Archive for Programming

what’s static?

Posted in Programming with tags , , on April 14, 2008 by wsjoung

First of all, the meaning of static in java is embedded or belong to a specific class not to its instance, and therefore there is no such static class.

  • There is no such static class
  • Embedded or belong to a specific class
  • Global variable
  • No need to load and create its instance

Most of methods and variables in Math class are static, because those are not need to be instanced in most of cases. we just use return value of PI or max(int a, int b).

class Bicycle {

private static int numberOfBicycles = 0;
private int id;

public Bicycle() {
id = ++numberOfBicycles;
}

public static int getNumberOfBicycles() {
return numberOfBicycles;
}
public int getID() {
return id;
}

public static void main(String[] args) {
Bicycle b1 = new Bicycle();
Bicycle b2 = new Bicycle();

System.out.println(“static result : “+getNumberOfBicycles());
System.out.println(“static result : “+numberOfBicycles);
System.out.println(“instance result : “+b1.getID());
System.out.println(“instance result : “+b2.getID());
}
}

Another characteristic of static is that even if we create instance of a class, jvm does not create instance variables for static resources. in above source code, there is no difference calling b1.getNumberOfBicycles(), b2.getNumberOfBicycles() and just getNumberOfBicycles() or directly numberOfBicycles
that mean we should call static variable with its class name because it’s belong to it. just like Bicycle.numberOfBicycles or in this example, getNumberOfBicycles() because we have a getter.
otherwise get confused.

javamail smtp with Gmail

Posted in Programming with tags , , , , on August 3, 2007 by wsjoung

package com.mycompany.emailsender;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.security.Security;
import java.util.Properties;

import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

/**
* Servlet implementation class for Servlet: GmailSender
*
*/
public class GmailSender extends javax.servlet.http.HttpServlet implements javax.servlet.Servlet {
/* (non-Java-doc)
* @see javax.servlet.http.HttpServlet#HttpServlet()
*/
public GmailSender() {
super();
}

/* (non-Java-doc)
* @see javax.servlet.http.HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}

/* (non-Java-doc)
* @see javax.servlet.http.HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
final String SMTP_HOST_NAME = “smtp.gmail.com”;
final int SMTP_HOST_PORT = 465;
final String SMTP_AUTH_USER = “XXXXXX@gmail.com”;
final String SMTP_AUTH_PWD = “XXXXXX”;
final String SSL_FACTORY = “javax.net.ssl.SSLSocketFactory”;

String emailFromAddress = null;
String[] sendTo = null;
String subject = null;
String content = null;

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

Security.addProvider(new com.sun.net.ssl.internal.ssl.Provider());

String address = request.getParameter(“address”);
sendTo = address.split(“;”);

emailFromAddress = request.getParameter(“sender”);
this.subject = request.getParameter(“title”);
this.content = request.getParameter(“content”);

try {
for (int i=0; i<sendTo.length; i++) {
sendSSLMessage(sendTo[i]);
}
System.out.println(“Sucessfully Sent mail to All Users”);

} catch (Exception e) {System.out.println(e);}
}

public void sendSSLMessage(String dest) throws MessagingException {

Properties props = new Properties();

props.put(“mail.transport.protocol”, “smtps”);
props.put(“mail.smtps.host”, SMTP_HOST_NAME);
props.put(“mail.smtps.auth”, “true”);
props.put(“mail.smtps.quitwait”, “false”);

Session mailSession = Session.getDefaultInstance(props);
mailSession.setDebug(false);
Transport transport = mailSession.getTransport();

MimeMessage message = new MimeMessage(mailSession);
message.setSubject(subject);
InternetAddress addressFrom = new InternetAddress(emailFromAddress);
message.setFrom(addressFrom);
message.setContent(content, “text/html”);

message.addRecipient(Message.RecipientType.TO,new InternetAddress(dest));

transport.connect(SMTP_HOST_NAME, SMTP_HOST_PORT, SMTP_AUTH_USER, SMTP_AUTH_PWD);

transport.sendMessage(message,message.getRecipients(Message.RecipientType.TO));
transport.close();
}
}

reference: http://www.rgagnon.com/javadetails/java-0083.html

Signed java applet

Posted in Programming with tags , , , on May 15, 2007 by wsjoung

Applet Security Basics

Below are the basic facts regarding applet security and Java Plug-in. More detail can be found in the next chapter, How RSA Signed Applet Verification Works in Java Plug-in.

  • All unsigned applets are run under the standard applet security model.
  • If usePolicy IS NOT DEFINED in the java.policy file, then a signed applet has the AllPermission permission if:
    Java Plug-in can verify the signers, and the user, when prompted, agrees to granting the AllPermission permission.
  • If usePolicy IS DEFINED, then a signed applet has only the permissions defined in java.policy and no prompting occurs.

Moreover, note that Java Plug-in now handles certificate management; i.e., the certificate verification task is no longer passed off to the browser.

keytool -genkey -keyalg rsa -alias MyCert
keytool -certreq -alias MyCert
keytool -import -alias MyCert -file VSSStanleyNew.cer

or

keytool -selfcert

and

jarsigner AppletName.jar MyCert
jarsigner -verify -verbose -certs AppletName.jar

String constant pool

Posted in Programming with tags , , on November 26, 2006 by wsjoung

Everybody knows about String constant pool which is for efficient memory management in java. Basically most of objects are managed on heap area but String object. In most of ordinary application, programmers use String object quite often and this String object quite frequently need to be changed or it occupies large amounts of memory. Therefore instead of managing String object on heap area, they introduced String constant pool.

One of important characteristic of String constant pool is that it doesn’t create same String object if there is already String constant in the pool.

String var1 = “This is String Literal”;
String var2 = “This is String Literal”;

For above two String objects, JVM creates only one object in the String constant pool and for the second string reference variable (var2), it points the string object which is created for var1. In this case, (var1 == var2) is true.

But one thing, people make confused is that. It works only when it encounter on String Literal with double quote.

String var3 = new String(“This is String Literal”);

In this case, a regular object will be created by new keyword on heap area and it will be placed in the String constant pool. Finally it will be assigned to the reference variable, var3. This process is just by passing from String constant pool management. Therefore, (var1 == var3) is false.

Read all files in a directory

Posted in Programming with tags , , , on November 17, 2006 by wsjoung

int fileCount = 0;
File dir = new File(inputDirName);
File[] strFilesDirs = dir.listFiles();

if (strFilesDirs == null) {
System.err.println(inputDirName +” is not valid directory name or there is no pdb file”);
System.exit(1);
}
for (int i=0; i
if (strFilesDirs[i].isDirectory()) {
//System.out.println(“Directory: “+strFilesDirs[i]);
} else if (strFilesDirs[i].isFile()) {
//System.out.println(“File: “+strFilesDirs[i]+”(“+strFilesDirs[i].length()+”)”);
fileCount = fileCount + 1;
}
}

beside, there is no such method which is changing directory.

Small Ajax example

Posted in Programming with tags , on November 17, 2006 by wsjoung

JavaScripts create XMLHttpRequest() object first.
req = new XMLHttpRequest();

once we create XMLHttpRequest() object, then we can request xml data
req.open(“GET”, url);
req.send(null);

then check onreadystatechange state
req.onreadystatechange = processReqChange;

it has 5 states (0-4)and 4 means completed retrieving data

function processReqChange() {
// only if req shows "complete"
if (req.readyState == 4) {
// only if  "OK"
if (req.status == 200) {
// ...processing statements go here...
response = req.responseXML;

if (response) {
// do something with XML contents
} else {
// no XML contents
}

} else {
alert("There was a problem retrieving the XML data:n" +
req.statusText);
}
}
}

Demo page

Java millisecond precision

Posted in Programming with tags , , on November 17, 2006 by wsjoung

There is another useful method under System class. usually I use just in, out, or err something like those methods under System. But,

long current = System.currentTimeMillis(); // Returns the current time in milliseconds.
definitely I need this to work with network program.

Class System