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Reading a Simple Class Diagram

publish date: 2026/06/08 08:10:21.638018 UTC

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The diagram below shows a simple UML class diagram with two classes and an association.

What does the "1" annotation at each end of the association line indicate?

The Patient class has only one attribute

There is only one type of patient in the system

The association is optional — a patient may or may not have a record

Each Patient object is associated with exactly one Patient Record object, and each Patient Record is associated with exactly one Patient

Correct Answer

Each Patient object is associated with exactly one Patient Record object, and each Patient Record is associated with exactly one Patient

Explanation

The "1" at each end of an association line in a UML class diagram represents a 1:1 multiplicity. It means each instance of Patient is associated with exactly one Patient Record, and each Patient Record maintains information about exactly one Patient. Multiplicity annotations show how many objects of each class are involved in an association.

Reference

Software Engineering, Ian Sommerville, 9th edition

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Structural Models