Eclipse: "The import java.io cannot be resolved"

Check two things

Eclipse preferences

1. Check your Eclipse preferences: Java -> Installed JREs
2. The one that you're using should be marked and it should be a JDK not just a JRE.

Project build path

1. Also check your project's build path: Right click on the project -> Properties -> Java Build Path
2. Check in the libraries folder whether the JRE System Library is present and ...
3. If not add it using Add library -> JRE System Library and then select the correct one (from an installed JDK).

I am using Maven an this is what I did to re-solve -->

1. Right click on project
2. Maven -> Update Project. Make sure you choose 'Force update of Snapshots/Releases'.
3. Also check the 'clean project', 'update project conf from pom' and 'refresh workspace resource options' and click ok.