android Multiple markers at this line - The type java.lang.Class cannot be resolved. It is indirectl

android Multiple markers at this line - The type java.lang.Class cannot be resolved. It is indirectly referenced from required .class files - The type java.lang.String cannot be resolved. It is indirectly referenced from required .class files  

Multiple markers at this line
 - The type java.lang.Class cannot be resolved. It is indirectly referenced from 
  required .class files
 - The type java.lang.String cannot be resolved. It is indirectly referenced from 
  required .class files
 - The type java.lang.Object cannot be resolved. It is indirectly referenced from 
  required .class files

package com.jay.test.activity;

提示如上的错误，并且注意到project中没有了android2.2的jar包了。

解决办法：在package explorer中右键自己的project名，build path，config build path，左边一列中选android，project build target中显示已经选中了android2.2，重新选择一个别的，比如android2.3.1，确认后再重新修改回android2.2，打开菜单栏的project，clean，选中自己的project名字，确定，清理会删除gen文件夹，然后自动重新build project（右下角有提示），完毕重新生成了gen文件夹，问题解决。