Class diagrams

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• Class diagrams shows the classes used in the system. Each class has some properties and operations. Relationships between the classes are shown in class diagram. Different types of relationships exist between the classes. It is a static view of system used to assist in modeling. 
• Association is allowing instance of object to perform an action on its behalf. E.g. A Student updates his Daily task. Here Student and Task are the classes while update is the relation.
• Aggregation is best described as “has a “relationship. E.g. an employee has a task.
• Composition is a more specific representation of Aggregation having a multiplicity. Eg. Employees of Project1 have a Login task. 
• Generalization relationship says that one class may have more related classes. The main class is the super class while the related classes are the subtypes. 
• E.g. Person is the main class. Employee and manager are the sub classes. 
• Realization exists between two classes when one of them must realize, or implement, the behavior specified by the other. E.g. Employee daily tasks are updated by the Manager. 
• A Dependency relationship is when any change made to one class/interface cause the behavior of some other class to change. E.g. changing task definition will change the employee status. 
• The multiplicity of the association denotes the number of objects that can participate in then relationship. E.g. an employee will update one task, but the task is updated by multiple employees. The common multiplicities are 0..1, 1, 0..*,1..*