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So what the heck is Java (besides coffee, that is)?. A programming language.Syntax and constructs are very similar to C A virtual platformJava Virtual Machine is a software
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1. Java Crash Course A Tutorial Introduction for C++ Programmers
2. So what the heck is Java (besides coffee, that is)? A programming language.
Syntax and constructs are very similar to C++
A virtual platform
Java Virtual Machine is a software “machine” or “hypothetical chip”
Since it’s “virtual”, it can be implemented on any hardware
Cross-platform distribution achieved via .class binary file of bytecodes (instead of machine-dependent machine code) ? Write Once, Run Anywhere
A class library
Standard APIs for GUI, data storage, processing, I/O, and networking.
3. Getting Java Brewing… Download the latest Java SDK from http://java.sun.com
The SDK is a command-line based set of tools
A Text Editor: our choice is, of course, Emacs! ?
Web-browser that’s java-enabled (optional)
IDE like BlueJ from http://www.bluej.org or JCreator from http://www.jcreator.com (optional)
Some introductory links/guides/tutorials:
http://developer.java.sun.com/developer/onlineTraining/Programming/BasicJava1/compile.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
4. Mechanics of Writing Java Programs Create a Java source file. Must have the .java extension and contain only one public class.
Compile the source file into a bytecode file. The Java compiler, 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. The Java 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.
5. Putting it all together
6. Applications vs. Applets A Java application:
Is a standalone program
Is interpreted by the Java Virtual Machine and run using the java command
Contains a main() method.
A Java applet:
Runs within a 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:
Create a Java source file (NameOfProgram.java) and an HTML file (NameOfHTMLFile.html)
Compile the source file (NameOfProgram.class)
Run the program (either using java NameOfProgram (application) or appletviewer NameOfHTMLFile.html (applet))
7. Java notes for C++ programmers Everything’s 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, implement interfaces
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()
8. Requisite First Program (Application Version) Put in HelloWorld.java:
public class HelloWorld {
public static void main(String args[]) {
System.out.println("Hello World");
}
}
9. Compiling and Running
10. So what’s going on? The Java bytecode and interpreter at work! Bytecode is an intermediate representation of the program (the class file)
Think of it as the machine-code for the Java Virtual Machine
The Java interpreter (java) starts up a new “Virtual Machine”
The VM starts executing the user’s class by running its main() method
11. 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:
<html>
<title>Test the applet</title>
<body>
<h3>Test the applet</h3>
<applet code=“HelloWorld.class” height=“200” width=“300”>
</applet>
</body></html> Requisite First Program (Applet Version)
12. 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
AudioClip getAudioClip(URL url): gets audioClip object (play with audioClip.play())
Image getImage(URL url): starts asynchronous image loading
java.net.URL constructor takes normal string argument (used to store 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, calls paint()
repaint(): passes events to Motif/Win32 ? don’t override/change this
13. 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:
conditional: if, if else, switch
loop: while, for, do
break and continue
14. 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 library of classes)
defines a namespace that contains all the classes.
Use the import keyword to include the 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 pointers ? everything’s a reference!
classes
arrays
15. Type Conversions (Skip) conversion between integer types and floating point types.
this includes char
No automatic conversion from or to the type boolean!
You can force conversions with a cast – same syntax as C/C++.
int i = (int) 1.345;
16. Exceptions When a program carries out an illegal action, an exception is generated.
Terminology:
throw an exception: signal (in the method header) that some condition or error has occurred but we want 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/catch block (next slide).
In Java, exception handling is necessary (forced by the compiler ? compilation errors!)
Except for RunTimeExceptions
17. 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
}
18. Exception Handling Exceptions take care of handling errors
instead of returning an error, some method calls will throw an exception.
Can be dealt with at any point in the method invocation stack.
But if no method in the hierarchy handles it, results in an unchecked exception which generates a compiler error (unless it’s a RunTimeException)
Forces the programmer to be aware of what errors can occur and to deal with them.
19. Defining a Class One top level public class per .java file.
Typically end up with many .java files for a single program with at least one containing a static public main() method (if they’re applications).
Class name must match the file name!
The compiler/interpreter use class names to figure out what the file name is.
Classes have these three features:
A constructor that’s used to allocate memory for the object, initiailize its elements, and return a reference to the object
Methods (function members)
Fields (data members)
20. 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;
}
21. Objects and new You can declare a variable that can hold an object: Point p;
But this doesn’t create the object! You have to use new: Point p = new Point(3.1,2.4);
new allocates memory and the garbage collector reclaims unused memory
22. 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 messages like button presses, mouse clicks, etc. (e.g., mouseUp(), mouseDown(), keyUp(), keyDown())
23. 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!”);
24. 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 different than with C++, much of the syntax is compatible.
however, creating an array requires new
Index starts at 0.
Arrays can’t shrink or grow.
e.g., use Vector instead.
Each element is initialized.
Array bounds checking (no overflow!)
ArrayIndexOutOfBoundsException
Arrays have a .length
25. 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<value.length;i++) {
total += values[i];
}
26. Array Literals You can use array literals like C/C++ (no need for new keyword):
int[] foo = {1,2,3,4,5};
String[] names = {“Joe”, “Sam”};
27. Reference Types Objects and Arrays are reference types
Primitive types are stored as values
Reference type variables are stored as references (pointers that we can’t mess with)
28. Primitive vs. Reference Types int x=3;
int y=x;
Point p = new Point(2.3,4.2);
Point t = p;
29. Passing arguments to methods Primitive types: the method gets a copy of the value. Changes won’t show up in the caller ? Pass by value
Reference types: the method gets a copy of the reference, the method accesses the same object ? Pass by reference
There is no pass by pointers!
30. Comparing Reference Types Comparison using == means:
“Are the references the same?”
Do they refer to the same object?
Sometimes you just want to know if two objects/arrays are identical copies.
Use the .equals() method
You need to write this for your own classes!
All objects and arrays are references!
31. Inheritance Use the extends keyword to inherit from a super (or parent) class
No multiple inheritance
Use implements to implement multiple interfaces (abstract, virtual classes)
Use import instead of #include (not exactly the same but pretty close) to include packages (libraries)
32. Using Documentation Comments Documentation comments are delimited by /** and */
javadoc automatically generates documentation
Copies the first sentence of each documentation comment to 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…
33. javadoc The Java Standard calls for every class, every method, every parameter, and every return value to have a comment
Write the method comments first!
If you can’t explain what a class or method does, you aren’t 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 an index.html file
Documenation is together with code!
34. A Gooey GUI!
35. It don’t mean a thing… Adding Swing to GUI applets/applications is easy:
Add import javax.swing.*; or import java.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 correct ContentPane by doing the following in the init() fxn:
Use JPanel contentPane; to declare it and contentPane = new JPanel(); to initialize a new contentPane
Could also have used Container contentPane = getContentPane() if our class extends JFrame
add everything to that contentPane using a statement like contentPane.add(button1);
Finally, do a setContentPane(contentPane); (all in the init() function) to make this the active contentPane
36. GridBagLayout Instead of the default FlowLayout (left-to-right), use the GridLayout or GridBagLayout to add buttons and other GUI elements along the grid
To call GridLayout, just use:
setLayout(new GridLayout(9,3)); for AWT or contentPane.setLayout(new GridLayout(9,3)); for Swing
GridLayout’s first argument is the number of rows (0 for unlimited) and the second argument is the number of columns
37. An example of a Swing-based GridLayout applet:
38. 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, you’ll implement the Runnable interface
How to implement the Runnable interface:
Add a public void start() function:
This is where you’ll initialize the thread and start() it
Add a public void stop() function:
This is where you’ll set the boolean stopFlag to true
Add a public void run() function:
This is where you’ll call repaint() to paint each new frame and handle any synchronized variables or methods
39. 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 put threads on hold and wake them up again (e.g., to implement a pause or suspend feature)
Must be called within a synchronized block
40. 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 applet’s 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
41. 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
