有时（我极少遇到），通过ctrl + shift + t找对应的类时，类明明存在，并且也在编译路径下，但就是查找不到，一个可能的原因就是eclipse为类建立的索引出了问题。
解决的方法是：找到项目所在工作空间下的.metadata/.plugins/org.eclipse.jdt.core文件夹，将里面的*.index文件和savedIndexNames.txt文件删除，重启eclipse，然后eclipse会重建索引，这时问题就可以解决了。
解决方法来自这里： http://www.java-forums.org/eclipse/6513-open-type-ctrl-shift-t-does-not-find-types.html
Sometimes, the indexes that Eclipse use for searching the types, get corrupted. You can clear the Open Type indexes by going to <Workspace>/.metadata/.plugins/org.eclipse.jdt.core and deleting all files there. The indexes will be re-built the next time you use open type.
Note that this is a empirical observation, and you may delete some JDT saved data that you don't want to delete. I observed no ill effects, but for safety you could try deleting just the *.index files and the "savedIndexNames.txt", and see if it works.