Eclipse code Error Hint: The import java.util cannot be resolve, how to solve?

Show the import java.util cannot be resolve, how to fix it?
When I was using eclipse, I accidentally changed the installation location (workspace), and now all the packages show that they cannot be imported: The import Java cannot be resolved how can I solve this problem?

------Solution 1--------------------
Solution:
Right-click Project-------BuildPath--------The bottom of the Configura ... Of
Select libraries
Find the JRE (this time you will find this JRE in front of it!). or a red X)
Check Remove off
Re-select a JRE for the project
Select project, Project----clean
OK, all the bags have been found.

------Solution 2--------------------
Install the jar in the project upstairs and configure it.
Because you changed the working environment and the jar packages in the project were the same.
You are now ready to configure the Java environment in your project.

Eclipse code Error Hint: The import java.util cannot be resolve, how to solve?