Are there any plugin for eclipse that allows to show only one method from class in a separate editor? Just like if one copied just one method from class into separate editor, however with ability to edit (save, use reference search, ctrl+click to navigate to the referenced object) this class. In a few words this should be the same java editor showing only one method.
Click on "Show source of selected element only" button on the toolbar.
If there isn't such a button do the following: right click on the toolbar -> "Customize Perspective..." -> select tab: "Tool Bar Visibility" -> menu: "Editor Presentation" -> check "Show Source of Selected Element Only"
It's not in a separate editor but gives you the features you want.