Activity diagram is used to document the logic of a single operation/method, a single use case or the flow of logic of a business process. The class diagram is the main building block of object-oriented modeling. State chart could contain states such as: entering, standing still, exit, arrived. There are various diagrams in UML such as class, object, use case, activity … State chart modeling is used to show the sequence of states that an object goes through, the cause the transition from one state to other and the action that result from a state change. You only have to look at the names. SEQUENCE DIAGRAM(contd..)Diagram: A sequence diagram of request and response between apatient and a receptionist 12. And, it is used for dynamic object modeling. Start with a use-case and elaborate it into activity, class, sequence, and communication diagrams. The actual differences lie in their purpose. Do PhD students sometimes abandon their original research idea?