About Techs

Reference variable vs. Real object

2008-11-18T15:15:00.000-08:00

This example shows what happens for the instance method, static method, variable in the inheritance.

======================== Code ========================
public class Sub extends Super{

int x = 15;
static int y = 25;
public static void main(String args[]){

//super reference and super object
Super t = new Super();
out.println(t.x + " " + t.y);
t.methodOne();
t.methodTwo();

//super reference and sub object
Super ts = new Sub ();
out.println(ts.x + " " + ts.y);
ts.methodOne();
ts.methodTwo();

//sub reference and sub object
Sub s = new Sub();
out.println(s.x + " " + s.y);
s.methodOne();
s.methodTwo();
}
static void methodOne() {out.println("Sub methodOne()");}
void methodTwo() {
out.println("Sub methodTwo()");
methodOne();
}
}

class Super{
int x = 10;
static int y = 20;
static void methodOne() {out.println("Super methodOne()");}
void methodTwo() {
out.println("Super methodTwo()");
methodOne();
}
}
================================================

======================== Answer ========================
10 20
A methodOne()
A methodTwo()
A methodOne()
10 20
A methodOne()
B methodTwo()
B methodOne()
15 25
B methodOne()
B methodTwo()
B methodOne()
================================================

======================== Conclusion ========================
static methods and variables are based on the reference type
Real object type determines which overridden method is used at runtime
================================================

SCJP

2008-11-11T17:01:00.001-08:00

Those are my study note of the SCJP. I used those notes to prepare the SCJP exam. Wish myself have luck for the exam.

Uploaded on authorSTREAM by maggie.zhou

Uploaded on authorSTREAM by maggie.zhou

Uploaded on authorSTREAM by maggie.zhou

Uploaded on authorSTREAM by maggie.zhou

SQA

2008-11-10T13:52:00.001-08:00

To understand what SQA doing, I create this note. The SQA is not same as quality control, it isn't testing. It is a process control to ensure the quality of software products. Hope my slides can explain the above points with details.

Uploaded on authorSTREAM by maggie.zhou

SQL

2008-10-28T21:48:00.001-07:00

One of my study notes about SQL. It covers all the concepts about SQL syntax, database design, query, etc.

Uploaded on authorSTREAM by maggie.zhou

IPSec

2008-10-28T21:03:00.001-07:00

I present the basic concept of IPSec. Let you know about what are differnets between the Transport mode and Tunnel mode, what are differents between ESP and AH, etc.

Uploaded on authorSTREAM by maggie.zhou

Ad hoc Network protocols

2008-10-08T01:45:00.000-07:00

This note presents three important Ad hoc network routing protocol. These three protocol are the typical routing types which inspired other protocols. Once you understood these three, you will easily get the key point from others.

Uploaded on authorSTREAM by maggie.zhou

Lost root password in MySQL

2008-10-08T00:23:00.000-07:00

If you lost the root password of MySQL, there are several to get over it. On Windows, you can use the following steps to reset a new password.

1. close currrent running MySQL: you can go to windows task manager(by press Ctrl+ALT+DEL), and end the process: mysqld-nt.exe

2, open a dos command window

3, set path as your mysql installation path, and into bin, mine is:

c:/program files/mysql/bin

4, Input command:

mysqld-nt --skip-grant-tables

if there is nothing shows up, it means everything is ok

5, Then, you need open another dos command window

6, Still, set the same path: c:/program files/mysql/bin

7, input:

mysql

then press ENTER key, to get into mysql

if you see:

mysql>

then so far so good

8. After the previous steps, please input:

use mysql;
update user set password=password("123456") where user="root";
flush privileges;

12345 is you new password for root user, please don't forget the semicolon and use double quote for both password and user name root

9. Quit mysql by input
\q

10, On Final step, you need to log out then login windows.

Now, you can use the new password to login mysql in the normal way.

A great Thread example from Nikos Pugunias(http://nikojava.wordpress.com/)

2008-09-16T00:05:00.000-07:00

Hi Zhou!
You should be 100% certain that wait() is a method of Object.Consider 2 threads: Thread A carries out a loooong task. Thread B must wait for A's result to go on with its job.This trick between threads happens with a single object that both threads can access: a token. it's like the piece of wood that the athletes running 4x100m in the Olympics: Runner 1 starts, Runner 2 waits for the token from Runner 1, as soon as he gets it Runner 2 starts and so on...

//Here's the first runner,
class Runner1 extends Thread { }

//that holds a wooden stick.

class Runner1 extends Thread {
Object stick;
public Runner1(Object stick) { this.stick = stick; }
}

//This runner has a loooong task to do,

class Runner1 extends Thread {
Object stick;
public Runner1(Object stick) { this.stick = stick; }
public void run() { // perform a loooooong task here }
}

//and then he will give the token to another runner.

class Runner1 extends Thread {
Object stick;
public Runner1(Object stick) { this.stick = stick; }
public void run() {
synchronized (stick) {
// perform a loooooong task here stick.notify();
}
}
}

//Here's the second runner
class Runner2 extends Thread { }

//that can also receive a wooden stick.

class Runner2 extends Thread {
Object stick;
public Runner2(Object stick) { this.stick = stick; }
}

//This second runner has also something to do,

class Runner2 extends Thread {
Object stick;
public Runner2(Object stick) { this.stick = stick; }
public void run() {
// I want to do something after
// the first runner has finished.
}
}

// but he'll have to wait for the token first.

class Runner2 extends Thread {
Object stick;
public Runner2(Object stick) { this.stick = stick; }
public void run() {
// wait for Runner1
synchronized (stick) {
try { stick.wait();
} catch (InterruptedException e) { }
}
// I want to do something
//after the first runner has finished.
}
}

//+++++++++++++++++++++++++++
//Here's the complete code.
//+++++++++++++++++++++++++++

public class Run {
public static void main(String[] args) {
Object token = new Object();
new Runner1(token).start();
new Runner2(token).start();
}
}

class Runner1 extends Thread {
Object stick;
public Runner1(Object stick) { this.stick = stick; }
public void run() {
synchronized (stick) {
// perform a loooooong task here stick.notify();
}
}
}

class Runner2 extends Thread {
Object stick;
public Runner2(Object stick) { this.stick = stick; }
public void run() {
// wait for Runner1
synchronized (stick) {
try { stick.wait();
} catch (InterruptedException e) { }
}
// at this point Runner1 is guaranteed
// to have finished his work
}
}

//*************** Conclusion: ***************//
Using a single object, two threads may schedule in what order they will run in accuracy.In the question you have posted, there two threads: Job and main. They schedule themselves based on the [B]thread[B] instance.It's just a choice to use this instance instead of any other object.Review:When thread A calls token.notify() it says: You may take the token.When thread B calls token.wait() it is waiting to take the token.Always synchronize on the token.

First Java step: set classpath

2008-09-13T16:58:00.000-07:00

From website: http://mindprod.com/jgloss/classpath.html

The CLASSPATH is an environment variable that tells the Java compiler javac.exe where to look for class files to import or java.exe where to find class files to interpret.

In contrast, the PATH is an environment variable that tells the command processor the where to look for executable files, e.g. *.exe, *.com and *.bat files. The Classpath is one of the most confusing things in Java. Unfortunately, you must master it to even compile HelloWorld.

CLASSPATH And File Naming RecipesHere are my simplified rules for using CLASSPATH and naming the files on the javac.exe and java.exe command line:

Put every class in a package. Don’t use the default package.
Use the latest JDK. It will be the one everyone you ask help from is familiar with. Two dangling prepositions in one sentence. Churchill would be proud.
Configure your SET CLASSPATH= in the environment to clear it out.
You don’t need to explicitly include rt.jar:
rt.jar system class files :
in C:\Program Files\java\jre1.6.0_07\lib\rt.jar in the JRE 1.6.0 on your local hard disk.
in J:\Program Files\java\jdk1.6.0_07\jre\lib\rt.jar in the in the JDK 1.6.0 on your local hard disk.
in C:\Program Files\java\jre1.5.0_16\lib\rt.jar in the older JRE 1.5.0 on your local hard disk.
in J:\Program Files\java\jdk1.5.0_16\jre\lib\rt.jar in the in the older JDK 1.5.0 on your local hard disk.

unless you are using Jikes, where you need to add it to the JIKESPATH, but not the CLASSPATH.

In all that follows, everything is strictly case sensitive.

To compile a HelloWorld.java app in the default package in C:\MyDir, use cd \MyDir
javac.exe -classpath . HelloWorld.java
To run a HelloWorld.class app, in the default package in C:\MyDir, use cd \MyDir
java.exe -classpath . HelloWorld
To compile a HelloWorld.java app in C:\com\mindprod\mypackage, in package com.mindprod.mypackage, use c:
cd \
javac.exe -classpath . com\mindprod\mypackage\HelloWorld.java
rem alternatively
e:
cd E:\com\mindprod\mypackage
javac.exe -classpath .;C:\ HelloWorld.java
To run a HelloWorld.class app in C:\com\mindprod\mypackage, in package com.mindprod.mypackage, use c:
cd \
rem not you do not do cd \com\mindprod\mypackage!!
java.exe -classpath . com.mindprod.mypackage.HelloWorld
To compile a HelloWorld.class app in C:\com\mindprod\mypackage, in into a jar called helloworld.jar in package com.mindprod.mypackage, use
To run a HelloWorld.class app in C:\com\mindprod\mypackage, in a jar called helloworld.jar in package com.mindprod.mypackage, use CD \dirwherejaris
java.exe -classpath helloworld.jar com.mindprod.mypackage.HelloWorld
rem or if your Main-Class manifest points to HelloWorld.class.
java.exe -jar helloworld.jar
rem or if your *.jar extension is set to invoke java.exe
helloworld.jar
If for any reason the examples shown do not work with your version of java.exe, try replacing the \ in com\mindprod\mypackage\HelloWorld with / mypackage/HelloWorld.If you stare long enough at those examples, you may understand the logic behind them, and then you can create variants. If you can’t, just slavishly copy the closest matching example.

Java.io.Console

2008-09-12T21:25:00.001-07:00

Make easy to accept input from and output to command line.

import java.io.Console;
Console c = System.console();
String readLine()
char[] readPassword(): for security purpose

== vs. equals() in Java

2008-09-12T15:29:00.000-07:00

In JAVA, you may be confused with "==" and equals() all the time. What are they all about?

"==":
For primitive: check the velue

For String:
1.if see any new(means String s = new String("xxx"); -->check if they pointing to same object
2.if see only ""(means String s = "xxx"; ) -->check the string content

For Wrapper:
1. same as String, see any new --> check if they pointing to same object
2, if there is not, then tricky things coming:
2.1, if it is byte, short, int and the value is <=127, -->check the value
2.2, otherwise check the if they pointing to same object

equals():
For primitive: no way to use this

For general Object:
1.must override it to compare the key attribute(s) of object;
2,if collection is Hashxxx, you need override hashCode() too, otherwise equals won't effect

For String and wrappers:
they have well overridden equals() and hashCode()

Start to blog all my techs articles

2008-09-12T15:04:00.000-07:00

I am always thinking to put all my tech related articles on a seperate blog. So from now on, I start to put all my research, study issues, results on this blog.