JavaExp: Java and Open Source Technology

Recently I found interesting issue regarding JVM and Linux Swapiness. I was getting java out of memory while compiling my Java application. I have 8GB/Linux, and I was getting Java out of memory while compiling it alone (Without any other memory intensive job), also the overall build time was rediculously high. I googled and tried some tricks on the machine, after that my build time was significantly better and  most importantly, I did not get any out of memory from JVM. What I did is that I set the swapiness of the system to 10 from 60. Swapiness is a value between 0 to 100 and it's a parameter to the linux kerner to take decision on how aggressively to take processes out of the RAM to Swap Space in the Hard disk. Swapiness with 0 tells the kernel to avoid using Swap as far as possible, but it does not mean Swap wont be used at all. Swapiness with value 100 tells the kernel to use Swap space aggressively. You can check swapiness of your linux system using the followi...

Recently I needed to print a receipt with Barcode. But the particular webpage was not showing the Barcode. I googled and found a very helpful page and I followed the steps and then I was able to achieve my goal. The link I am talking about is http://www.cb1.com/~john/notes/fedora-font-install.html . I followed the first option. First I donwloaded the font library and then installed using kfontview. type kfontview from the command line. if kfontview is not available then install it by typing yum install kdebase  from the command line. From the kfontview window, open the font you have downloaded. Click on the "Install". Thats all , now close all your browse instances and again open the page with Barcode . You should be able to view it properly

This discussion is very common , when to use interface and when to use Abstract Class. Before deciding we should know properties of Abstract Class and Interface in details ::Abstract Class:: It contains both abstract methods and non abstract methods Classes Extend Abstract Class Abstract Classes cannot be instantiated. Abstract can't be declared with Final Synchronised Native. It can have static and instance initialise block. Abstract Class has constructor. Abstract Class may not contain any method(any abstract method) ::Interfaces:: Interface contains all abstract methods. Classes implements Interface. all methods compulsory implemented by particular class which implement interface. Interface variables are implicitly public static and final. Interface variables are constants that cannot be overridden. They are inherited by any class that implements an interface. Interface may not contain any method(Marker Interface). Interface does not contain Constructor. I...

hostip.info provides Api's where if we pass the IP Address, it returns the Location details like Country Name, City Name , Lon and Lat etc. We can use PHP to process the response from the api call and get the required data. Some of the API provided by hostip.info are as follows, http://api.hostip.info/country.php US http://api.hostip.info/get_html.php?ip=12.215.42.19 Country: UNITED STATES (US) City: Sugar Grove, IL http://api.hostip.info/get_html.php?ip=12.215.42.19&position=true Country: UNITED STATES (US) City: Sugar Grove, IL Latitude: 41.7696 Longitude: -88.4588 Now I will show you how to use PHP to call and process similar Api's http://api.hostip.info/?ip=12.215.42.19 [use the URL above for an example - XML too long to paste below] Following is the PHP function, function countryCityFromIP($ipAddr) { //get the XML result from hostip.info $xml = file_get_contents("http://api.hostip.info/?ip=".$ipAddr); //get the city name inside the node and preg_mat...

Recently I have noticed that eclipse IDE on Fedora inserts the default system user name "root" in the @author attribute for java comments. I have found two ways to change the Default System User name to what ever you like, The First way is to The other one is just by  changing the associated templates in Window -> Preferences -> Java -> Editor -> Templates -> @author , and hard coding the author's name. The Other way is by editing the eclipse.ini file, For example , the following eclipse.ini file shows what I have done to set my name -showsplash org.eclipse.platform -framework plugins/org.eclipse.osgi_3.4.2.R34x_v20080826-1230.jar -vmargs -Dosgi.requiredJavaVersion=1.5 -Xms40m -Xmx512m -XX:MaxPermSize=256m -Duser.name="Siddhartha(Sidd)" Notice the  attribute Duser.name , that I have set to Siddhartha(Sidd),  after changing eclipse.ini we should restart eclipse so that changes can take effect. Now, when ever I want to a...

I have worked with Hibernate2 where we had to define all configuration as well as Class to Table mapping though XML files. Hibernate 3 goes one step ahead and provides new ways to work Where we can use annotation to define Class-Table mapping. Hibernate3 supports annotation and requires Java 5 or above to function properly. While trying to work with Hibernate you will notice it has several dependencies to other third party jars, like apache commons collections , apache commons logging. I have tried to put all the required file in a zip file  and named it   "hibernate3AllJars" . Since while playing around with codes which require some data base I prefer MySQL   since it is very easy to use and it is free. I have also putted the  MySQL JDBC Driver  mysql-connector-java-5.0.5-bin in the zip file. If needed you can email me regarding the zip file. For the Hands on I have created a Table in MySQL Database "sidd"  by executing the following command, CREATE TABLE `hone...

Design by Contract is a trademarked term of BertrandMeyer and implemented in his EiffelLanguage as assertions. Design By Contract (DbC) is a software correctness methodology. It uses preconditions and postconditions (Contracts) to programmatically assert the change in state caused by a piece of a program. The Preconditions and Postconditions that are in demand in DbC could summarized by the "three questions" that the designer must repeatedly ask:     * What does it expect?     * What does it guarantee?     * What does it maintain? Some attributes of Design By Contract... Classes should specify their invariants: what is true before and after executing any public method. Methods should specify their pre- and post-conditions: what must be true before and what must be true after their execution, respectively. Pre-conditions may be weakened by subclasses and post-conditions may be strengthened. The explicit definition of these "assertions" forces the designer to th...