ClassNotFoundException

The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project.

This problem is often encountered (especially by beginners). The solution is often quite simply: Just right click on the project, select the properties and then click on the the Java Build path tab. Scroll down to the bottom and change the java system library to Java version or add it if it is not included yet.