Begin java having used alice

A Very Simple Java Programpublic class JavaWorld { public static void mainString[] args { the end Open braces mark the beginning... Make the Program Run• Compile translate to java byte

Begin Java having used Alice

Pepper

- Some slides from Alice in Action

with Java

• Write some first Java programs

• Learn the basics of the BlueJ Integrated Development Environment (IDE)

• Begin making the transition from Alice to Java

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) {

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

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)

• 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

– 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?"

Design by test data

• Possible inputs and outputs:

Design by class diagram

DollarsToEuroConverter

main

Scanner

nextDouble Keyboard Input

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

euros

@author you

*/

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

*/

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

Check test data

• Possible inputs and outputs:

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 (*,/,+,-,^)