B.2. Instance Diagrams

A UML instance diagram, also known as a UML object diagram, shows a snapshot of the state of one or more objects. These diagrams are used extensively in Part I of this book, and are crucial to understanding the linked structures introduced in Chapter 6.

More than one instance of a class may be shown in an instance diagram (Figure B-8). Static fields and all methods are omitted, as they are the same for every instance. The name of the class of each instance is underlined, but not bold. A specific value is given for each primitive field.

Figure B-8. UML instance diagram showing two instances of the Starship class.

References are shown as arrows. In Figure B-9, the instance of class Outfit has four fields: price, top, bottom, and feet. Technically speaking, fields of type String are references to instances of the String class, but we omit this detail for clarity.

Figure B-9. An instance of class Outfit with references to three other objects.

A number of minor extensions to standard UML instance diagrams are used in this book.

First, a null reference is sometimes shown as a line ending in a large dot (Figure B-10). If including such lines would complicate the diagram excessively, null references are simply omitted.

Figure B-10. In this instance of class Outfit, the feet field contains a null reference.

Second, arrays are shown as rows of boxes (Figure B-11). Each box may contain a primitive value or a reference to an object or array.

Figure B-11. This instance of Menu contains two arrays, prices and items.

Third, variables, arguments, and constants are sometimes included in UML instance diagrams (Figure B-12).

Figure B-12. The variable target has the value 17. The constant ORIGIN is a reference to an instance of Point.

We end this appendix by noting that some special notation is used in Chapter 16, which deals with references in great detail.