摘自：https://blog.csdn.net/lazy_p/article/details/7452846
‘Clean’ Your Eclipse Project: Go to Project > Clean in Eclipse [This seems to work for me]
Refresh your project folder (right click on your project > refresh)
Re-build your project
Clean your builds (If using Ant or Maven – clean your builds)
Recreate your project in Eclipse
‘Switch’ Workspace – then Switch back (Eg Change to Debug, then switch back to Java)
‘Switch’ Workspace – then Switch back (Eg Change to Debug, then switch back to Java)
Remove and re-add your JRE:
1. Right Click on your project > properties
2. Click on the Libraries tab
3. Click on the JRE
4. Click remove, then OK
5. Repeat 1-3 again, but add the JRE again
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其中 project 中 clean功能解释如下：
由于eclipse的编译是基于时间戳的判断机制的。因此当你按build all的时候有些eclipse认为时间戳没有改变的类不会被编译。因此你可以先clean一下再编译。这个时候eclipse会将所有的以前编译信息清除然后编译。 eclipse --->project ----->clean... 选项将工程中的.class文件删除，同时重新编译工程，类似于jbuild中的rebuild
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