CODEWITHSUNDEEP
javaeventsgenerics
Amir.F
Amir.F

Reputation: 1961

java events using generics

I am learning java generics, and trying to implement it for events before someone asks, i have considered using polymorphism, but since I want every event to send different values to the handler's start method, I can't do this.

This is my event handler:

    public class MyHandler {
      public void startEventAction(int num) {
        //do something with num
      }  
    }

This is my template event:

     public abstract class SomeEvent<E> {
       protected List<E> handler_list = new LinkedList<E>();
       public SomeEvent() {
       }
       public Boolean addHandler(E handler) {
          return handler_list.add(handler);
       }
       public Boolean removeHandler(E handler) {
          return handler_list.remove(handler);
       } 
     }

and the event itself is: (now i know it's probably wrong to extend the SomeEvent like this, but it seems to work)

    public class MyEvent<MyHandler> extends SomeEvent<MyHandler> {
      public void executeHandlers() {   
        for (MyHandler handler : handler_list) {
            //Exception here!!
            handler.startEventAction(num);
        }
      }
    }

My question is how is it right to extend it and why does the function startEventAction doesn't exist for MyHandler in the last code block(for MyEvent) Answers will be very much appreciated.

Upvotes: 1

Views: 907

Answers (2)

assylias
assylias

Reputation: 328598

Simply change the MyEvent class declaration:

public static class MyEvent extends SomeEvent<MyHandler> { ... }

and it should be fine.

Upvotes: 0

Edwin Dalorzo
Edwin Dalorzo

Reputation: 78579

I think it should be

public class MyEvent<T extends MyHandler> extends SomeEvent<T> {
      public void executeHandlers() {   
        for (T handler : handler_list) {
            //Exception here!!
            handler.startEventAction(lvc_count);
        }
    }

If you use MyEvent<MyHandler> you're considering MyHandler as a type parameter, not as a type argument. That's why I typically rather use letters as type paramers instead of entire words, this avoids the confusion between them and classes and interfaces names.

Upvotes: 2

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