为项目添加 jre---- 项目右键，build path，configure build path，右面窗口的Libraries，add library...，jre system library--。
Eclipse “cannot be resolved to a type” error
http://chenxiaohu612.blog.163.com/blog/static/192214286201171004911955/
 http://zhaoningbo.iteye.com/blog/1137215
 
引言：     eclipse新导入的项目经常可以看到“XX cannot be resolved to a type”的报错信息。本文将做以简单总结。 正文：     （1）jdk不匹配（或不存在）     项目指定的jdk为“jdk1.6.0_18”，而当前eclipse使用的是“jdk1.6.0_22”。需要在BuildPath | Libraries，中做简单调整。     （2）jar包缺失或冲突     当找不到“XX”所在的jar包时，会报这个错。解决只需要找到（ctrl+点击，可能能看到jar包名称）对应jar包导入项目即可。     另外，出现相同的XX时也会报此错。可能需要调包、解包、选删。     （3）eclipse查找项目类型策略所致     eclipse下，上述两条都对比过了，没有任何问题，可偏偏还报这错。这时，需要操作一下 Project | Clean... ，问题即可解决。原因是，机制所致。因为某些特殊原因，eclipse没能自动编译源代码到build/classes（或其他classes目录），导致类型查找不到。 
Based on the comments here, I started checking how I could make sure that my Ant build 
wouldn’t interfere with my Eclipse build.
I found an option in Eclipse that clears up the problem (and is possibly more efficient than
 changing the 2 build system’s output folders).
Windows–>Preferences–>Java–>Compiler–>Building–>Output folder–>
”Rebuild class files modified by others”. This exists in Eclipse 3.5; I’m not sure about earlier versions.
Checking this seems to have fixed my problems.