The project was not built since its build path is incomplete

Right click on project -> Properties -> Java Build Path (From Left List) -> Libraries (Tab) -> Add Library (Button on right) -> JRE System Library -> Next -> Workspace default JRE -> Finish

This should solve at least one of your errors. Any others might require you to add further libraries.

Hope this helps.

Close eclipse and re-open worked for me. You may have to do this two or three times. Each time give it a minute for the auto build startup process (check bottom right hand corner) to complete, as these errors often disappear at this time.

Or try to clean the affected projects.

'Turn it off and on again'

It seems that you lack JDK (java.lang.Object comes from there )

The problem is that you used the same Android API on different PC. Eclipse is lost with path names that are differents but with same API.

To solve it, first re-select Android API version (set to another one and valid, then restore the on you want and validate it -> it will force Eclipse to reload API). Then go into project->properties-> java build path. Then remove "Unable to get system library for the project" then "Add library" and select "Android Classpath container".