to avoid building a class hierarchy of factories that parallels the class hierarchy of products; or

Transcription

1 Prototype

2 Intent Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype Applicability Use the Prototype pattern when a system should be independent of how its products are created, composed, and represented; and when the classes to instantiate ti t are specified at run-time; to avoid building a class hierarchy of factories that parallels the class hierarchy of products; or when instances of a class can have one of only a few different combinations of state. It may be easier to have the proper number of prototypes and clone them rather than instantiating the class manually each time

3 Structure

4 Implementation/Sample Code class Prototype { public Prototype t clone() { code to make a copy of current Prototype object return clone; // add what ever else you want the class to do class Protoplasm extends Prototype { public Prototype clone() { code to make a copy of current Protoplasm object return clone; // add more other stuff ClientCodeMethod( Prototype example ) { Prototype mycopy = example.clone(); // do some work using mycopy

5 Cloning Issues:How to in C++ - Copy Constructors t class Door { public: Door(); Door( const Door&); virtual Door* clone() const; virtual void Initialize( Room*, Room* ); // stuff not shown private: Room* room1; Room* room2; Door::Door ( const Door& other ) { //Copy constructor room1 = other.room1; room2 = other.room2; Door* Door::clone() const { return new Door( *this );

6 How to in Java - Object clone() protected Object clone() throws CloneNotSupportedException Creates a clone of the object. A new instance is allocated and a bitwise clone of the current object is placed in the new object. Returns: a clone of this Object. Throws: OutOfMemoryError If there is not enough memory. Throws: CloneNotSupportedException Object explicitly does not want to be cloned, or it does not support the Cloneable interface.

7 How to in Java - Object clone() class Door implements Cloneable { public void Initialize( Room a, Room b) { room1 = a; room2 = b; public Object clone() throws CloneNotSupportedException { return super.clone(); Room room1; Room room2;

8 Shallow Copy Verse Deep Copy Original Objects Shallow Copy

9 Shallow Copy Verse Deep Copy Original Objects Deep Copy

10 Example of Prototype JavaServer handles the network connection. Algorithm: 1. Wait for client to connect 2. Open connection 3. Create IO streams to client 4. Create Server Engine to handle request 5. Let engine handle the request Variations: JavaServer will want to multiple copies of server engine to handle more than one client at a time Would like to write JavaServer just once and use it for all possible different server engines

11 JavaSever With Factory Method For a DateServer class JavaServer { ServerSocket acceptor; public JavaServer( int portnumber ) { acceptor = new ServerSocket( portnumber ); public void run() { while (true) { Socket client = acceptor.accept(); InputStream cin = client.getinputstream(); OutputStream cout = client.getoutputstream(); processclientrequest( cin, cout ); private void processclientrequest( InputStream cin, OutputStream cout ) { DateServer handler = new DateServer( cin, cout); handler.processclientrequest();

12 JavaServer with Template Method class AirlineReservationJavaServer extends JavaServer { public AirlineReservationServer( int portnumber ) { super( portnumber ); private void processclientrequest( InputStream cin, OutputStream cout ) { AirlineReservation handler; handler = new AirlineReservation( cin, cout ); handler.processclientrequest();

13 Design Principle 1 Program to an interface, not an implementation Use abstract classes (and/or interfaces in Java) to define common interfaces for a set of classes Declare variables to be instances of the abstract t class not instances of particular classes

14 The Interface interface ServerEngine { public void processclientrequest(inputstream in, OutputStream out);

15 JavaServer with Factory Method abstract class JavaServer { ServerSocket S acceptor; public JavaServer( int portnumber ) { acceptor = new ServerSocket( portnumber ); public void run() { while (true) { Socket client = acceptor.accept(); InputStream cin = client.getinputstream(); OutputStream cout = client.getoutputstream(); ServerEngine requesthandler = makeserverengine(); requesthandler.processclientrequest( cin, cout ); abstract protected ServerEngine makeserverengine();

16 JavaServer with Factory Method class AirlineReservationJavaServer extends JavaServer { public AirlineReservationServer( int portnumber ) { super( portnumber ); protected ServerEngine makeserverengine() { return new AirlineReservation( ); class AirlineReservation implements ServerEngine { public void processclientrequest(inputstream in, OutputStream out) { //blah //etc.

17 JavaSever With Interface and Prototype For Any Server interface ServerEngine { public ServerEngine clone( InputStream in, OutputStream out ); public void processclientrequest();

18 JavaSever With Interface and Prototype For Any Server class JavaServer { ServerSocket acceptor; ServerEngine serverprototype; public JavaServer( int portnumber, ServerEngine acopy) { acceptor = new ServerSocket( portnumber ); serverprototype = acopy; public void run() { while (true) { Socket client = acceptor.accept(); accept(); InputStream cin = client.getinputstream(); OutputStream cout = client.getoutputstream(); ServerEngine requesthandler = serverprototype.clone( cin, cout); requesthandler.processclientrequest( cin, cout );

19 Driver Program class DriverProgram { public static void main( String args[] ) { ServerEngine aprototype = new DateServer(); JavaServer networklistener; networklistener = new JavaServer( 6666, aprototype ); networklistener.run();

20 Consequences Adding and removing products at run-time Specifying new objects by varying values Specifying i new objects by varying structure Reducing subclassing (from factory method) Configuring an application with classes dynamically y

21 Implementation Issues Using a prototype manager Implementing the Clone operation Initializing i clones