我遇到同样的问题，我在配置构建路径中删除了一个jar文件它开始为我工作正常。

I started using Eclipse today to develop my java programs (did not move/touch any settings).

I try to run the Java programs, and I get this message:

Error: Could not find or load main class CLASSNAME

This happens for ALL my java programs, including those I developed before (and ran without problems on Eclipse). However, this was not the case yesterday; I was able to run developed programs without problem.

I did not uninstall/change my Java installation. The developed programs run without problem when I use another IDE to run it...

How to fix this? Thanks!

I had same problem one of the jar files was missing which i removed in the configure build path it started working fine for me.