CS 635: Advanced Object-Oriented Design & Programming

References

CGI Environment Variables at: http://hoohoo.ncsa.uiuc.edu/cgi/env.html

Doc 21, Adapter Slide # 2

Servlets have greater functionality and are faster, but require special Web servers or servers with special extensions

To help write Java CGI programs there is class sdsu.net.CGI

Adapters would be useful in moving code between servers to avoid having to rewrite the code

There are various issues in writing such adapters
One issue is access to the CGI environment variables

There are about 20 common CGI environment variables

In servlets one has a HttpRequest class that has a getX() method for each CGI environment variable

sdsu.net.CGI class returns a hash table with one entry per CGI environment variable

Doc 21, Adapter Slide # 3

Do we have to rewrite all the CGI programs?

We have code that uses a class with get( "X" ) method

We can write a wrapper around HttpRequest to make it act like a hash table

Doc 21, Adapter Slide # 4

Doc 21, Adapter Slide # 5

sdsu.net.CGI cgiGateWay = sdsu.net.CGI.getInstance();

Hashtable cgiVariables =
     cgiGateWay.getCGIEnvironmentVariables();

To avoid more CGI details the complete code is not shown

Doc 21, Adapter Slide # 6

class HTTPAdapter extends HttpRequest
     {
     Hashtable CGIvariables;
     
     public HTTPAdapter( Hashtable CGIEnvironment )
          {
          CGIvariables = CGIEnvironment;
          }
     
     public String getAuthType()
          {
          return (String) CGIvariables.get( "AUTH_TYPE" );
          }
          
     public String getRemoteUser()
          {
          return (String) CGIvariables.get( "REMOTE_USER" );
          }
          
          etc. 
     }

Doc 21, Adapter Slide # 7

Adapter

• you want to use an existing class and its interface does not match the one you need

• you want to create a reusable class that cooperates with unrelated or unforeseen classes, that is classes that don't necessarily have compatible interfaces
  
  
• you need to use several existing subclasses, but it's impractical to adapt their interface by subclassing everyone. An object adapter can adapt the interface of its parent class
  

Class Adapter

Object Adapter

Doc 21, Adapter Slide # 8

Object Adapter

Doc 21, Adapter Slide # 9

Class Adapter Example

class OldSquarePeg {
     public:
          void squarePegOperation()
               { do something }
}

class RoundPeg {
     public:
          void virtual roundPegOperation = 0;
}

class PegAdapter: private OldSquarePeg, 
                    public RoundPeg  {
     public:
          void virtual roundPegOperation()  {
               add some corners;
               squarePegOperation();
          }
}
     
void clientMethod() {
     RoundPeg* aPeg = new PegAdapter();
     aPeg->roundPegOperation();
}

Doc 21, Adapter Slide # 10

class OldSquarePeg     {
     public:
          void squarePegOperation() { do something }
     }

class RoundPeg     {
     public:
          void virtual roundPegOperation = 0;
     }

class PegAdapter:  public RoundPeg     {
     private:
          OldSquarePeg* square;
          
     public:
          PegAdapter()  { square = new OldSquarePeg; }
               
          void virtual roundPegOperation()     {
               add some corners;
               square->squarePegOperation();
               }
     }

Doc 21, Adapter Slide # 11

• only adapts a class and all its parents, not all its subclasses

• lets Adapter override some of Adaptee's behavior

• does not introduce an additional pointer indirection

• lets a single Adapter work with many Adaptees

• makes it harder to override Adaptee behavior as the Adapter may not know with Adaptee it is working with

• How much adapting does the Adapter do?

• Pluggable adapters

• Using two-way adapters

Doc 21, Adapter Slide # 12

The Adapter may just map one operation to another

class PegAdapter:  public RoundPeg     {
     private:
          OldSquarePeg* square;
          
     public:
          PegAdapter()     { square = new OldSquarePeg;}
               
          void roundPegOperation()     {
               square->squarePegOperation();
          }
}

Doc 21, Adapter Slide # 13

This class would be given a list of keys to getX methods and an Adaptee object

HttpRequest CGIEnvironment = getHttpRequest();
PluggableHashAdapter sample = 
     new PluggableHashAdapter( CGIEnvironment );

sample.adapt( "AUTH_TYPE" , getAuthType );
sample.adapt( "REMOTE_USER" , getRemoteUser );
etc.

sample.get( "REMOTE_USER" );

Pluggable Adapters are common in Smalltalk, were it is easier to map strings to method calls

Doc 21, Adapter Slide # 14

class OldSquarePeg {
     public:
          void virtual squarePegOperation() { blah }
}

class RoundPeg {
     public:
          void virtual roundPegOperation() { blah }
}

class PegAdapter: public OldSquarePeg, RoundPeg {
     public:
          void virtual roundPegOperation() {
               add some corners;
               squarePegOperation();
          }
          void virtual squarePegOperation() {
               add some corners;
               roundPegOperation();
          }
}

Doc 21, Adapter Slide # 15

Adapter Verses State

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