Eclipse Java Package Explorer is not open! I don't know how to open it! I will write it for those who like. It will be a short story.
Click "Window"-> "View View"-> "Other"
Select "Java"-> "Package Explorer"-> click "OK"
You should see "Package Explorer" as shown below!
By the way, fixing the window of Package Explorer is OK if you click the upper right (red frame).
This time it's trivia, but it's necessary. Please remember it as a memo.