Eclipse - Errors - While coding

This section will gather all errors found when you were coding in Eclipse.
It is really hard sometimes to find what is the problem when you get an exception.

I will also give a solution of the problem.

Error 1. java.lang.ArrayIndexOutOfBoundsException

I had a hard reboot and when I came back to Eclipse (it was opened during this reboot), completion didn't work anymore.
I trying some stuff, but nothing.
Finally I erased the .metadata directory included in all projects generated by Eclipse.
I know it is a bit radical, but it works.