1 Composition A whole-part relationship (e.g. Dog-Tail) Whole and part objects have same lifetime –Whole creates instance of part in its constructor In Java code, involves 3 changes to whole class: –Declaration of instance variable of part class/type –Instantiation of part class in whole class constructor –Assignment of new part instance to instance variable

2 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }

3 Important points about composition Whole has responsibility for creating its parts (which is why instantiation of parts happens in constructor of whole). Whole can communicate with parts. This is why an instance variable is declared: to establish a name for the newly created object.

4 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Class definition is shown in green:

5 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Instance variable name is shown in green:

6 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Instance variable declaration is shown in green:

7 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Access control modifiers are shown in green: Note that access control modifier of _tail is private, not public.

8 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Constructor definition is shown in green:

9 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Header of constructor definition is shown in green:

10 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Access control modifier in header of constructor definition is shown in green:

11 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Name of constructor in header of constructor definition is shown in green:

12 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Parameter list in header of constructor definition is shown in green:

13 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Instantiation of class Tail is shown in green:

14 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } ‘new’ operator in instantiation of class Tail is shown in green:

15 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Use of constructor in instantiation of Tail class is shown in green:

16 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Argument list in instantiation of class Tail is shown in green:

17 Dog – Tail example in Java public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); } Assignment of new Tail instance to instance variable is shown in green: