Begin Java having used Alice Pepper - Some slides from Alice in Action with Java

Objectives Write some first Java programs Learn the basics of the BlueJ Integrated Development Environment (IDE) Begin making the transition from Alice to Java

Alice vs Java AliceJava 3D graphics let you visualize programming concepts Drag-and-drop coding reduces syntax errors Create new objects - Run Java programs across various platforms Build applets to make Web pages interactive Much of Alice is written in Java

A very simple Alice program

A Very Simple Java Program public class JavaWorld { public static void main(String[] args) { System.out.println ("This is my first Java program."); } Class header Method header statements Close braces mark the end Open braces mark the beginning

A First Program – What Does It Do? Prints the message This is my first Java program. Ends the line System.out.println ("This is my first Java program.");

Same steps to create a program AliceJava StorywriteDesign Code Compile PlayRun Test

Make the Program Run Compile (translate to java byte code) Run (interpreter for that OS reads java byte code and translates for machine) Source x.java Compile (javac x.java) Object x.class Execute (java x)

Development Environment Blue Jay Lets you enter code just like Alice did Download at: Instructions on syllabus

Exercise Add the line “Welcome to Java” When you run the program, you will see: This is my first Java Program. Welcome to Java. Extra: –Info: print() does not go to the next line –Try splitting Welcome to Java into two statements: “Welcome” and then “ to Java.”

A program that does something Convert Dollars to Euros Where to start?

Design possibilities By words : Nouns and Verbs  algorithm By test data : List some possible inputs and outputs before coding By class diagram : Design object pictures By flow chart : chart the sequential steps and decisions

Designing by words First step: write a user story to help setup structure User story for dollars-to-euros currency conversion –Query user for dollar amount to convert to euros –Read the dollar amount from the user –Query user for euros-per-dollar exchange rate –Read euros-per-dollar exchange rate –Compute corresponding number of euros –Display dollar and (computed euros values)

Nouns  objects ? = make a Class

Verbs -> Operations

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

Design by test data Possible inputs and outputs: Input Dollar Input euros per dollar Rate Output Euros

Design by class diagram DollarsToEuroConverter main Scanner nextDouble Keyboard Input

Design by flowchart Ask for dollars start Read Dollars Ask for rate Read rate Compute euros Print euros end

Let’s start coding – setup #1 Create a new project in BlueJ Create a new class Write the standard new World starter: public class DollarsToEuroConverter { public static void main(String[ ] args) { }

Comments – setup #2 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 */ Add comments to your class now at the top: /** DollarsToEurosConverter.java converts dollars to you */ Public class DollarsToEurosConverter

Import Packages – setup #3 Like importing other character blueprints– actually groups of characters Package Example: Scanner class is in java.util package Bring it in with an import statement –Example: import java.util.Scanner; - let’s us talk with the user – like world’s “ask user for” functions Do it now in your program between comment and class start: /** DollarsToEurosConverter.java converts dollars to you */ Import java.util.scanner Public class DollarsToEurosConverter

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) – Like SAY Ask now in your program, and then run it public class DollarsToEuroConverter { public static void main(String[ ] args) { System.out.print("How many dollars do you want to convert?”); }

Step 2 – Read dollars from screen Step 2 –Read dollars –This means we have to read from the screen. We need an object that knows how to do this.  Scanner. –Once we create him, we can ask him what the user typed on the screen. We can ask him as many times as we like once he is created. –Create a scanner in your program now Scanner kbd = new Scanner (System.in);

Step 2 read the dollars cont. Now we can ask scanner for the dollars, but we will need to save those dollars into a variable: double dollars; -> creates a variable dollars = kbd.nextDouble(); -> gets the input from the user and puts it into dollars

Step 3 & 4 – same thing for rate Step 3 –Display "What is the euros-per-dollar exchange rate?” –Use System.out.print(String query) System.out.print(“What is the Euros per dollar exchange rate?”); Step 4 –Read eurosPerDollar –Reuse the Scanner object from Step 2 double eurosPerDollar; -> creates a variable eurosPerDollar = kbd.nextDouble(); -> gets the input from the user and puts it into eurosPerDollar

Step 5 - compute Step 5 –Compute euros = dollars * eurosPerDollar –Assign the value in the expression to euros variable double euros; euros = dollars * eurosPerDollar;

Step 6 – tell result –Display dollars and euros, plus descriptive labels –Use System.out.println(String output) –Concatenation operator (+) combines String values Type: System.out.println(dollars + “dollars => “ + euros + “euros”); RUN YOUR Program

Check test data Possible inputs and outputs: Input Dollar Input euros per dollar Rate Output Euros

Testing a Java Program Using BlueJ 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

Print statement – a bit more 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

Some Java Statements (continued)

Step 6 – a bit better Type the statement to print the result to the screen in place of println: System.out.printf( “%.2f dollars => %.2f euros”,dollars, euros);

Your toolset Basic setup including comments and importing packages Print to screen with formatting Read from screen Create variables to hold decimal values Calculate (*,/,+,-,^)