Eclipse “cannot be resolved to a type” error

    （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.