Extra Java Crash Course

8/8/2019 Extra Java Crash Course

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Java Crash CourseA Tutorial Introduction for C++ Programmers


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Getting Java Brewing1. Download the latest Java SDK from http://java.sun.com

The SDK is a command-line based set of tools2. A Text Editor: our choice is, of course, Emacs!

3. Web-browserthats java-enabled (optional)

4. IDE like BlueJ from http://www.bluej.org or JCreator fromhttp://www.jcreator.com (optional)

5. Some introductory links/guides/tutorials: http://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/c

ompile.html

http://www.horstmann.com/ccc/c_to_java.pdf

http://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdf
http://java.sun.com/http://www.bluej.org/http://www.jcreator.com/http://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/compile.htmlhttp://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/compile.htmlhttp://www.horstmann.com/ccc/c_to_java.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.csd.uu.se/datalogi/cmtrl/oopj/vt-2000/slides/OOPJ-1-04.pdfhttp://www.horstmann.com/ccc/c_to_java.pdfhttp://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/compile.htmlhttp://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/compile.htmlhttp://www.jcreator.com/http://www.bluej.org/http://java.sun.com/


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Mechanics of Writing Java Programs

Create a Java source file.Must have the .java extension andcontain only one public class.

Compile the source file into a bytecode file.The Javacompiler, javac, takes your source file and translates its text into

instructions that the Java Virtual Machine(Java VM) can understand.The compiler puts these instructions into a .class bytecode file.

Run the program contained in the bytecode file.TheJava VM is implemented by a Java interpreter, java. This interpreter

takes your bytecode file and carries out the instructions by translating

them into instructions that your computer can understand.


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Putting it all together

public class Hello {

public static void main(String args[]) {

System.out.println(Hello, world!);

}

}

1. Put in: Hello.java

2. Compile with: javac Hello.java

Creates Hello.class

3. Run with: java Hello


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Applications vs. Applets A Java application:

Is a standaloneprogram

Is interpretedby the Java Virtual Machine and run using the javacommand

Contains a main() method.

A Java applet:

Runs withina Java-enabled Web browser

extends the Applet or JApplet class (Inheritance)

Contains an init() or a paint() method (or both).

To create an applet, you'll perform the same basic steps:

1. Createa Java source file (NameOfProgram.java) and an HTMLfile (NameOfHTMLFile.html)

2. Compilethe source file (NameOfProgram.class)

3. Runthe program (either using java NameOfProgram(application) or appletviewer NameOfHTMLFile.html

(applet))


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Java notes for C++ programmers Everythings an object

Every object inherits from java.lang.Object

No code outside of the class definition!

No global variables (use static variables instead)

Single inheritance only

Instead, implementinterfaces

All classes are defined in .java files

One top level public class per file

The file has to have the same name as the public class!

Syntax is similar (control structures are very similar).

Primitive data types are similar

But a bool is not an int

To print to stdout, use System.out.println()


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Requisite First Program (ApplicationVersion)

Put in HelloWorld.java:

public class HelloWorld {

public static void main(String args[]) {

System.out.println("Hello World");

}

}


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Compiling and Running

HelloWorld.java javac HelloWorld.java

java HelloWorld

HelloWorld.class

compile

run

bytecode

source code


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So whats going on?The Java bytecode and interpreter at work!

Bytecodeis an intermediate representation of theprogram (the class file)

Think of it as the machine-code for the Java Virtual

Machine

The Java interpreter (java) starts up a newVirtual Machine

The VM starts executing the users class byrunning itsmain() method


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Put in HelloWorld.java:

import java.awt.Graphics;public class HelloWorld extends java.applet.Applet {

public void paint(Graphics g) {

g.drawString("Hello World, 35, 15);

}

}

Put in test.html:

Test the applet

Test the applet

Requisite First Program (Applet Version)


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Cool Applet Methods

Basic methods on Applet

init(): called once for your applet start(): called every time you enter the page

stop(): called every time you leave the page

destroy(): called when your page is discarded

Funky methods on Applet

AudioClipgetAudioClip(URL url): gets audioClip object (playwith audioClip.play())

Image getImage(URL url): starts asynchronous image loading

java.net.URL constructor takes normal string argument (used tostore URLs)

void showDocument(URL url): tells browser to load new

document void showStatus(String msg): writes to browser status line

Applet repainting paint(): defaults to nothing

update(): clears screen, callspaint()

repaint(): passes events to Motif/Win32dont override/change this


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Java Language Basics

Data types same as in C++ (except bool)

bool,char,byte,short,int,long,float,

double,string, etc.

Operators (same as C++)

Assignment: =, +=, -=, *=,

Numeric: +, -, *, /, %, ++, --,

Relational: ==. !=, , =,

Boolean: &&, ||, !

Bitwise: &, |, ^, ~, ,

Control Structures more of what you expect:

1. conditional: if, if else, switch

2. loop: while, for, do

3.break and continue


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Classes, References, & Packages Classes and Objects

All Java statements appear within methods, and all methods are

defined within classes. Java classes are very similar to C++ classes (same concepts). Instead of a standard library, Java provides a lot of Class

implementations or packages

What are packages? You can organize a bunch of classes and interfaces into a package

(or libraryof classes) defines a namespacethat contains all the classes.

Use the import keyword to includethe packages you need import java.applet.*;

You need to use some java packages in your programs

java.awt (Abstract Windowing Toolkit), java.io (for Files, etc.), java.util(for Vectors, etc.)

References No pointerseverythings a reference! classes arrays


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Exceptions

When a program carries out an illegal action, anexceptionis generated.

Terminology:

throw an exception: signal (in the method header)

that some condition or error has occurred but wewant to pass the buck and not deal with it.

catch an exception: deal with the error (or whatever)ourselves inside the function/method.

Catch it using a try/catchblock (next slide).

In Java, exception handling is necessary (forced bythe compiler compilation errors!)

Except for RunTimeExceptions


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Try/Catch/Finally

try {// code that can throw an exception

} catch (ExceptionType1 e1) {

// code to handle the exception

} catch (ExceptionType2 e2) {// code to handle the exception

} catch (Exception e) {

// code to handle other exceptions

} finally {// code to run after try or any catch

}

This block is always run


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Exception Handling

Exceptions take care of handling errors

instead of returning an error, some method calls willthrow an exception.

Can be dealt with at any point in the methodinvocation stack.

But if no method in the hierarchy handles it, resultsin an unchecked exception which generates acompiler error (unless its a RunTimeException)

Forces the programmer to be aware of what errorscan occur and to deal with them.


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Defining a Class One top level public class per .java file.

Typically end up with many .java files for a singleprogram with at least one containing a staticpublic main()method (if theyre applications).

Class name must match the file name!

The compiler/interpreter use class names to figure outwhat the file name is.

Classes have these three features:

A constructorthats used to allocate memory for theobject, initiailize its elements, and return a referenceto the object

Methods(function members)

Fields(data members)


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A Sample Class

public class Point {

public Point(double x, double y) {

this.x = x; this.y=y;

}

public double distanceFromOrigin(){

return Math.sqrt(x*x+y*y);

}

private double x,y;

}


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Objects and new

You can declare a variable that can hold an object:

Point p;

But this doesnt create the object! You have to usenew:

Point p = new Point(3.1,2.4);

new allocates memory and the garbage collector

reclaims unused memory


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Using Java objects

Just like C++:object.method() or object.field

BUT, never like this (no pointers!)

object->method() or object->field

Event driven model:

Objects register to receive (and respond to) certain

messageslike button presses, mouse clicks, etc.(e.g., mouseUp(), mouseDown(), keyUp(),keyDown())


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Strings are special

You can initialize Strings like this:

String blah = "I am a literal ";

Or this ( + String operator):

String foo = "I love " + CET375";

Or this ( new operator):

String foo = new String(Yummy FooBars!);


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Arrays

Arrays are supported as a second kind of reference type(objects are the other reference type).

Although the way the language supports arrays is differentthan with C++, much of the syntax is compatible.

however, creating an array requires new Index starts at 0.

Arrays cant shrink or grow.

e.g., use Vector instead.

Each element is initialized. Array bounds checking (no overflow!)

ArrayIndexOutOfBoundsException

Arrays have a .length


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Array Examples

int x[] = new int[1000];

byte[] buff = new byte[256];

float[][] mvals = new float[10][10];

int[] values;

int total=0;

for (int i=0;i


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Array Literals

You can use array literals likeC/C++ (no need for new keyword):

int[] foo = {1,2,3,4,5};

String[] names = {Joe, Sam};


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Reference Types

Objects and Arrays are reference types

Primitive types are stored as values

Reference type variables are stored as references(pointers that we cant mess with)


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Primitive vs. Reference Types

int x=3;

int y=x;

Point p = new Point(2.3,4.2);

Point t = p;

There are two copies of

the value 3 in memory

There is only one Pointobject in memory!


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Passing arguments to methods

Primitive types: the method gets a copy of thevalue. Changes wont show up in the caller

Pass by value

Reference types: the method gets a copy of thereference, the method accesses the same objectPass by reference

There is no pass by pointers!


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Comparing Reference Types

Comparison using == means:

Are the referencesthe same?

Do they referto the same object?

Sometimes you just want to know if twoobjects/arrays are identical copies.

Use the .equals() method

You need to write this for your own classes!

All objects and arrays are references!


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Inheritance

Use the extends keyword to inherit from a super

(or parent) class

No multiple inheritanceUse implements to implement multiple interfaces

(abstract, virtual classes)

Use import instead of #include (not exactly

the same but pretty close) to include packages(libraries)


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Using Documentation Comments

Documentation comments are delimited by /**

and */ javadoc automatically generates documentation

Copies the firstsentence of eachdocumentationcommentto a summary table

Write the first sentence with some care!

For each method/class, supply:@param followed by the parameter name and a short

explanation@return followed by a description of the return

value

@author for author info, etc.

Need to use javadoc

author for this

j d


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javadoc

The Java Standard calls for everyclass, every

method, everyparameter, and everyreturn value tohave a comment

Write the method comments first!

If you cant explain what a class or method does, youarent ready to implement it!

How to create HTML documentation:Type: javadoc *.java in the directory containing

your source code

This produces one HTML file for each class and anindex.html file

Documenation is together with code!

Must come immediatelybefore the class, method,etc.

ooey What does an applet with multiple buttons and a text field look


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import java.applet.Applet;

import java.awt.*;

import java.awt.event.*;

public class ClickMe extends Applet implements ActionListener {

private TextField text1;

private Button button1;

public void init() {

text1 = new TextField(20);add(text1);

button1 = new Button (Click me please!);

add(button1);

button1.addActionListener(this);

}

public void actionPerformed(ActionEvent e) {

String msg = new String (Hello, world!);

if (e.getSource() == button1) {

text1.setText(msg);

}

}}

Add GUI elements in init()

Add components to the

current frame (AWT)

Register a class (this one) tolisten for button events

ooeylike? Well develop this in more detail in lab but lets just

see it (in all its gory detail) here in ClickMe.java:

It d t thi


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It dont mean a thing Adding Swing to GUI applets/applications is easy:

Add import javax.swing.*; or importjava.awt.swing.*;

Use JApplet instead of Applet, JButton instead of Button,JTextField instead of TextField, etc.

These are from the JFC (hence, all the J prefixes)

In addition, you need to add components to the correctContentPane by doing the following in the init() fxn:

1. Use JPanel contentPane; to declareit andcontentPane = new JPanel(); to initializea newcontentPane

Could also have used Container contentPane =getContentPane() if our class extends JFrame

2. add everything to that contentPane using a statement likecontentPane.add(button1);

3. Finally, do a setContentPane(contentPane); (all in theinit() function) to make this the active contentPane

G idB L t


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GridBagLayout

Instead of the default FlowLayout (left-to-right), use

the GridLayout or GridBagLayout to add buttonsand other GUI elements along the grid

To call GridLayout, just use:

setLayout(new GridLayout(9,3)); for AWTor contentPane.setLayout(newGridLayout(9,3)); for Swing

GridLayouts firstargument is the number of rows

(0 for unlimited) and the secondargument is thenumber of columns

n examp e o a w ng- ase r ayou app e :
http://java.sun.com/docs/books/tutorial/uiswing/layout/GridDemo.html


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import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

public class ButtonsArray extends JApplet implements ActionListener {

private JTextField text[] = new JTextField[3];private JButton button[] = new JButton[3];

private JPanel contentPane = new JPanel();

public void init() {

contentPane.setLayout(new GridLayout(3,2));

for (int i=0; i


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Concurrent Multi-threaded Programming Java is multithreaded!

Threads are easy to use.

Two ways to create new threads: Extend java.lang.Thread

Override run() method.

Implement Runnable interface

Include a run() method in your class. Usually, youll implement the Runnable interface

How to implement the Runnable interface: Add a public void start() function:

This is where youll initializethe thread and start() it Add a public void stop() function:

This is where youll set the booleanstopFlag to true

Add a public void run() function:

This is where youll call repaint() to paint eachnew frame

and handle any synchronizedvariablesor methods

Th h i d St t t


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The synchronized Statement

Instead of mutex (a binary semaphore), use

synchronized:synchronized ( object ) {

// critical code here

}

Also, declare a method as synchronized:synchronized int blah(String x) {

// blah blah blah

}

Can also use wait() and notify() to putthreads on hold and wake them up again (e.g., toimplement a pause or suspend feature)

Must be called within a synchronizedblock

Double buffering


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Double-buffering

Composing an image off-screen to avoid flickering

Not needed with Swing objects (already double-buffered!) Create an offscreen graphics image and context:

Image offscreen;

Graphics offgraphics;

Dimension offdim;

Set them in init() method:offdim = getSize(); // Get size of applet first:

offscreen = createImage(offdim.width,offdim.height); // Create offscreen image of same size:

offgraphics = offscreen.getGraphics(); // Setup offscreen graphics context

Add an update() method that simply calls paint(): Overriding update() prevents the OS from wiping off the applets previous

drawings and, instead, immediately repaints (since wiping off causes flickering,too). Called automatically when repaint() is called.

Do all drawing to offscreen graphics context in the paint() method: Erase the previous image (with a big blank rectangle) Do all new drawing to the offscreen graphics context Finally, draw the offscreen image to the screen like a normal image:gr.drawImage(offscreen,0,0,this); //offscreen is width of screen, so start at 0,0


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Some Random Links

http://java.sun.com/docs/books/tutorial/uiswing/components/components.html

http://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.html

http://java.sun.com/docs/books/tutorial/uiswing/components/layeredpane.html
http://java.sun.com/docs/books/tutorial/uiswing/components/components.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/components.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/layeredpane.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/layeredpane.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/layeredpane.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/layeredpane.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/components.htmlhttp://java.sun.com/docs/books/tutorial/uiswing/components/components.html

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