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Use case diagram and activity diagram

Describe Use case diagram
A use case diagram is a graphical representation where a user communicates with the system. A use case diagram has many use cases and actors in the system .The use cases are either represented by circles or ellipses.The actors shown in the form of stick figures
Main Reasons to draw a Use case diagram
The main reasons to draw a use case diagram is to capture the dynamic aspect of a system. However, this definition is too generic to describe, as other four diagrams(activity, sequence, collaboration and state chart) also have the same purpose.
Use case diagrams are used to gather the requirements of a system including internal and external influences. These requirements are mostly design requirements.Hence, when a system is analysed to gather its functionalities, use cases are prepared and actors are identified.
The main reason to draw a Use case diagram can be said to be as follows
– Used to gather the requirements of a system
– Used to get an outside view of a system
– Identify the external and internal factors influencing the system.
– Show the interaction among the requirements are actors

Symbols used in the use case diagram
-Actor- Actors are usually individuals interacting with the system. They are defined according to their roles.The Actor can be a human or other external system.
-Use case- A use case describes how actors uses a system to achieve a specific goal. Use cases are typically initiated by a user to fulfil goals describing the activities and variants involved in attaining the goal
-Relationship- The relationships are between the actors and the use cases
-System boundary- The system boundary defines the system of interest in relation to the world around it
Describe activity diagram
Activity diagram is basically a flowchart to represent the flow from one activity to another activity. The activity can be described as an operation of the system.The activity diagrams deals with all type of control flow by using different elements such as fork,join etc
Main reasons to draw an activity diagram
Activity is a particular operation of the system. Activity diagrams are not only used for visualizing the dynamic nature of a system, but they are also used to construct the executable system by using forward and reverse engineering techniques.
Activity diagram is sometimes considered as the flowchart.. It shows different flows such as parallel, branched, concurrent and single
The main reasons to draw an activity diagram can be described as
-Draw an activity flow of a system
-Describe the sequence from one activity to another
-Describe the parallel, branched and concurrent flow of the system
Symbols used in the activity diagram
-Action-A step in the activity wherein the users or software perform a given task. .
-Decision node-A conditional branch in the flow that is represented by a diamond. It includes a single input and two or more outputs.
-Control flows-Another name for the connectors that show the flow between steps in the diagram.
-Start node-Symbolizes the beginning of the activity. The start node is represented by a black circle.
-End node-Represents the final step in the activity. The end node is represented by an outlined black circle.


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