Class diagram technique is vital within Object Oriented functions. Class diagrams illustrate the kinds of objects in the system and the several static relationships among them. Class diagrams also define the methods and the attributes. Class diagrams have given possible relationships:
1. Association: A relationship between object instances of 2 classes.
2. Aggregation: An association in which one class refers to a collection. This is a part of a pure relationship where the phase may exist without the whole.
3. Composition: An association in which one class refers to a collection. This is a phase of a whole relationship where the part will not exist without the whole. If the whole is removed then the parts are deleted.
4. Generalization: An inheritance link indicating that one class is a parent class of the other. The Generalization expresses the "is a" relationship whereas the aggregation, association and composition describe the "has a" relationship.
5. Realization: defining of an interface.
6. Dependency: A dependency is a weak relationship where one class needs another class. The dependency refers the "uses" relationship.