Error:The project cannot be built until the build path errors are resolved.

when i develop program android with eclipse tool.
i find this error
The project cannot be built until the build path errors are resolved. 

to solve this error use this :

1. Open the project properties
2. Select Java Build Path > Libraries
3. build an temporary library --> Add a new, select "up to you" (to be deleted later) > OK
4. Wait for the workspace to refresh (or force a refresh of the project)
5. The error(s) will go away
6. Remove the temporary library 

And the error can go away