CSE 116 Introduction to Computer Science For Majors II 1

Syntax & terminology review

• While the following slides are not exactly what we did on the board (object diagrams are not shown here) they cover most of what we did, and also some details we did not.

CSE 116 Introduction to Computer Science For Majors II 2

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

CSE 116 Introduction to Computer Science For Majors II 3

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

CSE 116 Introduction to Computer Science For Majors II 4

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.

CSE 116 Introduction to Computer Science For Majors II 5

And now

the gory details

and

vocabulary review

CSE 116 Introduction to Computer Science For Majors II 6

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Class definition is shown in green:

CSE 116 Introduction to Computer Science For Majors II 7

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Instance variable name is shown in green:

CSE 116 Introduction to Computer Science For Majors II 8

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Instance variable declaration is shown in green:

CSE 116 Introduction to Computer Science For Majors II 9

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.

CSE 116 Introduction to Computer Science For Majors II 10

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Constructor definition is shown in green:

CSE 116 Introduction to Computer Science For Majors II 11

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Header of constructor definition is shown in green:

CSE 116 Introduction to Computer Science For Majors II 12

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:

CSE 116 Introduction to Computer Science For Majors II 13

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:

CSE 116 Introduction to Computer Science For Majors II 14

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:

CSE 116 Introduction to Computer Science For Majors II 15

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Body of constructor definition is shown in green:

CSE 116 Introduction to Computer Science For Majors II 16

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }

public void bark() {…}}

Suppose we define a public method “bark”.

CSE 116 Introduction to Computer Science For Majors II 17

member access operator

• Fields (instance variables) and methods are collectively known as “members”.

• In: Dog x = new Dog();x.bark();

“.” is the member access operator.

CSE 116 Introduction to Computer Science For Majors II 18

Dog – Tail example in Java

public class Dog { private Tail _tail; public Dog() { _tail = new Tail(); }}

Instantiation of class Tail is shown in green:

CSE 116 Introduction to Computer Science For Majors II 19

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:

CSE 116 Introduction to Computer Science For Majors II 20

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:

CSE 116 Introduction to Computer Science For Majors II 21

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:

CSE 116 Introduction to Computer Science For Majors II 22

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:

CSE 116 Introduction to Computer Science For Majors II 23

Class instantiation

• process by which objects are created

• example

new JButton()

CSE 116 Introduction to Computer Science For Majors II 24

Class instantiation

• new + constructor

new JButton()

• new: operator• JButton(): constructor call

CSE 116 Introduction to Computer Science For Majors II 25

Class instantiation

109500

109501

109502

109503

109504

109505

109506

109507

109508

109509

• new + constructor

new JButton()

• new: operator• JButton(): constructor call

CSE 116 Introduction to Computer Science For Majors II 26

Class instantiation

109500

109501

109502

109503

109504

109505

109506

109507

109508

109509

• new JButton() is an expression whose value (in this particular example) is 109500, the starting address of the block of memory storing the representation of the JButton object just created.