I do not see the Design tab when opening a java file with a userinterface.
If in Eclipse, you can see the Package Explorer, but a java file with a userinterface does not get the Design tab, close that file. Then re-open it by right-clicking it in the Package Explorer and choose Open With > WindowBuilder Editor.
If the Package Explorer is not visible: reset all views of Eclipse: Window > Perspective > Reset Perspective.
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It is also possible, the WindowBuilder is missing from your Eclipse install. Then, read here how to add it – Fjodor van Slooten Apr 25 at 11:27