Structural diagrams illustrate components of a system. The six diagrams listed here are structural, and are briefly described in the subsections that follow:
The class diagram is the most commonly used UML diagram, as it provides a visual description of a system's objects. Consider that, in Java, everything is an object, so you can see the relevance and reason as to why this particular diagram is so widely used. Class diagrams do more than just display objects—they visually depict their construction and relationships with other classes.
As you can see here, the basic component of the class diagram is a rectangle, divided into three sections. Each overall rectangle represents a class, and the class name appears using a bold typeface in the top section. The middle section contains attributes that correlate to variable fields. The third section contains operation data which, in Java, means functions and methods:
A simple example of a class diagram for a Kennel is displayed in the following diagram. The class name is Kennel, and there are three attributes (animal, breed, and name) and two operations (intake and discharge):
We will further explore class diagrams using our Kennel class example later in this chapter.
Component diagrams provide a visual representation of a system's physical components. The following example illustrates the three physical components of the system:
The relationships between these components are annotated with dotted lines in the following component diagram:
The Composite structure UML diagram shows the runtime structure of a system. This diagram can be used to show the internal components of a class. The following example shows a microwave with four structures:
We can also represent a composite structure with a rectangular box, as illustrated in the following diagram:
Deployment diagrams provide a visual representation of a system's hardware and software. Physical hardware components are illustrated, along with the particular software components that are on them. The hardware components are represented as nodes, and software is represented as an execution environment.
The nodes are drawn as three-dimensional rectangles and represent hardware for a software object, such as a database. As illustrated in the following example, associations are annotated with lines marked with the type of communication protocol used. Our example shows TCP/IP as the communication type:
Deployment diagrams can be more involved than the example provided here. Our example does, however, provide sufficient insight for you to progress through the proceeding chapters.
Object diagrams have an unfortunate name, as it does not aptly describe this UML diagram's purpose. The object diagram visually communicates a set of class instances. In fact, the instances have mostly optional components and are often only partially depicted. Therefore, a more apt name for this diagram might be a loosely defined instance UML diagram.
The following example depicts four objects and their hierarchy. Because each object is an instance of a class, it is underlined and followed by a colon and the class name:
Package diagrams are used to provide a high-level visual depiction of large systems. These diagrams are simplistic and simply show how a system's components are grouped. The following example illustrates nested packages, starting with Java and drilling down to the ArrayList:
