In the graphical representation of class diagrams, classes are represented as subdivided rectangles. The classes are arranged in groups and contain the class name, attributes and methods or operations. The connection of the class boxes takes place with lines, which have arrows at one of the two ends. They are used to establish the relationships between the classes. A simple relationship consists in the association, another one consists in the fact that one class is included in another. This relationship is called aggregation. And the third type of relationship is specialization.