The drag and drop concept could be important in order to track the
activity diagram elements
used in more than one diagram.
It
is not recommended to use the same activity diagram element in
different diagrams, but
it could be important for DSL modeling to have this option.
In an Activity diagram, you can drag and drop both Java and UML
elements
from:

1. From the Package Explorer you can
drag and drop the following Java
elements:
The classes or interfaces from the Package Explorer
to
the Diagram
Editor will display a new instance in your Activity Diagram.
2.
From the Model Editor you can
drag
and drop the following Java or UML (just
a metamodel element not related to Java) elements: