Java the easy way

James A. Rome

Tennessee Governor's Academy

August 2010

Why Java? Java is the most in-demand

language (= job opportunities) It is a modern object-oriented

language It is similar to C and C++

It leaves out pointers, multiple inheritance, operator overloading

Write once, run anywhere From supercomputers to cell

phones, Macs, Windows, Linux It is faster than any language other

than Fortran or Assembler There are oodles of libraries Free excellent development tools

Java programmers are in demandhttp://www.eweek.com/c/a/IT-Management/Programming-Development-Skills-In-Demand-505665

In the WSJ recently, there was a longarticle on how the language you useaffects how you think.

Computer languages foster logicalthought.• But many scientific calculations do not fit nicely into existing computer languages• Remember that digital computers do discrete mathematics and computations: calculations are done at a finite number of points. -> round-off and truncation errors -> most floating point numbers cannot be represented exactly in binary

Tools you will need (installed I hope)

The Java development kithttp://www.oracle.com/technetwork/java/javase/downloads/index-jsp-138363.html

The NetBeans IDE. Some people prefer Eclipsehttp://www.netbeans.org

A programmer's text editor such ashttp://www.crimsoneditor.com/

We may get to databases, so I asked for MySQLhttp://www.mysql.com/downloads/mysql/ http://dev.mysql.com/downloads/workbench/#downloads

Our foray into Java

Purpose: Get your feet wet so you can understand how scientists use computers and teach yourself more Java. The book is really good, so please read it. I want to concentrate on what is not in the book

How to do a graphical user interface (GUI) How to model a real problem and get it into a form a

computer can solve How to teach yourself

Along the way we will use and learn many Java Classes and techniques

Some basic syntax Assignment uses an equals sign

int b = 4; // assigns 4 to the integer variable b Testing for equality uses ==

if(b == 4) { /* do something when b is 4 */} // Comments to the end of the line /* . . .*/ Comments blocks, which can be many lines /** . . . */ A JavaDoc comment

Java is self documenting if you put these comments into your code they get converted to nice html

Formatting is optional but important for readability All statements must end in a semicolon Braces {} group lines of code and define a scope

Things defined in the braces are not accessible outside them. Names must start with a letter

Java syntax

Built-in types for variables: char, int, short, byte, long, float, double, boolean

These are not Objects. But all have wrapper classes that turn them into Objectsint a = 5;Integer aInt = new Integer(a);

Everything else is an Object (think noun, its a thing) Unlike in C or C++, these are the same on all platforms

Otherwise (without ints, floats,…) , you could not writea = b + c;

Loops

int j = 0; for(int i = 0, j = 5; i < 3; i++) { j += i; // becomes 5, 6, 8 (same as j = j + i) } // j = 8 at end of loop

// i no longer defined here because outside the {} while(j < 10) { // Test at start of loop System.out.println("j=" + ++j); // preincrement } // Prints out j=9, j=10

do { // Test at end of loop System.out.println("j=" + j++); } while (j < 20); // prints out j=10, ...j=20

starting values

do while

at end of loop do this

Class methods or fieldsare referenced using "."System.out returns aPrintStream object,which has a method println()

Logic (make decisions)

int a = 3;

while(a < 12) {

if(a == 5) // == is a test for equality

System.out.println("Hit a=5");

else if(a < 10)

System.out.println("a < 10");

else

System.out.println("a >= 10");

a++; // if you forget this, you get an infinite loop!

}

// Ternary operator

int b = ((a > 7) ? 0 : 1); // Usually a good idea to put

if(b != 5) b += 7; // Parens around the condition

Case statements

char c = 'b';

switch(c) {

case 'a':

System.out.println("a");

break; // if break is missing, this “falls through”

case 'b':

System.out.println("b");

break;

default:

System.out.println("not a or b");}

The argument you switch on must be an int, char, short, or byte. Used to improve logic and speed decision trees with a hash.

Everything in Java is part of a classClasses represent objects from the real world.Think of a class as “The definition of a set where each member of the set exhibits the same behavior and has the same attributes and associations.” Good programmers decide what classes they need to write before

starting to write a program! Classes in Java may have methods and attributes.

Methods define actions that a class can perform. Attributes (or fields) describe properties of the class

Classes are described in JavaDocs that are generated from comments in your code. (http://docs.oracle.com/javase/7/docs/api/)

You need to learn what the basic classes do by reading JavaDocs: String, Collection, HashMap, Vector, BufferedReader, BufferedWriter, Integer, Math, File, …

Call with ClassName.method() or ClassName.attribute

Creation of a Pet

public class Pet { int age; // with no public, private, these are float weight; // these are all protected float height; String color; /** Method to put pet to sleep */ public void sleep() { System.out.println("Good night. See you tomorrow"); } /** Method to feed pet */ public void eat() { System.out.println("Feed me a cookie!"); } /** Method to let pet speak */ public String say(String aWord) { String petResponse = "OK!! OK!! " + aWord; return petResponse; }}

The returned type

file:///home/jar/TGA/Presentations/FermiAcceleration.ppt

What is an IDE?

You write, compile, run, and profile your program within the Integrated Development Environment (IDE).

It makes your life simpler: A code-friendly editor with context highlighting, code

hints, and code completion It makes it really easy to create a graphical user

interface It write lots of boiler-plate code for you

NetBeans is also great for C/C++, Python, Ruby, Fortran

It is extendable with numerous plugins NetBeans is my favorite IDE by far

You might like Eclipse or JDeveloper

NetBeans should be installed

Start NetBeans IDEStart/All Programs/

Tools/Plugins/Installed Tab Make sure that

Java SE is activated

Set your NetBeans optionsUnder Tools/Options In General: Pick your favorite

Web Browser Under Options/Formatting,

set the indent and tab sizes to 3 characters

You can look at the other options at your leisure

Install the JavaDocs The Java documents come in

a zip file: jdk-7u??-docs.zip[download it from the JDK page]

NetBeans is smart enough to use it in its zipped form, but you might also want to use it in your Web browser, so copy the file from the cd to your Documents folder into a Java subdirectory. Unzip it by double-clicking on it.

Add the zip file to NetBeansTools, Java Platforms

You could add the source code here also

Create a Pet in NetBeans

File/New Project Create a new Project called “MyPets” Do not create a Main Class

Make a Package for your project

A package is a way of specifying a set of classes that have special privileges because they belong with each other. They avoid name collision By convention, you use a

reverse DN (e.g., com.sun.java.utility) We will use mypets (in lc)

Right-click the MyPets Source Package name and select “New, Java Package”

Create the Pet Class

Right-click the mypets Package and select “New, Class”

• Note that the file is alwaysclassname.java, and it is storedin the package directory tree• By convention, Classes startwith an uppercase letter.

Pet Code

Elements of our class: Four class attributes Three class methods One class constructor

In Java, things have visibility: public — visible and usable to all private — only accessible by this

class protected (default) —accessible by

classes in this package

Visibility keeps code clean and secure. If you want something visible, create getter and setter methods So the attributes are private

Classes must be instantiated

In order to do anything with a class you must “create an instance of the object.”

This uses the class definition to make a copy of the object described by the class and to put it in the computer’s memory.

Pet myPet =

new Pet();

myPet is the name of the instance and is a member of the class Pet.

We need code to instantiate Pet

public class PetMaster {

public static void main(String[] args) {

String petReaction;

Pet myPet = new Pet();

myPet.eat();

petReaction =

myPet.speak("Tweet!! Tweet!!");

System.out.println(petReaction);

myPet.sleep();

}

}

class signaturemain methodLocal variable

Make a Pet

Call the Pet methods

Make a PetMaster Class

Run the projectThe project needs to know where the main() method is.• Right-click the Project, and select Properties• Note that you must specify the package as well as the class name in the Run settings

Run your project by either clicking the green arrow, or selecting Run Main Project on the Run Menu

Inheritance

Our class Pet will help us learn yet another important feature of Java called inheritance. In real life, every person inherits some features from his or her parents. Similarly, in the Java world you can also create a new class, based on an existing one (but only one).

A special keyword extends does the trick: class Fish extends Pet {

}

Fish is a subclass of the class Pet.

Class Pet is a superclass of the class Fish. Fish myLittleFish = new Fish();

myLittleFish.sleep(); (Fish inherits all of the methods of Pet)

Making a subclass

Creation of subclasses in NetBeans is a piece of cake!

Right-click the package and select New, Class, and type Fish as the name of the class.

Then put in “extends Pet” before the first {

Let’s not forget, however, that we’re creating a subclass of a Pet to add some new features that only fish have, and to reuse some of the code that we wrote for a general pet.

Everything inherits from Object

Fleshing out our Fish

public class Fish extends Pet { private int currentDepth = 0; // keeps track of the depth

public int dive(int howDeep) { // a new method for fish currentDepth = currentDepth + howDeep; // Incremental System.out.println("Diving for " + howDeep + " feet"); System.out.println("I’m at " + currentDepth + " feet below sea level"); return currentDepth; } @Override // An annotation. This overrides Pet.speak public String speak(String something) { return("Don’t you know fish do not talk!"); }}

Need to change PetMaster

public class PetMaster { public static void main(String[] args) { String petReaction;// Your previous code can stay here....... Fish myFish = new Fish();

myFish.dive(2); myFish.dive(3); petReaction = myFish.speak("Glug"); System.out.println(petReaction); myFish.sleep(); }}

Run the code again with your fish

How to run without NetBeansTaken from README.TXT in the MyPets/Dist folder:

When you build an Java application project that has a main class, the IDEautomatically copies all of the JARfiles on the projects classpath to your projects dist/lib folder. The IDEalso adds each of the JAR files to the Class-Path element in the applicationJAR files manifest file (MANIFEST.MF).

To run the project from the command line, go to the dist folder andtype the following:

java -jar "MyPets.jar"

To distribute this project, zip up the dist folder (including the lib folder)and distribute the ZIP file.

User interfaces

Note the thousands of identical and unlabeled controls

© Pixar

The cartoon Lifted was shown along with Ratatouille. It is all about user interfaces.

It is even worse under pressure

© Pixar

Making an application with a GUI

There are several methods of making graphical user interfaces in Java — AWT, Swing, SWT. We will concentrate on Swing. Swing is built into all modern JDKs

A Swing book is on your CD

It is an easy-to-use high-level java interface Swing components are Java Beans

A Java Bean Object has get and set methods It adheres to certain naming conventions Java Bean properties can be manipulated in GUIs by

tools such as NetBeansBut you must always remember the user!

Making a GUI application using NetBeans Open NetBeans and do File,

New Project Select Java Application

from the Wizard and hit Next

Uncheck Create Main Class

Enter the project Name YourName and hit Finish

The YourName project will appear in the left Package Window, with a package, files, and a blank GUI

NetBeans has a new GUI framework

Make a new Java package

Create a new GUI class based uponA JFrame by right-clicking the yournamePackage.

Your new GUI project

NavigatorLets youset parametersof eachcomponentand rearrangethem

Palette hascomponentsyou can placeonto the GUI

Set theproperties ofeach com-ponent here

Add a prompt label

Click the Label in the Palette (top-right) Swing controls, and then click near the top-left of the design area

Use the dotted guidelines to space from the top and left edges.

Double-click jLabel1 to edit the text and hit Enter (or else it will not "stick")

The solid semi-circlesindicate that thiscomponent is pinnedto the edge if thecomponent is resized.

Add a TextField for data input

Add a TextField. Note the three spacing option lines and the lower edge alignment markers

Drag the right edge of the TextField to the right edge until the vertical dotted line appears. This pins the field to the right edge.

Double-click “jTextField1”, delete it, and hit Enter. Right-clicking and selecting “Edit text” may work better.

Try to run your project.

Rename your swing objects It is very important to replace the default names in a more

complicated project so that you know which text field (or other object) is which.

Right-click each label and change its name I used “enterNameLabel” and “enterNameTextField” Note that by convention, variables start with a lower-

case letter, and I retained the Swing type in the name Do this to all your Swing objects in the future...

Run the app

Notice how the TextField is pinned to the top and right edges as the window is resized.

Windows Apps do not have this nice property, which is due to using a Layout Manager

What is a Java Bean?

A class that exposes its accessor methods: For each public class member, there is a get and set

method It must be serializable. This allows the instance of

the class to be written to a file and read back again. class ClassName implements Serializable {....}

It must use member names that start with lowercase private String myString = “Boo!”

public String getMyString(void) {return String;}public void setMyString(String s) { myString = s;}

All Swing components are Java Beans Therefore, we can make our own GUI components by

extending a Swing class

Eliminate the StatusBar

Click StatusPanel anddelete it

Source view

Delete the statusBar code

At the end of YourNameView, delete private final Timer messageTimer;

private final Timer busyIconTimer; private final Icon idleIcon; private final Icon[] busyIcons = new Icon[15]; private int busyIconIndex = 0;

In the constructor, delete all but public YourNameView(SingleFrameApplication app) {

super(app); initComponents(); }

Fix the App name

Expand the yourname.resources package Open Application.description Change the Title (to "Your Name" ) and you can replace the

other fields (e.g., Vendor or the Description) if you wish.

Save your projectSave your project

Add an OK Button & output Label

Add a Button aligned to the bottom-right edge guides (which pins the button to this corner as the frame is resized).

Rename it to okButton, and change the displayed name to “OK”.

Add a Label for the output text called “outputLabel”. Stretch it to fill the horizontal width up to the right guideline. Delete the text (the label will collapse to a line).

I made the whole frame smaller verticallyThe Label ispinned to theleft edge and thetop of the button

Save your projectSave your project

Change the font for the outputLabel

Double-click the ... to openthe font dialog. Pick a fontyou like!

Save your projectSave your project

Add an action event to the JButton

The GUI is done, but it does not do anything yet! An ActionPerformedEvent occurs when a button

is pressed. At that point we can get the text the user typed and display it.

Right-click the okButton and add the event.

Save your projectSave your project

ActionEvent code Source view

w

The yellowbars indicateunused imports.Right-click in thecode andchoose "Fiximports"(They belongedto the progressbar that wedeleted.

Code completion and JavaDocs helps

private void okButtonActionPerformed(java.awt.event.ActionEvent evt) { // TODO add your handling code here: String name = "Your name is " + enterNameTextField.getText(); outputLabel.setText(name); }

Save your projectSave your project

Strings are immutable.They are stored andnever changed.

Run the code

Defects in the application: It would be nice to let the user hit “Enter” in addition to

clicking the button Can make it prettier. (e.g., Colored text) This is a possible security hazard because we do not

check what the user typed in. (It would be for sure in a Web Application.)

Adding a key event

private void enterNameTextFieldKeyTyped(java.awt.event.KeyEvent evt) { // TODO add your handling code here: }

Right click the enterNameTextFieldand do Event, Key, keyTyped.

Edit the JTextField KeyListener

'\n' is generated whenthe user hits Enter.Note that chars are insingle quotes.

This code appearstwice. It is a goodcandidate for a newclass method. Thenany changes to thiscode only have to bedone once.How????

Run your code and verify that hitting Enter is the same as clicking OK

Homework assignmentMake a Magic Fortune-Telling Machine Make a fortune-telling GUI– press

the button and get an answer. Answers: Yes, no, maybe, unlikely,

focus and ask again, cannot foretell now, looks like yes, indications say yes, consult me later, can't say now, absolutely, chances aren't good, don't bet on it, answer unclear ask me later, prospect good, very likely…

Make your own list Put in random delays before

answering Put in intermediate messages

("I'm thinking")

Code issues: Will need Math.random()

It returns a double from 0.0 to <1.0

For example, if there are 10 possible answersint j = (int)10.0*Math.random();

Will want to put answers in an array:String [] answers = {"Yes", "No",…}; // 10 entries

So, to get the random answer, just use answers[j]

One way of getting a delay is to make a for loop in which you do something that takes time [e.g., Math.sin()] and repeat it a random number of times.

Project: A RPN binary calculator

RPN = reverse Polish notation Easier logic and faster to use Example: 1+1=2

1 Enter 1+ // the result is displayed

Example: 1 + 5 / 6 = 1 1 Enter 5 + 6 / // the result is displayed

When you start to enter a number, the old result is erased from the field

A RPN binary calculator

My GUI for RPN Binary Calculator-- you make one you like

This has the samecomponents you usedin the last project.

Questions to ponder:• Does the code calculate in binary or decimal?• Where does the user enter numbers?• What does each button do?• How do you handle negative numbers?• Do you enter the binary digits from right-to-left, or from left-to-right?

Calculator logic Calculators have at least 2

registers: Accumulator accepts the

current entry Result stores the running

result The +-*/ keys

They preform their operation: Result +-*/ Accumulator

Display result Set clear flag and clears if

it was on

The number keys If first one, erases display

string Appends number to end

of display string The Enter key

Replaces the result with contents of Accumulator

Sets clear flag The Clear key

1st: Clears accumulator 2nd: Clears result

Code snippets for the calculator // New variables for Class // The accumulator private int accum = 0; // The running result private int result = 0; // The binary answer private String display = ""; private String binaryOut = ""; // Define constant variables for the operations // The entry field needs clearing private boolean clearEntry = true;

private void oneButtonActionPerformed(java.awt.event.ActionEvent evt) { if(clearEntry) { clearEntry = false; accum = 0; display = ""; } accum = accum * 2 + 1; display = display + "1"; binaryTextField.setText(display); decimalTextField.setText(new Integer(accum).toString());}

private void clrButtonActionPerformed(java.awt.event.ActionEvent evt) { // Make this so that the first time it is pressed,

// it clears the entry, // and the second time it clears everything if(accum != 0) accum = 0; else result = 0; display = ""; decimalTextField.setText(new Integer(accum).toString()); binaryTextField.setText(display); }

private void enterButtonActionPerformed(java.awt.event.ActionEvent evt) { result = accum; clearEntry = true;}

private void plusButtonActionPerformed( java.awt.event.ActionEvent evt) { // Calculate the result result += accum; clearEntry = true; binaryTextField.setText(toBinary(result)); decimalTextField.setText(new Integer(result).toString());}

/** * Convert a decimal number to binary and return it as a String * @param decimal - the input integer * @return The returned String binary representation of the decimal number */ private String toBinary(int decimal) { String binaryOut = ""; // Convert answer to binary while(true) { binaryOut = decimal%2 + binaryOut; //Move right-to-left decimal /= 2; if(decimal == 0) break; } return binaryOut; }

Decimal to binary conversion

Suppose we want to convert 9 to binary:Must start with the lowest power of 2(right-most bit) 9/2 = 4 with remainder 1

There must be a 1 in 2^0 right-most bit

Divide the number by 2 which moves the binary number one-place to the right, so we are checking the 2^1 place

4/2 = 2 + 0 remainder There are no 2^1 Answer so far = 01

Divide 4 by 2 again to check the 2^2 place

2/2 = 1 with 0 remainder No 2^2 Answer = 001

Divide by 2 again to check the 2^3 place

1/2 = 0 with 1 remainder Answer = 1001

Additions and improvements

Handle negative numbers Need to define the number of allowed places and the

sign bit Will have to check for user input errors!

Still calculate in decimal, but use two's complement in the display

Allow the user to input a positive binary number, but change it to two's complement with a +/- key

Possibly display the current result register Add a memory register