Java Example Person.java shows how the association relationship shown in Figure VG.3 between Person and Vehicle can be implemented in Java. Each reference to a Vehicle object is kept in a Vector. Person.java The Person class simply holds the vehicles for a person. import java.util.*; /** * The Person class contains all data and logic for a person * in the system. */ public class Person { public Vector vehicles = new Vector(); /** Adds a vehicle to this person. */ public void addVehicle(Vehicle v) { this.vehicles.addElement(v); } } Guidelines Association is the default long-term relationship between objects. If you are in doubt as to which long-term relationship to use, use association. Aggregation Aggregation indicates a long-term relationship, with the additional restriction that some of the objects are part of another object. It is this whole-part nature of the rela- tionship that distinguishes aggregation from association. UML Example To continue the example, each Vehicle object may contain zero or one Engine objects. There is a clear whole-part relationship, as the engine is part of the car or truck. Figure VG.4 shows a modified association from Vehicle to Engine, with the hollow diamond at the Vehicle indicating aggregation. The hollow diamond is always drawn next to the enclosing whole. 50 Visual Glossary 267783 WS08.qxd 5/5/03 9:19 AM Page 50 Figure VG.4 Aggregation example. Java Example The following Java files show how the UML model in Figure VG.4 can be implemented in Java. Only the files that have changed from the previous running example are shown. Vehicle.java Vehicle no longer determines whether it is running or not. Instead, this behavior is delegated to an Engine object. /** * The Vehicle class contains the data and behavior that * is common to all Vehicles. */ public abstract class Vehicle { private Engine engine; /** Sets the engine */ public void setEngine(Engine e) { this.engine = e; } /** Answers the noise made when the Vehicle goes. Must be overridden by all concrete implementations of Vehicle. */ Vehicle + go() : String + startEngine() : void + stopEngine() : void + isEngineOn() : boolean Truck + go() : String + loadCargo() : void Engine + start() : void + stop() : void + isOn() : boolean <<Interface>> ICargoTransport + loadCargo() : void Car + go() : String Person + addVehicle(v : Vehicle) : void 1 * 0 * 0 1 1 Visual Glossary 51 267783 WS08.qxd 5/5/03 9:19 AM Page 51 public abstract String go(); /** Starts engine. */ public void startEngine() { if (this.engine != null) { this.engine.start(); } } /** Stops engine. */ public void stopEngine() { if (this.engine != null) { this.engine.stop(); } } /** Answers true if engine is started. */ public boolean isEngineOn() { if (this.engine != null) { return this.engine.isOn(); } return false; } } Engine.java Engine.java provides very simple behavior for starting, stopping, and checking the current value. /** * The Engine class contains the data and behavior for all engines * for use with Vehicles. */ public class Engine { private boolean on; /** Starts this engine. */ 52 Visual Glossary 267783 WS08.qxd 5/5/03 9:19 AM Page 52 public void start() { this.on = true; } /** Stops this engine. */ public void stop() { this.on = false; } /** Answers true if the engine is running. */ public boolean isOn() { return this.on; } } Guidelines Aggregation requires a clear whole-part relationship. Any uncertainty about the need for aggregation or ambiguity over which object is the whole and which is the part should lead you to use association instead. Composition Composition is an even stronger relationship, with one object essentially owning the other object or objects. The subordinate objects are created when the whole is created, and are destroyed when the whole is destroyed. Also, an object cannot play the role of a subordinate part in two composition relationships. UML Example Every engine contains many wheels, cogs, and gears that are integral and indivisible parts of the greater whole. Figure VG.5 shows that each Engine object contains zero or many Cog objects. The filled-in diamond next to the enclosing class indicates the com- position relationship. Java Example The following Java file shows how the UML model in Figure VG.5 can be implemented in Java. Only the files that have changed from the previous running example are shown. Visual Glossary 53 267783 WS08.qxd 5/5/03 9:19 AM Page 53 Figure VG.5 Composition example. Engine.java The Cog objects are created when the Engine is created, and become eligible for garbage collection along with their enclosing Engine. import java.util.*; /** * The Engine class contains the data and behavior for all engines * for use with Vehicles. */ public class Engine { private boolean on; private Vector cogs = new Vector(); public Engine() { this.cogs.addElement(new Cog()); this.cogs.addElement(new Cog()); } /** Starts this engine. */ public void start() { Vehicle + go() : String + startEngine() : void + stopEngine() : void + isEngineOn() : boolean Truck + go() : String + loadCargo() : void Engine + start() : void + stop() : void + isOn() : boolean <<Interface>> ICargoTransport + loadCargo() : void Car + go() : String Cog Person + addVehicle(v : Vehicle) : void 1 * 0 * 0 1 1 1 * 54 Visual Glossary 267783 WS08.qxd 5/5/03 9:19 AM Page 54 this.on = true; } /** Stops this engine. */ public void stop() { this.on = false; } /** Answers true if the engine is running. */ public boolean isOn() { return this.on; } } Guidelines As with aggregation, when in doubt, do not use composition. Dependency Objects often need to use another object. An object may receive a reference as a param- eter to a method, or it may create the object, use it, and lose it before the end of the cur- rent method. The key idea is that the dependent object acquires, uses, and forgets the object within a single method. UML Example Continuing the example, people use gas pumps to get gas, but most people do not keep track of every pump that they have used. The Person object receives a reference to a GasPump object as a parameter to the purchaseGas method. The reference is used within the method; it is not kept. The resulting dependency relationship can be seen as the dashed line from Person to GasPump in Figure VG.6. Java Example Person.java shows how the dependency relationship between Person and GasPump can be implemented in Java. Visual Glossary 55 267783 WS08.qxd 5/5/03 9:19 AM Page 55 Figure VG.6 Dependency example. Person.java Now, the Person class has a purchaseGas method that accepts a reference to a GasPump object as a parameter. import java.util.*; /** * The Person class contains all data and logic for a person * in the system. */ public class Person { public Vector vehicles = new Vector(); /** Adds a vehicle to this person. */ public void addVehicle(Vehicle v) { this.vehicles.addElement(v); } public void purchaseGas(GasPump pump) { // use pump // forget about pump } } Vehicle + go() : String + startEngine() : void + stopEngine() : void + isEngineOn() : boolean Truck + go() : String + loadCargo() : void Engine + start() : void + stop() : void + isOn() : boolean <<Interface>> ICargoTransport + loadCargo() : void Car + go() : String GasPump Cog Person + addVehicle(v : Vehicle) : void + purchaseGas(pump : GasPump) : void 1 * 0 * 0 1 1 1 * 56 Visual Glossary 267783 WS08.qxd 5/5/03 9:19 AM Page 56 Guidelines Dependency should be used whenever an object is used and forgotten within a single method. Visual Glossary 57 267783 WS08.qxd 5/5/03 9:19 AM Page 57 267783 WS08.qxd 5/5/03 9:19 AM Page 58 . VG .6. Java Example Person .java shows how the dependency relationship between Person and GasPump can be implemented in Java. Visual Glossary 55 267 783 WS08.qxd 5/5/03 9:19 AM Page 55 Figure VG .6. Visual Glossary 267 783 WS08.qxd 5/5/03 9:19 AM Page 50 Figure VG.4 Aggregation example. Java Example The following Java files show how the UML model in Figure VG.4 can be implemented in Java. Only. 1 1 1 * 56 Visual Glossary 267 783 WS08.qxd 5/5/03 9:19 AM Page 56 Guidelines Dependency should be used whenever an object is used and forgotten within a single method. Visual Glossary 57 267 783