Download Javadoc Javadoc 2 Jar-files Applets An Applet is a Component The

Javadoc
Javadoc 2
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• Special comments that can be used to create
documentation for the classes we write
• /** denotes the start of a javadoc comment
• Is written above a class, attribute, constructor or
method declaration
• The first line should be a general description of
the method etc.
• The general description ends starting at the first
line containing a @ character.
@author
@version
@param
@return
@exception
@see
@since
@serial
@deprecated
(only
(only
(only
(only
(also
classes and interface)
classes and interface)
methods and constructors)
methods)
@throws since Javadoc 1.2)
(or @serialField or @serialData)
• The Java API description is built using javadoc
• For more info see:
http://java.sun.com/j2se/javadoc/
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Jar-files
Applets
• Are used to package java files to make them
easier to distribute.
• Eclipse has the ability to export a project to
a jar file
• The jar file can be made executable by
specifying witch class contains the main
method
• Applets
– Are loaded over a network connection
– Runs in a browser window (or appletviewer)
• The browser (Netscape, Explorer etc)
– downloads the applet from the server when it finds the
<APPLET>-tag
– Supplies a graphical environment
– Creates a object from the Applet class.
– Does not permit certain operations (for security
reasons)
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An Applet is a Component
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The lifecycle of an Applet
• Is created by the browser (Constructor)
• Is Initialized (init)
– only once (in theory…)!
• Is started (start)
– each time the page is shown
• Is stopped (stop)
– when the user leaves the page
• Final cleanup (destroy)
– when the page is removed from memory
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The anatomy of an applet
init()
• public void init()
• public void start()
• public void stop()
• public void destroy();
• We usually don’t supply a constructor since
the applet is not fully functioning at that
time
• Should contain the initialization of the
applet
• Usually contains the code that would
otherwise be in the constructor.
• Is called as soon as the browser has done its
initialization of the Applet (setting up
graphics environment and so on)
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start()
stop()
• Om start omdefinierats så är det bäst att
ta med stop också
• Anropas när användaren bläddrar bort från
sidan
• Stoppar beräkningar trådar animeringar
(ingen kan se sidan ändå)
• Is called each time the page is shown
Performs the actual work (or more likely
starts up a separate Thread)
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A small Applet
Doodle
import java.awt.*;
import java.applet.*;
• Doodle.java
public class SmallApplet extends Applet
{
Label field;
public void init()
{
field= new Label("Hej !");
setLayout(new FlowLayout());
add(field);
validate();
}
}
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What you can’t do in an Applet
Starting an applet from a webpage
<APPLET CODE=AppletSubclass.class
WIDTH=anInt HEIGTH=anInt>
</APPLET>
<APPLET CODE=“AppletSubclass.class”
ARCHIVE=“file1, file2”
WIDTH=anInt HEIGTH=anInt>
</APPLET>
• Parameters:
<APPLET CODE=AppletButton.class CODEBASE=example
WIDTH=350 HEIGTH=60>
<PARAM NAME=titel VALUE=“Lets try!”>
</APPLET>
String title=getParameter(“titel”);
• An Applet can not (without some extra work)
– call ‘native methods’
– read and write files on the client computer.
– setup network connections to other computers except
the server from where it originated
– Start programs on the client
– read certain system properties
– Windows opened by applets usually have a different
look then other windows.
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Threads
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Threads in a program
• A Thread
– Is a process in a program
– Shares resources with other threads in the same
application.
– Can execute at the same time as another thread
– Can be created in two ways
• The ’Garbage collector’ is executed in a
separate thread
Two Threads
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Two ways of creating a new
thread
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Extend Thread
• Extend the class Thread and override the
method run
• Implement the Runnable interface
public class SimpleThread extends Thread {
public SimpleThread(String str) {
super(str);
}
public void run() {
for (int i =0;i<10;i++) {
System.out.println(i+” ”+getName());
try{
sleep((long) (Math.random()*1000));
} catch (InterruptedException e) {}
}
System.out.println(”Done !” +getName());
}
}
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Testing
Implement Runnable
public class Clock extends Applet
implements Runnable
{
private Thread clockThread =null;
public class TwoThreadsTest {
public static void main(String[] args) {
new SimpleThread(”Jamaica”).start();
new SimpleThread(”Fiji”).start();
}
}
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public void start() {
if (clockThread==null)
{
clockThread=new Thread(this,"Clock");
clockThread.start();
}
}
Jamaica
Fiji
Jamaica
Jamaica
Jamaica
public void run() { …
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Synchronizing threads
• Every object in Java has a built-in lock, which means that it
has at its disposal a boolean locked variable and a queue for
blocked threads.
• Mutual exclusion code blocks are created using the
synchronized keyword as follows:
Object x = new Object();
synchronized(x) {
// mutual exclusive access for all threads which
share x
}
• A thread can only run in the synchronized(x) region if it has
acquired x's lock, otherwise if it tries to run, it is blocked.
• When a thread leaves the region it gives up x's lock and
wakes up a single blocked thread (if any).
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Synchronizing threads
• The other way to create mutual exclusion regions
using the synchronized keyword is as a qualifier for
member functions like this:
class MyClass {
public synchronized void foo() {
// ...
}
}
• In this case, if multiple threads share the object
MyClass c = new MyClass()
then all calls to
c.foo()
are mutually exclusive.
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