Alice in Action with Java Chapter 7 From Alice to Java

Alice in Action with Java

Chapter 7From Alice to Java

Alice in Action with 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


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


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 what’s new in the current edition

• The Eclipse IDE is divided into several regions


Starting Eclipse (continued)


Writing a Java Program Using Eclipse

• Creating a project– Select File->New->Project 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


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->New->Class

• 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






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


Compiling a Java Program Using Eclipse (continued)


Running a Java Program Using Eclipse

• Running your program the first time– Choose the Run->Run... 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


Running a Java Program Using Eclipse (continued)






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


Testing a Java Program Using Eclipse (continued)


Java Basics

• Strategy: compare the first program to the algorithm

• References– Algorithm developed in Section 7.1– DollarsToEurosConverter.java in Figure 7-20


Java Basics (continued)


Comments

• Comments: explanatory remarks

• Comments are ignored by the Java compiler

• Three different kinds of comments– Inline: begins comment with // and ends at line’s 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


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


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


The Simplest Java Program (continued)


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– String[]args 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


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


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;



• 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 => %.2f euros“

• Placeholder %.2f provides precision and type information




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


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 program’s application

• Step 2: extract the objects from the noun phrases

• Step 3: extract the methods from the verb phrases


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




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


Writing the UnitPricer Program (continued)


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


Testing the UnitPricer Program (continued)


Testing the UnitPricer Program (continued)


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


Solving the Problems (continued)


Solving the Problems (continued)

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Alice in Action with Java Chapter 7 From Alice to Java