Alternatives
In UML 1.x, an alternative to activity diagrams is the UML state machine diagram, in which states can be used to represent activities (namely the state of that activity being performed), and transitions represent the start or completion of activities.
This mapping of activity diagram features to state diagram were formalized in UML 1.x, leaving activity diagrams little more than the use of state machines to capture behavior and partially concurrent behavior within a work flow.
Despite the (small) advantages of using UML 1.x activity diagram over state machines, such as the ability to cover the behaviors of collaborating elements, while state machines are limited to a single element, many modelers continued using state diagrams. State machines were attractive to use as the states may also represent conditions that hold before or after the activities. Activity diagrams and state diagram had similar abilities to express concurrency, though the run-to-completion (RTC) semantics of State machines limited the expressiveness of the concurrency.
Perhaps the most important reason why state machines were more popular in UML 1.x over activity diagrams was that the tool vendors were slow to implement robust activity diagram capabilities.
With UML 2.x, the foundation of activity diagram changed from being based on state-machine semantics to now being based on Petri net semantics. This vastly expanded the number of circumstances where activity diagrams are more appropriate to capture the paths of workflow in a system. Now with the advent of robust activity diagram implementations, activity diagrams are now generally used more than state machine diagrams.
Read more about this topic: Activity Diagram
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