solving an error: Eclipse does not show projects in Project Explorer

I have installed Eclipse with PDT pluging on my virtualbox instance which had Ubuntu installed on it.
The problem was that I was getting this error :

"org.eclipse.core.runtime.CoreException: The file does not exist"

and my projects were not listed in PHP explorer in my eclipse. The problem was that projects existed on my Hard disk. At first I was importing the projects into my workspace , but when it happened for the third time I had enough!

I went to the project directory first: (These files are hidden so you have to use "ls -a" to see them) and there was couple of configuration files and directories:
.buildpath , .project and .settings/ but none of them seemed to have a problem.
I googled it and found this blog post:

http://ahtik.com/blog/2008/01/15/eclipse-workspace-fails-to-start-after-crash-quick-fix/

as he suggests, these are the steps that should be followed:

1- exit Eclipse
2- delete this directory :
        $ rm -r <<workspace_dir>>/.metadata/.plugins/org.eclipse.core.resources/
3-start eclipse with -clean switch:
        $eclipse -clean
4- go to File->Import-> General->Existing Projects into workspace

and it worked well for me! The directory <<workspace_dir>>/.metadata/.plugins/org.eclipse.core.resources/ is made automaticly again!

LXDE a good option for Ubuntu inside Virtual Box

When running guest operating systems inside a host operating system using virtualization, it is important to assign a correct memory allocation to the guest. If your computer for example has a 3 GB RAM , and you are running 3 virtual box at the same time allocating 1 GB to each you can not run any other application in your host OS. 

Ubuntu is a very popular Linux distribution because of its simplicity and also rich repositories. After Canonical decided to use Unity as their default user interface from release 10.10, Ubuntu uses a lot of system RAM just to boot and run in normal mode.  However, If you are using Virtual Box or VMware for virtualizing with the help of Ubuntu Linux instances,your system will be so slow that you can continue working anymore!

A very recommended solution to this problem is using LXDE desktop environment.  LXDE stands for "Lightweight X11 Desktop Environment". It consumes the least memory and CPU resources compared to other desktop environments(Gnome, KDE and Xfc).

It is also a very good option for old computers or also weak notebooks. When battery life matters it should be considered as the first option. I have installed it also on my ubuntu on my old Asus Eeepc netbook because it was impossible to work with Unity on that netbook.

To install it on your ubuntu system just enter this command in your shell :

$ sudo apt-get install lxde

when you login to your ubuntu don`t forget to select lxde as your desktop environment!