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

Alice Java

•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 markthe end

Open braces markthe beginning

A First Program – What Does It Do?

Prints the messageThis is my first Java program.

Ends the line

System.out.println("This is my first Java program.");

Same steps to create a program

Alice Java

Storywrite Design

Code Code

Compile

Play Run

Test Test

Make the Program Run

• Compile (translate to java byte code)

• Run (interpreter for that OS reads java byte code and translates for machine)

Sourcex.java

Compile(javac x.java)

Objectx.class

Execute(java x)

Development Environment

Blue Jay

Lets you enter code just like Alice did

Download at:

http://www.bluej.org/download/download.html

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

10 100 1000

10.55 100 1055

10 .055 .55

0 100 0

100 0 0

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 BlueJCreate a new classWrite 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 euros.

@author 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 euros. @author you*/Import java.util.scannerPublic 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 variableeurosPerDollar = 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

10 100 1000

10.55 100 1055

10 .055 .55

0 100 0

100 0 0

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 (*,/,+,-,^)