如果项目已经从项目导航器中删除，那么右键单击项目导航器，并选择refresh以刷新工作空间。然后重新添加已删除的项目。

您的工作区中可能有一个同名的项目。尝试在删除同名的另一个工作区后刷新(file->refresh)您的工作区。 当您希望重新导入已删除的项目时，会出现典型的情况。

Eclipse工作区中的项目必须是唯一的。但是请注意，项目名称不必与项目的目录/文件夹名称相同，因此您可以删除具有相同名称的任何现有项目，也可以重命名现有项目。

如果项目已经从项目导航器中删除，那么右键单击项目导航器，并选择refresh以刷新工作空间。然后重新添加已删除的项目。

检查您是否已将Eclipse配置为显示所有工作集。我曾经遇到过完全相同的问题，结果是我不小心将项目导入了Other Projects工作集。而且我的Eclipse配置没有在包资源管理器中显示该工作集，这使我相信项目还没有导入。

更新@Mawg:

首先，单击选择工作集…

然后，选择No Working Sets:

这将有效地禁用工作集并显示所有项目。

我刚接触Eclipse和Android开发，这让我困扰了很长一段时间。以下是我做错的几件事，也许将来会对别人有所帮助:

I was downloading code examples and assuming project name would be the same as the folder name and was looking for that folder name in the project explorer, not finding it, re-importing it, then getting the error message it already existed in the workspace. Yeah. Not proud of that. Didn't click on 'Copy projects into Workspace' and then searched in vain through the workspace when it didn't appear in the project explorer BECAUSE The 'Add project to working sets' option in the Import Projects tab isn't working as far as I can tell, so was not appearing in the project explorer for the active working set (refresh made no difference). Adding project to the working set had to be done after successfully importing it.