The purpose of Use case diagram is that it shows the interaction between external actors and actions performed within a system. Use case diagram is developed in the analysis phase of the object oriented system development life cycle. It is done in the early stages of system development to help developers gain a clear understanding of the functional requirement of the system, without worrying about how those requirements will be implemented. In object development orientation, use cases are used to find objects and often generate new requirements as the system is analyzed and the design takes shape. It serves as the foundation for developing system test cases. Use Case diagram allows users to participate in a way that is seldom possible using the abstractions of Class diagram alone and helps the analyst get to grip with specific user needs before analyzing the internal mechanics of a system. Use cases provide a basis for early prototyping and readily identifiable units for incremental delivery. They also provide a means of traceability for functional requirements upstream in the process and for constructing test plans downstream in the process.
The purpose of a class diagram is to depict the classes within a model. In an object oriented application, classes have attributes, operations and relationships with other classes. Class diagrams are useful throughout the software lifecycle for a variety of team members such as analysts, business modelers, developers and testers. Class diagrams are the key activity in object oriented