You can drag Java classifiers onto your diagram to create objects. Any view with Java elements can be the source. In this example the Project Explorer is used, but the Package Explorer or the Hierarchy, Outline, Search and Call Hierarchy views work just as well.
Note that if you drop a Java classifier onto the yellow header of an existing object, the object's classifier is updated and any operations that are unavailable will be removed from their messages. If you don't want this behavior, please drop classifiers further down into the white lifeline area.
You can drag one or more objects and move them to a different position, thereby changing their order. An object is represented by the selectable yellow header of a lifeline. As usual, multiple objects can be selected by pressing the 'Ctrl' key before left-clicking on them. It is also possible to resize an object by changing its width.