Java程序辅导

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A Java Crash Course
Dan Wallach, Princeton University
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Outline
n Applet “Hello, World”
u graphics
u widgets
n AWT event model
n Multithreaded programming
n Networking
n Utilities and tricks
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Starting resources
n If you own only one book...
Java in a Nutshell, David Flanagan (O’Reilly &
Associates)
http://www.ora.com/catalog/javanut/examples/
(examples online!)
n “Whenever possible, steal code.” [Duff]
http://www.developer.com (formerly gamelan.com)
http://www.acme.com/java/software/
http://java.sun.com
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Normal Hello World
n Put in:  Hello.java
n Compile with:  javac Hello.java
ò Creates  Hello.class
n Run with:  java Hello
public class Hello {
    public static void main(String args[]) {
        System.out.println(“Hello, world.”);
    }
}
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Applet Hello, world #1
n paint() called by system when refresh is necessary
n Graphics class has lines, polygons, text, images,
etc.
import java.applet.*;    // Don’t forget these import statements!
import java.awt.*;
public class FirstApplet extends Applet {
    // This method displays the applet.
    // The Graphics class is how you do all drawing in Java.
    public void paint(Graphics g) {
        g.drawString("Hello, world.", 25, 50);
    }
}
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Hello, world #2
n Make a scrolling text area where you can do terminal-
like output
import java.applet.*;
import java.awt.*;
import java.io.*;
public class HelloWorld2 extends Applet {
    TextArea textarea;
    // Create a text area to send our output
    public void init() {
        textarea = new TextArea(20, 60);
        this.add(textarea);
        Dimension prefsize = textarea.preferredSize();
        this.resize(prefsize.width, prefsize.height);
    }
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Hello, world #2 (cont.)
n text printed after program is done
n TextArea widget redraws itself
    public void start() {
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        PrintStream ps = new PrintStream(os);
        
        try {
            go(ps);
        } catch (Throwable t) {}
        textarea.setText(os.toString());
    }
    
    public void go(PrintStream ps) {
        // your program goes here
        ps.println(“Hello, world.”);
    }
}
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Applets in HTML
n codebase/archive tags are optional
n argument-passing through param tags





    
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Applet class
n java.applet.Applet
u you extend this for your applet
n java.awt.Panel
n java.awt.Container
u applet widget can contain other widgets
n java.awt.Component
u lots of interesting methods here
n java.lang.Object
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Basic methods on Applet
n init()
u called once for your applet
n start()
u called every time you enter the page
n stop()
u called every time you leave the page
n destroy()
u called when your page is discarded
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Funky methods on Applet
n AudioClip getAudioClip(URL url)
n Image getImage(URL url)
u starts asynchronous image loading
n void showDocument(URL url)
u tells browser to load new document
u optional second argument for frames
n void showStatus(String msg)
u writes to browser status line
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Applet repainting
n paint()
u defaults to nothing
n update()
u clears screen, calls paint()
n repaint()
u passes events to Motif/Win32
u don’t mess with this
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Applet event handling
n boolean handleEvent(Event evt)
u mouse, keyboard, all widget events
u checks event type, then calls...
n mouseUp() / mouseDown() / keyUp() / keyDown()
n action(Event evt, Object arg)
u evt.target - specific widget
u arg - widget-specific result (i.e., new state of a
checkbox)
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Applet event handling
n Centralized event management
u add standard buttons, widgets as children of the top-
level applet
u custom action() method, checks evt.target
n Distributed event management
u subclass buttons, widgets
u custom action() methods in subclasses
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Java and threads
n One lock per object plus one per class
n synchronized keyword on a method
n Mesa-style monitors
u wait() / notify() / notifyAll()
u must be called within a synchronized block
n System classes already thread-safe
u HashTable, OutputStream, AWT, etc.
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Thread-safe Message Passing
 Exercise for reader: barrier sync, bounded-buffer
queue, etc.
public class SafeBuffer {
  private Object buffer;
    
  public SafeBuffer() {}
    
  synchronized public void put(Object o) {
    while(buffer != null) {
      try {
        wait();
      } catch (InterruptedException e) {}
    }
    buffer = o;
    notifyAll();
  }
  synchronized public Object get() {
    while(buffer == null) {
      try {
        wait();
      } catch(InterruptedException e) {}
    }
    Object tmp = buffer;
    buffer = null;
    notifyAll();
    return tmp;
  }
}
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Starting Threads
n Thread constructor takes any object which implements
Runnable
class client implements Runnable {
  private SafeBuffer b;
    
  public client(SafeBuffer b) {
    this.b = b;
  }
    
  public void run() {
    String s;
    ...
    s = (String) b.get();
    ...
  }
}
...
SafeBuffer sb = new SafeBuffer();
new Thread(new client(sb)).start();
sb.put("Hello, world.");
  
...
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Networking
n Applet restrictions
u Same IP address which loaded applet
u UDP support is flakey
n Using the browser’s cache
u java.net.URL constructor takes normal string argument
u InputStream toStream()
u only current way to get SSL support
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Networking
n client: java.net.Socket
u constructor takes DNS name, port
u getInputStream() / getOutputStream()
n server: java.net.ServerSocket
u constructor takes local port number
u Socket accept()
F blocks until success -- use multithreading!
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Utilities and tricks
n StringBuffer vs. String
u strings are immutable
u StringBuffer append() is cheaper
n java.util.Hashtable
u uses Object.hashCode() and Object.equals()
n java.util.StringTokenizer
u split string on whitespace / separator chars
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Use Javadoc
n Literate programming for Java
u document as you write code
u generates pretty, cross-linked HTML
    /** 
     * Creates an absolute URL from the specified protocol,
     * host, port and file.
     * @param protocol the protocol to use
     * @param host the host to connect to
     * @param port the port at that host to connect to
     * @param file the file on that host
     * @exception MalformedURLException If an unknown protocol is 
     * found. 
     */
    public URL(String protocol, String host, int port, String file)
throws MalformedURLException {
    ...
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Useful tools
n Debugging / Development
u Microsoft Visual J++ / Symantec Visual Café
F Debuggers integrated with IE / Netscape
u Kaffe - free JVM with JIT (http://www.kaffe.org)
u Jikes - fast Java compiler from IBM
http://www.alphaworks.ibm.com (?)
u cc-mode for Emacs understands Java
ftp://ftp.python.org/pub/emacs/cc-mode.tar.gz
n When in doubt…
u http://www.developer.com/