java - How find/invoke appropriate handler for interface implementation? -

suppose there interface several implementations:

//methods , implementations omitted public interface myrequest {} 

and several handlers extended interface:

public <t extends myrequest> inteface handler<> {     myresult handle(t request) }  public xmlhandler implements handler<xmlrequest> {     @override     myresult handle(xmlrequest request) {         //...     } }  public jsonhandler implements handler<jsonrequest> {     @override     myresult handle(jsonrequest request) {         //...     } } 

up clear, problem that, when requests parsed parser returns interface:

public void  somemethod () {     xmlhandler  xmlhandler = ...     jsonhandler  jsonhandler = ...     myparser parser = ...     myrequest request = parser.parse(someobejct);      if (request instanceof jsonrequest) {         jsonhandler.handle(request);     }      if (request instanceof xmlrequest) {         xmlhandler.handle(request);     }  } 

instanceof bad here. how avoid this?

my initial thought use visitor pattern here, e.g. put handlers visito , define dozen methods each request type invoke handler.handle method. , problem of course lies word dozen.

is there more elegant way "select" handler request call (or more elegant pattern visitor)?

p.s. way, visitor pattern suitable here?

yes, visitor pattern solution problem.

public interface myrequestvisitor {     public void handle(jsonrequest request);     public void handle(xmlrequest request); }  public interface myrequest {     public void handle(myrequestvisitor visitor); }  public class jsonrequest implements myrequest  {     public void handle(myrequestvisitor visitor) {         visitor.handle(this); // calls first handle method     } }  public class jsonrequest implements myrequest  {     public void handle(myrequestvisitor visitor) {         visitor.handle(this); // calls second handle method     } } 

then, in somemethod:

public void  somemethod () {     xmlhandler  xmlhandler = ...     jsonhandler  jsonhandler = ...     myparser parser = ...     myrequest request = parser.parse(someobejct);     request.handle(new myrequestvisitor() {         public void handle(jsonrequest request) {             jsonhandler.handle(request);         }         public void handle(xmlrequest request) {             xmlhandler.handle(request);         }     }); 

if add new subclass called newrequest, compiler forces implement newrequest.handle(myrequestvisitor), forces add new myrequestvisitor.handle(newrequest request) method, forces add anonymous class too.

you can't not implement it, safe, multi-way if-instanceof not.