I'm using the Chain of Responsibility design-pattern in Java. The chain as a whole represents a request for objects of certain types. Each "Handler" in the chain is responsible to handle the requested units of 1 type.

All requests are handled in essentially the same way so I tried making the "Handler"-class generic.

So in the Handle-class I need a method like this (the handling itself is simplified because it would only obfuscate my problem):

public class Handler{

int required;

Handler> next;

public void handle(Object O){

if(o instanceof T){

required --;

}else{

next.handle(o);

}

}

}

The problem is that an instanceof like this is impossible. Because the type T isn't explicitly stored during run time (or that's what I understood during my research on the internet). So my question is: what is the best alternative?

解决方案

Implement handlers using generics by using a constructor parameter to define the class the handler supports:

public class Handler {

private int required;

private Handler> next;

private Class extends T> c;

public Handler(Class extends T> c) {

this.c = c;

}

public void handle(Object o) {

if (c.isInstance(o)) {

required--;

} else {

next.handle(o);

}

}

// ...

}