Inheritance in Java

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Inheritance is represented using a solid line and a hollow triangular arrow For the simple Employee class, which is subclass of Person, we write the following code:
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public class Employee extends Person { public Employee (String name) { super (name); } public String getJob() { return "Research Staff"; } }
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This is represented in UML as shown in Figure 23 Figure 23 UML diagram showing Employee derived from Person
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Note that the Employee class name is not in italics This is because the class is now a concrete class because it includes a concrete method for the formerly abstract getJob method While it has been conventional to show inheritance with the arrow pointing up to the superclass, UML does not require this, and sometimes a different layout is clearer or uses space more efficiently
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Interfaces
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An interface looks much like inheritance, except that the arrow has a dotted line tail, as shown in Figure 24 Figure 24 ExitCommand implements the Command interface
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Note that the name <<interface>> is shown enclosed within double angle brackets (or guillemets)
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Composition
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Much of the time, a useful representation of a class hierarchy must include how objects are contained in other objects For example, a Company might include one Employee and one Person (perhaps a contractor)
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public class Company { Employee emp1; Person per1; public Company() { } }
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We represent this in UML as shown in Figure 25 Figure 25 Company contains instances of Person and Employee
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The lines between classes show that there can be 0 to 1 instances of Person in Company and 0 to 1 instances of Employee in Company If there can be many instances of a class inside another, such as the array of Employees shown here,
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public class Company1 { Employee[] emp1; public Company1() { } }
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we represent that object composition as a single line with either an * or 0, * on it, as shown in Figure 26 Figure 26 Company 1 contains any number of instances of Employee
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21 Team-Fly
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Some writers use a hollow and a solid diamond arrowhead to indicate containment of aggregates and a circle arrowhead for single object composition, but this is not required
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Annotation
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You will also find it convenient to annotate your UML or insert comments to explain which class is calling a method in which other class You can place a comment anywhere you want in a UML diagram, either enclosed in a box with a turned-down corner or just entered as text Text comments are usually shown along an arrow line, which indicates the nature of the method that is called, as shown in Figure 27 Figure 27 A comment is often shown in a box with a turned-down corner
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UML is a powerful way of representing object relationships in programs, and the full specification contains more diagram features The previous discussion covers the markup methods used in this text
JVISION UML Diagrams
All of the UML diagrams in this book were drawn using the JVISION program from Object Insight This program reads in the actual compiled classes and then generates the UML class diagrams Many of these class diagrams have been edited to show only the most important methods and relationships However, the complete JVISION diagram files for each design pattern are stored in that pattern's directory on the accompanying CD-ROM Thus you can run your copy of JVISION and read in and investigate the detailed UML diagram starting with the same drawings you see here in the book
Visual SlickEdit Project Files
All of the programs in this book were written using Visual SlickEdit 40 using the project file feature Each subdirectory on the CD-ROM contains the project file for that project so that you can load the project and compile it as we did