Use case diagram is high level diagram and it is end user perspective. Use case diagram must not have alternate flow, any conditions & any repetition. Use case diagram consists of The actors, the systems, the use cases, the lines that represent relationships between elements
The Actors – The actors interacts with the system and it is of two types primary actor and secondary actors. Primary actor always be on the left hand side of the system.
The System – It is a rectangular box and all the use cases must be drawn within the confines of the system.
The Use Cases – It is a list of actions or event steps typically defining interactions between an actor and a system. It is of two types essential use case also known as parent class and supporting use case also known as child class. Supporting use case can not exist without a essential use case.
Generalization – Parents class will not exist without any of the one child class or multiple child class and direction of arrow will head towards parent class.
Include – Parent class must exist with at least one child class or multiple child class
and direction of arrow will head towards child class
Extend – Parent class may or may not exist with a child class or multiple child class and direction of arrow will head towards parent class.
Note : Direction of arrow will always head towards the dependency.