GWT - JSNI , Facing error while using String[] as return type.

I worked with JSON and JSNI to retrieve data from server and sending the data back to client. The server returns the following JSON as a response to a request   

[{"message": "MMMM","word": "abcd","options": ["I","H","G","F","E"],"answer": 0}]

My class that results in by the eval(json) function is the following

public class Question extends JavaScriptObject{
      
    protected Question() {
    }
      
    public final native String getMessage()/*-{ return this.message ; }-*/;
    public final native String getWord()/*-{ return this.word ; }-*/;               
    public final native String[] getOptions()/*-{ return this.options ; }-*/;
    public final native int getAnswer()/*-{ return this.answer ; }-*/;
    

}

then calling the getOptions() function inside the code greeted me with the following exception

java.lang.ClassCastException: com.google.gwt.core.client.JavaScriptObject$ cannot be cast to [Ljava.lang.String

After that I changed 

public final native String[] getOptions()/*-{ return this.options ; }-*/;

to

public final native JsArrayString getOptions()/*-{ return this.options ; }-*/;

and make necessary update in code to work with JsArrayString ( It is more or less like ArrayList in java ) . Then the code worked nicely.

Using JDO with MySQL

I recently worked with Java Data Object( JDO ) on my machine to store and retrieve data from a local MySQL database. Following is the procedure how I did it. I used Eclipse IDE for writing JAVA codes.

*** Important: To build and run the project this post is useful.

First I created a property file that contains necessary information for connecting to the local MySQL and used by JDO api to create PersistenceManagerFactory class. Put the file in project's root folder.
File    : datanucleus.properties
Code : http://pastebin.com/0npQb4ag
Update username , password ,db-name according to yours.

Then I created a class named "UserInfo" to store username , password , email-address.

File    : UserInfo.java

Code : http://pastebin.com/CAMth7db

For the annotaions' guideline visit here 
Then I created a class named PMF.java to make sure that only one instance of PersistenceManagerFactory  is used through out the application. That file uses datanucleus.properties
file.
File    : PMF.java
Code : http://pastebin.com/24tcVgGk
Finally to test everything works I write TestJDO.java to test every thing works fine.
File    : TestJDO.java
Code :  http://pastebin.com/zeJgpcp9

Eclipse , JDO and MySQL Integration using Ant

I did it in a Linux machine( Ubuntu 10.10 ).I am assuming the following programs are already installed in the machine.

Eclipse JAVA IDE

MySQL

DataNucleus is used as an implementation for JDO( Java Data Object ). Follow the link and download

datanucleus-accessplatform-rdbms-<version>.zip from the latest release (and extarct) . I worked with version= 3.1.0-release.

Also download JDO-API from apache. Find the Latest version and download jdo-api-<jdo-version>.jar.

MySQL JDBC driver is also needed. 

Now create a Java Project in Eclipse and add the following jars to the project's CLASSPATH.

From ....../datanucleus-accessplatform-rdbms-<version>/dep/ folder
       asm-<asm-version>.jar
       log4j-<log4j-version>.jar

From ....../datanucleus-accessplatform-rdbms-<version>/lib/ folder
       datanucleus-api-jdo-<version>.jar

       datanucleus-core-<version>.jar

       datanucleus-enhancer-<version>.jar

       datanucleus-rdbms-<version>.jar

....../jdo-api-<jdo-version>.jar

....../mysql-connector-java-<mysql-version>-bin.jar (For MySQL JDBC driver).

Now Export the created Java project to General > AntBuildFiles . Click next and finish nothing much is needed to be done. A build.xml file has been created in the project's root directory. Open it with Eclipse Ide.

Add following snippet of xml to it

<target
        name="-post-compile"
        depends="init" >

        <taskdef
            name="datanucleusenhancer"
            classname="org.datanucleus.enhancer.tools.EnhancerTask"
            classpathref="<ProjectName>.classpath" />

        <echo message="start datanucleusenhancer" />

        <datanucleusenhancer
            classpathref="<ProjectName>.classpath"
            dir="bin"
            verbose="true" >

            <fileset dir="bin" >

                <include name="**/*.class" />
            </fileset>
        </datanucleusenhancer>

        <echo message="end datanucleusenhancer" /> 

</target>  

add this under <project> tag . Change <ProjectName>  according to yours.

Find <javac> tag and add the following line inside <javac></javac> tag.
<compilerarg line=" -processor org.datanucleus.enhancer.EnhancerProcessor" />
Add an attribute verbose="on" to javac tag.

Now find the following xml

<target
        name="build"
        depends="build-subprojects,build-project"/>

Turn it to

<target
        name="build"
        depends="build-subprojects,build-project,-post-compile" />

Now go to project --- properties -- builders and select new. Then select "Ant Builder" . Add a name and in "Buildfile" tab point the build.xml file. Mark only the newly created builder in the "Builders" list.

Configure Eclipse and Java 3D in Linux( Ubuntu 10.10 )

At the very beginning we need to install Java 3d.

Installation:

- Open the terminal ( Applications >> Accessories >> Terminal  )

- Type the following command

       sudo apt-get install libjava3d-java

After that we need to copy some files.
Copying Files:
- Copy the following jar files 
                j3dcore-1.5.2+dfsg.jar , 
                j3dcore.jar , 
                j3dutils-1.5.2+dfsg.jar , 
                j3dutils.jar , 
                vecmath.jar
to  $JAVA_HOME/jre/lib/ext folder. I found ( hope you also will ) the above files in 
/usr/share/java folder. You can use the following commands to do that
       sudo cp -i /usr/share/java/j3d*.jar $JAVA_HOME//jre/lib/ext
       sudo cp -i /usr/share/java/vecmath.jar $JAVA_HOME//jre/lib/ext
In my case , $JAVA_HOME = /usr/lib/jvm/java-6-sun-1.6.0.24 .


- Copy the libj3dcore-ogl.so to any of the directory that has been pointed out in LD_LIBRARY_PATH . The file is located in /usr/lib/jni/ folder . (This folder may already be in your LD_LIBRARY_PATH. Then copying is not needed ) .  I copied it to 
$JAVA_HOME/java-6-sun-1.6.0.24/jre/lib/i386. You can use the following command 
        sudo cp -i /usr/lib/jni/libj3dcore-ogl.so $JAVA_HOME//jre/lib/i386.


Eclipse Integration:
- Create a new Java Project.
- At project's build path select "Add external archive".
- Point out the following jar files 
       j3dcore.jar , 
       j3dutils.jar , 
       vecmath.jar         


Testing:
You can find a sample program in the tutorial hosted in site.




  

        

Clob datatype , oracle and java

Recently I worked with clob data of oracle. Using java , I found the following code works for the following purpose
- inserting clob data to a database table 
- retrieve clob data from database .


Assuming that reader has already executed the following sql command

create table testclob( _id numeric(5) , data clob );

variable name is not important . What is important is the table structure.

For inserting the following code suffices -

        
PreparedStatement pstmt = conn.prepareStatement("insert into testclob values(?,?)");
        
pstmt.setInt(1, 1);
pstmt.setCharacterStream(2, new FileReader(new File("hello.txt")));

pstmt.execute();
        
        
For retrieving data, the code is given below

         
Statement stmt = conn.createStatement();
ResultSet result = stmt.executeQuery("select * from testclob");

while( result.next() )

{
     Clob clob = result.getClob(2);
     Reader in = clob.getCharacterStream();
     char[] buffer = new char[(int)clob.length()];
            
     in.read(buffer);
     System.out.println(buffer); 

} 

As oracle driver , ojdbc6.jar is used.

Can not start AVD after creating - Android

Android system is emulated in the computer using Android Virtual Devices(AVDs). In my ubuntu ,When I create a new AVD using the AVD and SDK Manager it creates a file named

<avd-name>.ini

in the directory below  

~/.android/avd

When we try to start a virtual machine it tries to read the corresponding .ini file from that directory. Recently I created an AVD and when i tried to run it ,the AVD manger greeted me with the following error massage

PANIC: Could not open: ~/.android/avd/<avd-name>.ini

Just because of I typed
 
    sudo ./android

to run the  SDK and AVD Manager . For that it created the .ini file in
 
   /root/.android/avd

directory instead of the location desired by the Start command. Running
 
   ./android

command and cretaing new AVD there solved my problem.

Resuming cancelled download of Google Chrome

Say you are downloading a large file and it gets cancelled before finishing . But you want to resume that file and there is no such option in the chrome's default downloader . In linux, you can do the following -

Chrome usually saves the content of the file it is downloading to a file with following file-name pattern
         <filename>.<extention>.crdownload
in the directory where it usually saves the file after download. If you want to know which directory it is
follow the steps below-

• Click the "wrench" icon on the upper right corner of the browser
• Next Go to Preferences >> Under the hood >> Downloads   

So as you have a cancelled download , you most probably get a similar file with .crdownload extension.

Remove the .crdownload portion from the filename. (For safety you can make a copy of that file then remove the extension ) .

Then run a shell and type

wget -c <link to the file>

Here  <link to the file> is that chrome used to download the file . You can get it from the downloader of chrome.

If you do not find wget commnad ( in rare case ) type 

sudo apt-get install wget 

to install it in your machine.

After running the download command, wget will report how much of the target file it has already downloaded. By observing this you can be sure if it is really working.    

Integrating PeerSim ( P2P Simulator) with Eclipse

In this article I will explain how to integrate PeerSim with Eclipse IDE .
I am using peersim version 1.0.5 and Eclipse Helios IDE

Step 1: Download peersim-1.0.5 and extract it in a folder you like . Say the downloaded zip file was saved in C:\My Documents\Downloads. Extract it here and you will find a folder named peersim-1.0.5 .


Step 2: At first create a JAVA project . ( Go File -> Project -> JAVA Project )
Step 3: In window that popped up , unmark the option Use default location. A text box will be alive and show there the directory where peersim-1.0.5 was extracted . According to Step 1 that should be C:\My Documents\Downloads\peersim-1.0.5


Step 3: Press next to see everything is included correctly .


Step 4: Right Click on project name in the project explorer. Select Build Path -> Configure Build Path . In the window that popped up ,select the Libraries tab . Remove the jars named peersim-1.0.5.jar and peersim-doclet.jar.

Step 5: Right Click on project name in the project explorer. Select Run as -> Run Configuration. In the explorer at the left double click on JAVA Application.You will see a new run configuration window is on the right. Give any name you like at the Name field. Hit  Search to find the main class.
The class name is Simulator under peersim package.

Step 6: Go to the Arguments tab. In the program arguments field input "example/config-example1.txt". The press Run. That will simulate a protocol configured in the example/config-example1.txt file.

There are also some other configuration files in the "example/" directory . Change in the "Argument" tab to check the others.