units are in the first item?
• Read the number of units in the first item
• Perform the first four actions for the second
  item
• Compute and display the unit prices of the two
  items
• Use of a generic item broadens programs
  application
• Step 2 extract the objects from the noun phrases
• Step 3 extract the methods from the verb phrases

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Designing the UnitPricer Program (continued)
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Designing the UnitPricer Program (continued)
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Designing the UnitPricer Program (continued)

• Step 4 develop the UnitPricer algorithm
• The algorithm is the blueprint for the program
• Generalization broadens application of a program
• Generalization in the UnitPricer algorithm
• The use of item in place of cereal box and MP3
  player
• Item is a generic term embracing a variety of
  objects

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Designing the UnitPricer Program (continued)
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Writing the UnitPricer Program

• A summary of the steps
• Create a new Java project in Eclipse
• Create the UnitPricer class
• Implement the algorithm in main()
• Figure 7-24 presents the final version

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Writing the UnitPricer Program (continued)
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Testing the UnitPricer Program

• Conduct testing using easy-to-verify values
• The printf() message revisited
• Begin format-string with n to advance cursor one
  line
• w.pf placeholder used to specify width and
  precision
• Format-string "nItem 1 unit price 7.2f
• n advances the cursor to the next line
• 7.2f 7 spaces, two decimal places for a real
  number

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Testing the UnitPricer Program (continued)
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Testing the UnitPricer Program (continued)
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Solving the Problems

• Solve the MP3 player problem
• Reminder the 60-gigabyte model cost 150
• Reminder the 80-gigabyte model cost 190
• MP3 player solution 80-GB model is a better
  value
• Solve the corn flakes problem
• Reminder the 12-ounce regular size cost 2.90
• Reminder the 15-ounce economy size cost 4.00
• Corn flakes solution regular size box is a
  better value

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Solving the Problems (continued)
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Solving the Problems (continued)