Java visitor pattern example

The visitor is the most complex design pattern. This pattern shows us how to "visit" the elements of one structure without changing the code of element's. Add a new visitor when you want to add a new operation for these elements.

Java BigDecimal example

In Java do not use float or double to do decimal arithmetic. Use BigDecimal instead of float and double. If you want to know the details click here.

CentOS remove press any key to enter the menu

After fresh installing CentOS and starting the system there is a sentence on the screen "Press any key to enter the menu, Booting boot in __seconds". Editing the /etc/grub.conf to remove the sentence and start CentOS quickly. The content is like following.

Enable network after centos minimal install

The network is disabled after minimal install CentOS. You can use the following command to enable the network. service network start. The network service does not start after rebooting the system. Change the config file to make the network automatic start. The path of configuration file is /etc/sysconfig/network-scripts/ifcfg-eth0. The content of configuration file is like following.

Linux alias command permanent

The command alias helps us us short command instead of long command and parameters. The problem is when you next time login the shortcut is disappeared. You need to change the .bashrc file to define your specific aliases. The path of .bashrc is ~/.bashrc. For example, you want us vim instead of vi command. Edit your .bashrc file like following.

Linux often-used commands tutorial

This page will show you often-used commands in Linux.

Linux alias command example

alias can help you make long command as a shortcut. For example, you have to type mysql -h host_address -u user_name -p when you need access mysql remotely. alias can make this efficiently. Create a shortcut for remote accessing by using the command below. Next time when you want to access database just type mysql-remote.

Virtualbox install Linux unable to boot

After creating virtual machine I want to install CentOS6 (32bit). I got the error message like following.

Jackson JsonMappingException: Object is null

When I try to convert net.sf.json.JSONObject to json I got the JsonMappingException. After checking the stack information I know I forget init the value of user. The detail of exception is like following.

MySQL export data with where condition

Last blog I have showed you how to export data from database. If you don't want to dump all data from database you can add --where parameter. If you want dump user info where age bigger then 27 the command is like following.

Java bean mapping to wrapper bean example

We need to convert java beans to another java bean when pass it from one layer to another. For example, you need to convert PO(persistent object) to BO(business object) when pass it from DAO layers to business layers. I recommend you to use Dozer instead of converting them by yourself.

Java mediator pattern example

If you want to reduce the relationship between a set of objects mediator pattern is a good choice . In mediator pattern objects don't know each others . All object to send the " event " to the mediator . Mediator knows every object and invoke the method of them if needed. In this page I will show you how "button" and "text" works in mediator pattern.

Java zookeeper example

In this page I will show you how to use zookeeper as a configuration service. I use a standalone zookeeper for the demonstration. If you want to build a high availability configuration service click here for more detail.

Java command pattern example

Four roles in command pattern are client, invoker, receiver and command. Command invokes a method of Receiver to finish one command. Invoker knows which command will be executed. Invoker and Receiver do not know each other. After packaging the command you can repeat or undo it.

Java iterator pattern example

The best example of iterator pattern are source code of util package in JDK. If you want to learn more, I recommend you read the source code of JDK. (Including but not limited to:Iterator, ArrayList and HashMap). In this page I will make an easy example to show how iterator works. The structure of classes is like following.