"The type java.lang.Object cannot be resolved." with Eclipse

There can be many cases for this:

1) You have lost .classpath and other eclipse files. If you use maven, then do "mvn eclipse:eclipse:.

2) Your .classpath may contain (org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/java5) Just remove that to yield

<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>