Interview Questions, Tips and Tricks, Tutorial

Singleton in Java

What is Singleton?

Singleton is a class that cannot have more than one instance.

In Java, there are many way to create singleton class, one of the example is:

class SingletonClass {

    private static volatile SingletonClass INSTANCE = null;

    private SingletonClass() {   }

    public SingletonClass getInstance() {
         if (INSTANCE == null) {
            synchronized (SingletonClass.class) {
                 if (INSTANCE == null) {
                    INSTANCE = new SingletonClass();
         return INSTANCE;

When to use singleton?

Singleton is used when you need system wide “global value” that need to be accessed by many part of systems. Example: Session Value, Logger, Cache, etc

Information Technology, Tips and Tricks

Change JSESSIONID in URL parameter

Having problem with JSESSIONID in URL ?

Well, this is not a bug. When the server received a request from client, it is not sure whether the client support cookie or not. So, it generate both of them in the URL. When it comes back for second time, and the server detects the cookie is present, it will drop JSESSIONID from URL automatically. However, when the cookie is not present, the server will continue to use it.

Having JSESSIONID in URL actually is not bad at all, however it has a few drawbacks:

  • Security Risk

By exposing SESSIONID in the URL, it allows attacker to attack victim and steal some information store in the session

  • SEO

Because SESSIONID is unique, search engine both may not recognise different pages that having unique SESSIONID.

How to solve this?

There are few ways to avoid JSESSIONID in URL

  1. Implementing Servlet Filter which will disable/skip URL based on SESSION ID generation.
  2. Since Servlet 3.0, we can use SessionTrackingMode Enumeration to specify how SessionTracking should be.
    So, in web.xml we just need to add the following:


So, who said it is difficult and complicated? Happy coding 🙂

Articles, News Around The World

Facebook, Twitter, YouTube and Microsoft agree to remove hate speech across the EU — TechCrunch

Facebook, Twitter, Google’s YouTube, Microsoft as well as the European Commission unveiled a new code of conduct to remove hate speech in less than 24 hours across these social media platforms. The EU has ramped up efforts leading to this code of conduct following the recent terrorist attacks in Brussels and Paris. Read More

via Facebook, Twitter, YouTube and Microsoft agree to remove hate speech across the EU — TechCrunch

Information Technology, Tutorial

Testing an API

Some consideration in defining testing scope for an API:

  1. Output value based on input parameter
  2. Output value based on incorrect/missing parameter
  3. API behaviour when there is no output or expected to return nothing
  4. Whether API trigger another event/process
  5. Whether API modifying certain resources 

Scenario 1: Output Value based on Input Parameter

This test scenario will test API functionalities based on API Specifications. Example: function add(int x, int y). We can test that API by passing any int value, then validate the return value.

Scenario 2: Output Value based on Incorrect/Missing Parameter

This test scenario will test API functionalities by passing incorrect/missing parameters against API Specifications. Example function add(int x, int y). We can test that API by passing x value only, or passing x value as double, then validate the behaviour of an API (eg: will it return an error message?)

Scenario 3: API behaviour when there is no output or expected to return nothing

This test scenario will test API functionalities boundary. One of the boundary need to consider is no output expected. Example, function search(String keyword). We can test that function with non-existence keyword, then observe API return value (eg: return empty result, etc).

Scenario 4: Whether API trigger another event/process

This test scenario will test API functionalities that will trigger another event/process. Example: function backup() to execute backup shell script on server. We can test that API, and then validate whether backup script are executed after API call.

Scenario 5: Whether API modifying certain resources

This test scenario will test API functionalities that modifying certain resources (eg: creating new file, deleting file, updating file. Example: function export(Date startDate, Date endDate) will export data for certain period in CSV.  We can test that API and validate if intended resource modification (eg: CSV file is produced) successfully executed.

Beside these 5 factors, there are many other factor that’s good to be consider:

  1. API performance
    • To check API performance during high load to the API Server.
  2. Output / Response
    • To check how application behaviour when the output/response from API does not according the specifications (unstructured data), etc



Apps Reviews, My Comments

MessageMe App: The Good and Bad

MessageMe is a new mobile free messaging app. It comes with a lot of visual features and personalization. Through MessageMe app, people are able to send SMS text messages and make voice calls to their friends for free using WiFi and the phone’s data plan.

Sounds good, isn’t it? But wait … there are a lot of rumor going around that say when you receive an SMS from MessageMe, and you click the link, the same SMS will be sent out to ALL your contacts stored in your mobile phone.

WOW ….. what does it means?

Imagine if you have over 1000 contacts in your mobile phone …. it means over 1000 SMS will be sent from your mobile phone to all your contact … at the end you may incurred HUGE phone bill to pay those SMS … so be careful before clicking to any link

Also, a lot of people complaining that they are receiving a call from +375602605281, +37127913091 or any number starting with a +375, or +371 number. So please, don’t answer or call back, or you will be charged of International call, which most likely will be very costly.

A lot of rumor over the internet said that, if you call back, they will be able to steal your information store in your phone, but I believe this is an HOAX (See this:

Finally, it is up to you to decide … you also can review other people comment on iTunes here:

Big Data, Information Technology, Tips and Tricks

Install MongoDB on CentOS 6.4

MongoDB can be easily to be installed on CentOS 6.4 (64 bits) with with the following 2 steps:

  1. Add/Create 10 gen repo into the system#> vi /etc/yum.repos.d/10gen.repo

    name=10gen Repository

  2. Install Packages#> yum install mongo-10gen mongo-10gen-server

Note: You may need to be root to do the installation.

Once the download complete, your MongoDB is ready to be used. You can start/stop MongoDB instance with the following command:

# >  service mongod start
# >  service mongod stop

and, to restart

# > service mongod start

You may want to let the MongoDB auto start when the system up, with the following command:

#> chkconfig mongod on

Now, your MongoDB is installed and ready to use. You can start to using MongoDB shell, with the following command:

# > mongo

Happy trying and using with it.


Information Technology, Tips and Tricks

Steps to create MySQL datasource in JBoss 7 (not jdbc 4 compliant driver)

  1. Create the directory structure for the MySQL module.
    Example: how to create a module for MySQL driver:

    cd $JBOSS_HOME
    mkdir modules/com/mysql
    mkdir modules/com/mysql/jdbc
    mkdir modules/com/mysql/jdbc/main

  2. Copy the driver(s) jar into the created directory
  3. Create a module.xml file to define dependencies and resources. For our mysql example it looks like this:
    jboss:module:1.0" name="com.mysql.jdbc"

    Note: replace the JAR name accordingly with the version that you have in your local machine

  4. Add driver to the standalone.xml
                        <driver name="h2" module="com.h2database.h2">
    		    <driver name="mysql" module="com.mysql.jdbc"> 
  5. Restart your JBoss
  6. Go to JBoss console http://localhost:8080/console, select “Connector –> DataSources” from menu on the left, and then click button “Add” on the top right corner. Afterwards, follow the steps and you will that “mysql” is now available as driver option.
Gadgets Reviews, My Comments

What will “iphone 5S” or “the new iPhone” look alike?

With much controversy of longer iPhone 5 (instead of wider), I think Apple won’t make “the new iPhone” keep longer and longer. So, what will the Apple do in order to make “the new iPhone” able to put more content (as what they told us when releasing iPhone5) into their screen?

It’s easy, I think they will remove the “home” button and keep the screen as per now (iPhone 5) or back to the original design of iphone 4/4S. We knew that since iOS5, Apple had introduced what they called as “Assistive Touch”. Is it a sign that Apple actually doing some research on-screen “home” button/gesture? Or Apple actually had done some research that  their “home” button is easy to be broken. So they are going to remove it soon

Well, some people will argue that “home” button is still needed for recovery mode. If that so, Apple still able to provide another “reset” button on the left side (just below the volume button), or use the volume button to trigger recover mode, or create another button on the right side. or just create a pin-hole “reset” button at the bottom next to their new USB charger connector (that probably also one of the reason why they make the USB connector smaller now 😛 )

However, if Apple really going to throw away their “home” button, the more interesting question here, will Samsung sue them because of copyright design of no “home” button?

Well, let’s see in the couple of months, what will “the new iPhone” look a like?

Also,  last not least, will they named it “iPhone 5S” or “the new iPhone”? I think “the new iPhone” will be more suitable if they remove the “home” button design …