Android: dealing with "Unable to open class file R.java" error

Here's another common error on Eclipse environment.
Unable to open class file R.java
I googled for solutions, there were many varieties but none worked so far.
But for me, the solution was very plain simple.

Go to Menu -> Project -> Clean

That solved the issue for me.
To all Android enthusiasts.

Android: dealing with debug certificate expired error

I haven't done any Android development recently.
I've been busy playing with new iPad and iPhone4.
But I'm an Android enthusiast at heart. Today I've gone back into developing Android apps.
However, I got stuck with a couple of problems before I could actually start developing again.
After updating all the SDKs, I got this error "debug certificate expired".

The solution was right in the developer page.

To fix this problem, simply delete the debug.keystore file. The default storage location for AVDs is in ~/.android/avd on OS X and Linux, in C:\Documents and Settings\\.android\ on Windows XP, and in C:\Users\\.android\ on Windows Vista.

Signing Your Applications



My problem was fixed.
My coworker however couldn't find the debug.keystore file.
I found this section "I can't compile my app because the build tools generated an expired debug certificate" in the trouble shooting page.


Here's the quotes from the page.

1. First, delete the debug keystore/key already generated by the Android build tools. Specifically, delete the debug.keystore file. On Linux/Mac OSX, the file is stored in ~/.android. On Windows XP, the file is stored in C:\Documents and Settings\<user>\.android. On Windows Vista, the file is stored in C:\Users\<user>\.android
2. Next, you can either
   • Temporarily change your development machine's locale (date and time) to one that uses a Gregorian calendar, for example, United States. Once the locale is changed, use the Android build tools to compile and install your app. The build tools will regenerate a new keystore and debug key with valid dates. Once the new debug key is generated, you can reset your development machine to the original locale.
   • Alternatively, if you do not want to change your machine's locale settings, you can generate the keystore/key on any machine using the Gregorian calendar, then copy the debug.keystore file from that computer to the proper location on your development machine.
   
    

Basically, she had to change the locale, and restart the computer.
Then her development environment started working as well.
To all Android enthusiasts.