1. Composition - 1 J. Sant.

2. Agenda Define Composition Reuse and Composition. Composition in Java. Composition Exercise with simple containment.

3. Composition A programmer can specify that a new class is composed of instances of previously defined classes. A Car class may be composed of instances of an Engine class, a Chassis class and a DriveTrain class.

4. Object-Oriented Code Re-use Composition is used to represent “has a” relationship. A car HAS AN engine. A gas station HAS pumps. Inheritance is used to represent “is a” relationships. A savings account IS A type of account. A barrel type IS A type of container.

5. Composition – Why? Composition is probably the easiest method of improving code reuse. All that is necessary is to define instance variables that are objects then construct them in the constructor. Composition is generally preferred to inheritance as a code reuse method (see Bloch).

6. Composition Example class Course { private int id; // attribute that is itself an object. private TextBook textbook; }

7. public class Gun { public static final int DEFAULT_CLIP_SIZE =7; public static final int DEFAULT_RANGE =700; private int clipSize = DEFAULT_CLIP_SIZE; private int range = DEFAULT_RANGE; private int currentRounds = DEFAULT_CLIP_SIZE; private String name; public Gun(String name, int clipSize, int range ){ this.name = name; this.clipSize = clipSize; this.range = range; this.currentRounds = clipSize; } public void setName(String name){ this.name = name; } public String getName(){ return this.name; } public int getClipSize(){ return this.clipSize; } public int getRange(){ return range; } public int getCurrentRounds(){ return currentRounds; } public void fire( int noRounds ){ if( noRounds <= currentRounds ){ currentRounds -= noRounds; } else { currentRounds = 0; } public void reLoad(){ currentRounds = clipSize; } Gun.java

8. public class Player { private Gun armament; private String name; public Player( String name, Gun gun ){ this.name = name; armament = gun; } public String getName(){ return this.name; } public void switchGun( Gun newGun ){ armament = newGun; } public void fire( int noRounds ){ armament.fire(noRounds); } public int roundsLeft(){ return armament.getCurrentRounds(); } Player.java

9. Player-Gun Discussion. Why is Player a composite class. Player uses a strategy called delegation where is completes a task by delegating it to one of the contained objects. Where does this occur.

10. Your turn. Define a simple class, Leg. This defines a leg in a trip with a single stopover. Leg should store only the name of the origin city, the destination city and an integer value for the length of the leg in kilometers. Define a composite class TripWithStopover. This stores an initial leg and a final leg. It should be able to return the total distance of the trip and also return the starting city and the destination city.

11. Summary. Composition allows a programmer to save work by using pre-defined classes to compose new classes. In Java, you implement composition by defining new classes using pre-defined classes. Delegation is when a containing (composite) class completes a requested operation by passing on the request to a contained object. ance.