为什么我的myeclipse项目jre使用sun jdk1.6才不报错

新手求指教，自己新建的项目，jre只有使用sun jdk 1.6 才行，换成自己安装的jdk1.8过后就报错。
错误信息：Description Resource Path Location Type
The project was not built since its build path is incomplete. Cannot find the class file for java.util.Map$Entry. Fix the build path then try building this project BigData Unknown Java Problem
正确的时候如下：

错误的时候如下：

jre: