CS596: Doc 6, Conceptual OO Definition

	CS 596 Java Programming Fall Semester, 1998 Conceptual OO Definition
	To Lecture Notes Index © 1998, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 22-Oct-98

Contents of Doc 6, Conceptual OO Definition

Reference

Conceptual Level Definition of OO

Some Typical Beginer Mistakes

Information Hiding - Physical and Logical

Example - Linked List of Floats

First Java Linked List

Second Node Class

Reference

Object-Oriented Design Heuristics , Riel, Addison-Wesley, 1996

Heuristics 2.1, 2.8, 2.9, 3.3

S-sept16 3mins Doc 6, Conceptual OO Definition Slide # 2

Conceptual Level Definition of OO

Abstraction

“Extracting the essential details about an item or group of items, while ignoring the unessential details.” Edward Berard

What is it?
Can you describe it with a simple sentence?
It contains data and operations (behavior) on the data

Encapsulation

Enclosing all parts of an abstraction within a container

Information Hiding

Hiding parts of the abstraction

Hierarchy

Abstractions arranged in order of rank or level

S-sept16 8mins Doc 6, Conceptual OO Definition Slide # 3

Stack Example
Definition

"A stack is a linear list for which all insertions and deletions are made at one end of the list"

Operations

pop, push, isEmpty, isFull

S-sept16 2mins, Q-oct22 3mins Doc 6, Conceptual OO Definition Slide # 4

Stack Class

The following Stack class is not a production level class.

class Stack  {
   private float[] elements;
   private int topOfStack = -1;
   
   public Stack( int stackSize )  {
      elements = new float[ stackSize ];
   }
   
   public void push( float item )  {
      elements[ ++topOfStack ] = item;
   }
   
   public float pop()  {
      return elements[ topOfStack-- ];
   }
   
   public boolean isEmpty()  {
      if ( topOfStack < 0 )    return true;
      else               return false;
   }
   
   public boolean isFull()  {
      if ( topOfStack >= elements.length )    return true;
      else                           return false;
   }
}

Using the Stack

   Stack me = new Stack( 20 );
   me.push( 5 );
   me.push( 12 );
   System.out.println( me.pop() );
   System.out.println( me.pop() );
S-sept16 3mins, Q-oct22 1min Doc 6, Conceptual OO Definition Slide # 5

A Struct Version

struct Stack 
{
   float stack[100];      
   int topOfStack;
};
void push(Stack& it, int item) 
{  
   it.stack[(it.topOfStack)++] = item;   
}
float pop(Stack& it)
{  
   return it.stack[--(it.topOfStack)];   
}
main()
{
   Stack tryThisOut; 
   Stack yours, mine;
   tryThisOut.topOfStack = 0;
   yours.topOfStack = 0;
   push( tryThisOut, 5.0 );
   push( yours, 3.3 );
   push( tryThisOut, 9.9 );
   cout << pop( tryThisOut ) << endl;
}

Doc 6, Conceptual OO Definition Slide # 6

Problems with the Struct Version
Encapsulation

Data is encapsulated
Operations are not encapsulated with data
The boundaries of what it is become fuzzy

Information hiding

Data is not hidden
Data is not safe!

   Stack troubleAhead;
   troubleAhead.topOfStack = 13;
   troubleAhead.stack[ 8 ] = 29;

S-sept16 36secs Doc 6, Conceptual OO Definition Slide # 7

Some Typical Beginner Mistakes

Struct-like Class
One can make a class that is just a struct dressed as a class. Avoid this doing this.

class StackData
{
   public  float[]  elements = new float[100];
   public  int   topOfStack      = 0;
}
class Test
{
   static void push(Stack it, int item) 
   {  
      it.stack[ ( it.topOfStack )++ ] = item;   
   }
   static float pop(Stack it)
   {  
      return it.stack[--(it.topOfStack)];   
   }
   public void static main( String[] args)
   {
      Stack yours, mine;
      push( yours, 3.3 );
      push(  mine, 9.9);
   }

S-sept16 3mins, Q-oct22 2mins Doc 6, Conceptual OO Definition Slide # 8

Some Typical Beginner Mistakes
The Long Way Struct-like Class
This example has all fields hidden. But the methods do not support the stack abstraction. The logic of operating the stack is not in the stack class. This means that the topOfStack field can be improperly set by some one outside the class.

class StackData
{
   private  float[]  elements = new float[100];
   private  int        topOfStack      = -1;
   public int getTopOfStack()
   {
      return topOfStack;
   }
   public void setTopOfStack( int newTop )
   {
      topOfStack = newTop;
   }
   
   public float getElement( int elementIndex )
   {
      return elements[ elementIndex ];
   }
   public void setElement( int elementIndex, float element )
   {
      elements[ elementIndex ] = element;
   }
}
S-sept16 1min Doc 6, Conceptual OO Definition Slide # 9

Some Typical Beginner Mistakes
The God Class
If we have a lot of struct-like classes, then the logic of manipulating the data needs to be placed somewhere. One bad solution is to combine the logic of manipulating the data of all(most) struct-like classes in on big class. This "god" class then knows everything about your program, hence the name. This make the program hard to maintain because to understand the "god" class one has to understand the entire program.

S-sept16 to end Doc 6, Conceptual OO Definition Slide # 10

Some Typical Beginner Mistakes
Mixing IO and an Abstraction
Adding IO to the push and pop methods make the stack class unusable. This often happens in student assignments.

class Stack {
   private float[] elements = new float[ 100 ];
   private int topOfStack = -1;
   
   public void push( )  
   {
      float item = Console.readFloat( "Type a float to push");
      elements[ ++topOfStack ] = item;
   }
   
   public void  pop()  
   {
      Console.println( "Top stack item: " + 
         elements[topOfStack--];
   }
}

class Test
{
   public void static main( String[] args)
   {
      Stack yours ohNo = new Stack();
      ohNo.push(  );
      ohNo.push(  );
      ohNo.pop(  );
   }

S-sept21 5mins, Q-oct22 56secs Doc 6, Conceptual OO Definition Slide # 11

Design Heuristics

All data should be hidden within its class
A class should capture one and only one key abstraction
Keep related data and behavior in one place
Beware of classes that have many accessor methods in their public interface. This implies data and behavior are not in one place

Applying these heuristics to your classes will go a long way to help you avoid typical beginner mistakes in designing classes

S-sept21 1min Doc 6, Conceptual OO Definition Slide # 12

Information Hiding - Physical and Logical

Physical information hiding
Physical information hiding is when a class has a field and there are accessor methods, getX and setX, setting and getting the value of the field. It is clear to everyone that there is a field named X in the class. The goal is just to prevent any direct access to X from the outside. The extreme example is a struct converted to a class by adding accessor methods. Physical information hiding provides little or no help in isolating the effects of changes. If the hidden field changes type than one usually ends up changing the accessor methods to reflect the change in type.

class  PhysicalHiding
{
   private int a;
   private int b;
   
   public int getA()
   {
      return a;
   }   
   public int getB()
   {
      return b;
   }   
   public int setA( int newA )
   {
      a = newA;
   }   
   public int setB( int newB )
   {
      b = newB;
   }   
   
}
S-sept21 3mins Doc 6, Conceptual OO Definition Slide # 13

Logical Information Hiding
Logical information hiding occurs when the class represents some abstraction. This abstraction can be manipulated independent of its underlying representation. Details are being hidden from the out side world. Examples are integers and stacks. We use integers all the time without knowing any detail on their implementation. Similarly we can use the operations pop and push without knowing how the stack is implemented. Given the following interface to a Point class is there any way to determine how it is implemented? Does it use polar coordinates internally or not?

class Point
public int getX()
public int getY()

public int setXY( int newX, int newY )

public int getRadiusVector()
public int getPolarAngle()

public int setRadiusVector( float newRadius)
public int setPolarAngle( float newAngle)

Doc 6, Conceptual OO Definition Slide # 14

Example - Linked List of Floats

What operations do we want on a linked-list?

addFirst( float data )
addLast( float data )
contains( float data )
get( int index)
getFirst()
getLast()
indexOf( float data)
insert( int index, float data)
contains( float data )
removeFirst()
removeLast()
remove( int index)
remove( float data)
removeAll( float data )
size()

How to implement? In C we would use a struct

struct Node
{
   float data;      
   struct Node*  next;
};

then use it to implement the above functions.

S-sept21 3mins Doc 6, Conceptual OO Definition Slide # 15

First Java Linked List

package MyLinkedList;
public class LinkedList {
   
   private Node frontOfList = null;
   
   public void addFirst( float data )
      {
      Node newNode = new Node();
      newNode.data = data;
      newNode.next = frontOfList;
      frontOfList = newNode;
      }
      
   public String toString() {
      String listAsString = "";
      Node current = frontOfList;
      while (current != null ) {
         listAsString = listAsString + " " + current.data;
         current = current.next;
      }
      return listAsString;
   }
   
   // Other methods deleted
}
class Node {
   float data;
   Node next = null;
}

S-sept21 3mins Doc 6, Conceptual OO Definition Slide # 16

Using the First Linked List
import MyLinkedList;

public class Test
   {
   public static void main( String args[] ) throws Exception 
      {
      //new ClassMethodView( Test.class );
      LinkedList test = new LinkedList();
      test.addFirst( 1);
      test.addFirst( 2);
      test.addFirst( 3);
      System.out.println( test );
      }
   }

Output

3.0 2.0 1.0

What is Wrong with the Node Class?
Clearly the Node class does what an earlier slide says not to do. So what? Most textbooks will have Node code that looks like the first example. The Java library source code has many instances of linked lists that use this type of Node class. So what is wrong with Node class? Put another way how can it be improved?.

Think about this before going on to the next slide.

S-sept21 2mins Doc 6, Conceptual OO Definition Slide # 17

Second Node Class

class Node {
   private float data;
   private Node next = null;
   private Node previous = null;
   public Node( float initialData) {
      this( initialData, null, null );
   }
   public Node( float initialData, 
                  Node previousInList, 
                  Node nextInList )   {
      data = initialData;
      next = nextInList;
      previous = previousInList;
      if ( next != null )
         next.previous = this;
      if ( previous != null )
         previous.next = this;
   }
   
   public void append( float data ) { 
      new Node( data, this, next); 
   }
   public void prepend( float data ) { 
      new Node( data, previous, this); 
   }
      
S-sept21 4mins Doc 6, Conceptual OO Definition Slide # 18

// Node Continued

   public void remove() {
      if ( previous != null )
         previous.next = next;
      if ( next != null )
         next.previous = previous;
   }
      
   public Node getNext() { 
      return next; 
   }
   public Node getPrevious() { 
      return previous; 
   }
   public float getData() {
      return data;
   }
   
   public void setData( float newData ) {
      data = newData;
   }
}
S-sept21 6mins Doc 6, Conceptual OO Definition Slide # 19

Second Linked List

public class LinkedList {   
   private Node frontOfList = null;
   private Node endOfList = null;
   
   public void addFirst( float data ) {
      frontOfList = new Node( data, null, frontOfList );
      if (endOfList == null )
         endOfList = frontOfList;
   }
   public void addLast( float data ) {
      endOfList = new Node( data, endOfList, null );
      if (frontOfList == null )
         frontOfList = endOfList;
   }
      
   public float removeFirst( ) {
      Node oldFirst = frontOfList;
      frontOfList = frontOfList.getNext();
      oldFirst.remove();
      return oldFirst.getData();
   }
   public String toString() {
      String listAsString = "";
      Node current = frontOfList;
      while (current != null ) {
         listAsString = listAsString + " " + current.getData();
         current = current.getNext();
      }
      return listAsString;
   }
}
Doc 6, Conceptual OO Definition Slide # 20

Is the Second List Better?
What is the difference between the first and second Node classes?

Is the second Node better? If so how?

S-sept21 5mins Doc 6, Conceptual OO Definition Slide # 21

Computation as Simulation Procedure programming consists of procedures acting on data

Object-oriented programming consists of objects interacting

Main() creates web of objects and starts them interacting

Doc 6, Conceptual OO Definition Slide # 22

The OO Process
Step One

Identify the data and the operations on the data
These start to form the classes

Step Two

Determine how classes interact

Step N

Create program on top of the classes

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