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11.6: UML class diagrams

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  • One challenge of working with the code in this chapter is that we have several classes that depend on each other. Here are some of the relationships between the classes:

    • MyLinearMap contains a LinkedList and implements Map.
    • MyBetterMap contains many MyLinearMap objects and implements Map.
    • MyHashMap extends MyBetterMap, so it also contains MyLinearMap objects, and it implements Map.
    • MyFixedHashMap extends MyHashMap and implements Map.

    To help keep track of relationships like these, software engineers often use UML class diagrams. UML stands for Unified Modeling Language (see A “class diagram” is one of several graphical standards defined by UML.

    In a class diagram, each class is represented by a box, and relationships be- tween classes are represented by arrows. Figure \(\PageIndex{1}\) shows a UML class diagram for the classes from the previous exercise, generated using the online tool yUML at

    UML diagram for the classes in this chapter.

    Figure \(\PageIndex{1}\): UML diagram for the classes in this chapter.

    Different relationships are represented by different arrows:

    • Arrows with a solid head indicate HAS-A relationships. For example, each instance of MyBetterMap contains multiple instances of MyLinearMap, so they are connected by a solid arrow.
    • Arrows with a hollow head and a solid line indicate IS-A relationships. For example, MyHashMap extends MyBetterMap, so they are connected by an IS-A arrow.
    • Arrows with a hollow head and a dashed line indicate that a class implements an interface; in this diagram, every class implements Map.

    UML class diagrams provide a concise way to represent a lot of information about a collection of classes. They are used during design phases to communicate about alternative designs, during implementation phases to maintain a shared mental map of the project, and during deployment to document the design.