CS 696: Advanced OO
Spring Semester, 1997
Doc 13, State and Strategy

Doc 13, State and Strategy Slide # 1

State

Intent

Applicability

An object's behavior depends on its state, and it must change its behavior at run-time depending on that state.

Operations have large, multipart conditional statements that depend on the object's state. Often, several operations will contain this same conditional structure.

Consequences

2. It makes state transitions explicit

3. State objects can be shared

Doc 13, State and Strategy Slide # 2

Implementation

2. A table-based alternative

3. Creating and destroying State object.

4. Using dynamic inheritance

Doc 13, State and Strategy Slide # 3

Example - SPOP

SPOP supports the following command:

• USER <username>
  
• PASS <password>
  
• LIST
  
• RETR <message number>
  
• QUIT

USER with a username must come first
PASS with a password or QUIT must come after USER

If the username and password are valid, then the user can use other commands

Returns: size of message in octets

If it contains an optional message number then returns the size of that message

Otherwise return size of all mail messages in the mail box

Doc 13, State and Strategy Slide # 4

Returns: the mail message indicated by the number

Updates mail box to reflect transactions taken during the transaction state, then logs user out

If session ends by any method except the QUIT command, the updates are not done

Doc 13, State and Strategy Slide # 5

class SPop
     {
     static final int QUIT = 1;
     static final int HAVE_USER_NAME = 2;
     static final int START = 3;
     static final int AUTHORIZED = 4;

     private int state = START;
     
     String userName;
     String password;
     
     public void user( String userName )
          {
          switch (state)
               {
               case START: 
                    {
                    this.userName = userName;
                    state = HAVE_USER_NAME;
                    break;
                    } 
               default:
                    {
                    // invalid command
                    sendErrorMessageOrWhatEver();
                    endLastSessionWithoutUpdate();
                    userName = null;
                    password = null;
                    state = START;
                    }
               }
          }

Doc 13, State and Strategy Slide # 6

public void pass( String password )
          {
          switch (state)
               {
               case HAVE_USER_NAME: 
                    {
                    this.password = password;
                    if ( validateUser() )
                         state = AUTHORIZED;
                    else
                         {
                         sendErrorMessageOrWhatEver();
                         userName = null;
                         password = null;
                         state = START;
                         }
                    }
                    
               default:
                    {
                    // invalid command
                    sendErrorMessageOrWhatEver();
                    endLastSessionWithoutUpdate();
                    state = START;
                    }
               }
          }
     etc.     
     }

Doc 13, State and Strategy Slide # 7

Outline of the State Pattern Implementation

class SPop
     {
     private SPopState state = new Start();
          
     public void user( String userName )
          {
          state = state.user( userName );
          }

     public void pass( String password )
          {
          state = state.pass( password );
          }

     public void list( int messageNumber )
          {
          state = state.list( massageNumber );
          }

     etc.

Doc 13, State and Strategy Slide # 8

abstract class SPopState
     {
     public SPopState user( String userName )
          {
          put default action here 
          }

     public SPopState pass( String password )
          {
          put default action here 
          }

     public SPopState list( int massageNumber )
          {
          put default action here 
          }

     public SPopState retr( int massageNumber )
          {
          put default action here 
          }

     public SPopState quit(  )
          {
          put default action here 
          }
     }

Doc 13, State and Strategy Slide # 9

class Start extends SPopState
     {
     public SPopState user( String userName )
          {
          return new HaveUserName( userName );
          }
     }
     
class HaveUserName extends SPopState
     {
     String userName;
     
     public HaveUserName( String userName )
          {
          this.userName = userName;
          }
          
     public SPopState pass( String password )
          {
          if ( validateUser( userName, password )
               return new Authorized( userName );
          else
               return new Start();
          }
     }

Doc 13, State and Strategy Slide # 10

Issue: Who defines the state transitions?

• If the criteria changing states is fixed then SPop (Context) could
  

class SPop
     {
     private SPopState state = new Start();
          
     public void user( String userName )
          {
          state.user( userName );
          state = new HaveUserName( userName );
          }

     public void pass( String password )
          {
          if ( state.pass( password ) )
               state = new Authorized( );
          else
               state = new Start();
          }

Doc 13, State and Strategy Slide # 11

• It is more flexible to let State subclasses specify the next state

class SPop
     {
     private SPopState state = new Start();
     
     protected changeState( SPopState nextState )
          {
          state = nextState;
          }
          
     public void user( String userName )
          {state.user( userName, this ); }
     etc
     }

class Start extends SPopState
     {
     public SPopState user( String userName, SPop mailServer )
          {
          mailServer.changeState( HaveUserName( userName ) );
          }
     }

Doc 13, State and Strategy Slide # 12

Issue: Sharing State Objects

A state object can have no instance variables if:

• the object has no need for instance variables or

• the object stores its instance variables elsewhere

Storing Instance Variables Elsewhere

class SPop
     {
     private SPopState state = new Start();
     
     String userName;
     String password;
          
     public void user( String newName )
          {
          this.userName = newName;
          state.user( newName );
          }

     public void pass( String password )
          {
          state.pass( userName , password );
          }
Doc 13, State and Strategy Slide # 13

class SPop
     {
     private SPopState state = new Start();
     
     String userName;
     String password;
          
     public String userName() { return userName; }

     public String password() { return password; }

     public void user( String newName )
          {
          this.userName = newName ;
          state.user( this );
          }

     etc.

class HaveUserName extends SPopState
     {
     public SPopState pass( SPop mailServer )
          {
          String useName = mailServer.userName();
          etc.
          }
     }

Doc 13, State and Strategy Slide # 14

Issue: Creating and Destroying State Objects

1. Create state object when needed, destroy it when it is no longer needed

2. Create states once, never destroy them, use as needed

Doc 13, State and Strategy Slide # 15

Strategy

Intent

Strategy lets the algorithm vary independently from clients that use it

Example: STL Container Adaptors

STL has three basic container classes:

Vector

O(1) access time per object

O(1) growth at the bottom

O(N) access time per object

O(1) growth anywhere

O(2) access time per object

O(1) growth at the bottom or top

Doc 13, State and Strategy Slide # 16

Stack, queue, and priority queue

One can implement a Stack with either a Vector, linked-list or a deque

Under different situations each can be better than the other two

class Stack
     {
     Container myStorage;
     
     public Stack( Container storage )
          {
          myStorage = storage;
          }
          
     public void push( Object item )
          {
          myStorage.append( item );
          }

     public Object push( Object item )
          {
          return myStorage.removeEnd(  );
          }
     }
     
Doc 13, State and Strategy Slide # 17
abstract class Container
     {
     public append( Object item );
     public Object removeEnd();
     etc.
     }

class Vector extends Container
     {
     public append( Object item );
          { add the code here }
     public Object removeEnd()
          { add the code here }
     
     }

Doc 13, State and Strategy Slide # 18

Applicability

• you need different variants of an algorithm

• an algorithm uses data that clients shouldn't know about

• a class defines many behaviors, and these appear as multiple switch statement in the classes operations

• many related classes differ only in their behavior
  

Consequences

Alternative to subclassing

Eliminates conditional statements

Gives a choice of implementations

Clients must be aware of different Strategies

Communication overhead between Strategy and Context

Increase number of objects

Doc 13, State and Strategy Slide # 19

Implementation

How does data flow between them

Strategies as template parameters

Making Strategy objects optional