Same Ideas, Different Terms
 

• classes, objects,factory object
• instances, objects
• message passing, method lookup,memberfunction invocation,method binding
• methods, member function, method function
• inheritance, sub classing

Encapsulation and Instantiation
 

Encapsulation: purposeful hiding of information, thereby reducing the details to be remembered/communicated
 
Instantiation: create multiple instances of an abstraction
 

Internal and external views

 

 Push                       |            const limit = 300
                                |
 Pop                         |            var current Top : 0 ..Limit
                                |
 Top                         |             var : array [ 1 .. limit ] of integer
 

                      State - Instance Variables
 
Instance -  a representative    or example of a class
Instance Variable - the state, or data values, maintained internal to a class

Record information about instance variables on the back of the CRC card
           - clients can see front of CRC card
           - implementors can see on the back of the CRC card

Example - Playing Card

|--------------------------------------------------------------|
|      Card                                           ;       |
|     Maintain and return suit and rank                         |
|    Return color of face                                         &n bsp;      |
|    Maintain face-up or face-down status              |
|    Draw card on playing surface                                |
|     Erase card on playing surface                                |
|-------------------------------------------------------------|

                         |-------------------------------------------------------------|

| Card -implementation information         |
|                                       &n bsp;                     |
|        int suit value -0- heart, 1- club                         |
|             2- diamond , 3- spade                                     |
|          int rank value -1 through 12                             |
|          boolean faceup                                        & nbsp;         |
|                                        ;                                          &n bsp;   |
|-------------------------------------------------------------|
 
the back can record about the internal state of the instance.
 
 
 Revision of Card

|----------------------------------------------------------------|
|  Card                                     &nb sp;                |
|    suit( ) - return suit value                                       &nbs p;   |
|    rank( ) - return rank value                                        |
|     color( ) - return color value                                       |
|     erase(x,y ) - erase card image                                    |
|    draw( x,y) - draw card image                                       |
|    faceUp( ) , flip ( ) - test or flip card                            |
|--------------------------------------------------------------- |

we refine the behavior by providing explicit function names and arguments types.

                        Interface and implementation

Distinction between interface and implementation is manifest in different ways:
 

•  within class ,separation between interface and implementation
        Example :  public and private fields in c++ and java
 
•   Interface descriptions and implementation bodies appearing in separate places
        Example : .h files and *.cpp files in c++
 
Varieties of classes
 
Data manager : principal responsibility is to maintain data values
       example: playing card - maintain suite value information

Data sink or source : interface to data generator or consumer, no data itself
       example : input/output stream objects - files, i/o devices

View or observer class: display objects graphically, not the object itself

•      Viewing tasks are different from data manipulation tasks
•      an object might have different views
Facilitator or helper class: maintain little/no state, but assist with complex tasks
       Example: painting assistant ( Pen , Canvas, Edit box ) for graphics output device
 
        Classes and Methods in java
Java is similar to c++ , but
 
• no preprocessor, global variables, enumerated data types
• methods always defined directly in  the class definitions
• Keywords public private applied individually to features
• Boolean data type is boolean , not bool
• Object references instead of pointers
• All methods must specify a return type
 
Playing Card in java
 
class Card {
          // static values for colors and suits
final public int red =0;
        final public int black =1;
        final public int spade =0;
        final public int heart =1;
        final public int diamond =2;
        final public int club =3;

            // constructor
Card (int sv, int rv) { s= sv; r= rv; faceup = false; }

        // mutator
public void flip ( ) { faceup = ! faceup }

      // accesors
public int rank ( ) { return r; }

public int suit ( ) { return s; }

public int color ( ) {

 if (suit( ) = = heart || suit = = diamond )
         return red;
         return black; }

public boolean faceup ( ) { return faceup; }

 // perform other actions

public void draw( Graphics g, int x,int y ) {

. . ./* omitted */. . .

// data fields

private boolean faceup;
private int r;
private int s;

};