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User can provide any binary, empty file or broken keystore as existing keystore file in "Add keystore" dialog. And this keystore will be added to the list, so user can see just locked keystore with "Unlock Keystore" button. But it's impossible to unlock it and "Keystores Manager" doesn't inform user that this keystore is invalid.
Please see description for issue http://www.netbeans.org/issues/show_bug.cgi?id=162693. There is no way to determine is keystore file is valid/invalid without password. One cannot try ANY password for checking correctness of keystore file. If file is really not keystore file ( was not created as keystore ) then you will not be able to unlock it. And we cannot inform user about its validity because there is no way to check this. You can just remove file that you SUGGEST invalid ( because even you cannot affirm that file is not keystore file )