Java - asynchronous methods with return value

Question

I have a question about asynchronous method calls in java, especially about the response value of the async method.

The situation is the following:

The async method I want to call is..

public void getSpeed(IResponseListener listener) {
....
}

The interface of the listener is...

interface IResponseListener {
    public void response(ResponseEvent event);
}

This method is called when the async method has a response value

My problem now is that the class ResponseEvent has an attribute response that can be of any type (boolean,float,String...)and in the implementation of the interface IResponseListener I have to cast the value...

IResponseListener listener = new IResponseListener {

    public void response(ResponseEvent event) {
        float f = (float)event.response;
    }
}

Is this a good solution to handle this? I think the bad thing is that the response listener HAS to know the type of the response! Is there a better solution to handle asynchronous calls that want to give a response even if the response can be of any type?

You should create a getter for the response in the ResponseEvent and parameterize (use generics) for the ResponseEvent using one of the boxing classes (Float in this case). — Maarten Bodewes
– Maarten Bodewes, Commented Oct 1, 2012 at 20:11
On what basis does the datatype of the response property vary? — midhunhk
– midhunhk, Commented Oct 1, 2012 at 20:13
Your problem seems to be with Java's typing rules. The fact that there's an async method involved is irrelevant. — Mike Baranczak
– Mike Baranczak, Commented Oct 1, 2012 at 20:16
I'm just curious, but why would the return type be of different type? Are you reusing the same listener class in multiple places? — midhunhk
– midhunhk, Commented Oct 1, 2012 at 20:56

Daniel B. Chapman · Accepted Answer · 2012-10-01 21:16:45Z

2

I think a lot of these answers are starting to look like this->

public interface Function<Args, Value>
{
  public Value call(Args args);
}

Your return type doesn't matter--if it can return multiple types, then the "multiple types" are a type...might I suggest a type of JSON considering what you're looking at?

The reality is you can't expect your handler to know the type in advance so you need to specify that. Whether this is with the return type or the class is up to you.

I could easily see doing a class hierarchy:

public class ResponseString implements Function<Args, String>; 
public class ResponseNumber implements Function<Args, Number>;
...
public class ResponseType implements Function<Args, Type>;

or simply creating a type that has all the information you need. The long and short is that the method can DEFINE what it expects for the types and you have the ability to extend them. Keep in mind that response could also be a Function which could be executed. There's nothing wrong with knowing what to DO with something and not knowing what it is ->

Example->

//Admittedly I'd probably have a "Procedure or VoidFunction interface as well".
public yourMethod(Function<String args, Function<String,?> handler)
{
  String hello = "hello";
  Function<String,?> function = handler.call(hello);
  function.call(hello); 
}

I hope this helps. Sometimes there's no reason to go this far, and sometimes there is. You don't know the type--it seems like maybe you're hoping someone else will provide it and this can do that for you while remaining strict.

EDIT: the example of have for this in one framework is:

  Applcation.openDialog(Dialog dialog, Callable<Boolean> onClose);

This returns true of the dialog cleans up and closes and false if not. I don't really care what happens here, I do care that it tells me yes, close it, or no don't.

edited Oct 1, 2012 at 21:16

answered Oct 1, 2012 at 20:49

Daniel B. Chapman

4,68734 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Daniel B. Chapman Over a year ago

A side note--if you tell us what you're trying to do with the response you'll see less complex answers!

Tobi Weißhaar Over a year ago

thank you for your great effort but I think the response of @nekman comes closest to my needs ;)

Daniel B. Chapman Over a year ago

@TobiWeißhaar His answer is a very solid answer, you don't need functional abstraction unless you really need it (and then it can make something stupidly simple)

ndeverge · Accepted Answer · 2012-10-01 20:11:52Z

1

Use Java generics:

interface IResponseListener<T> {
    public void response(T response);
}

Then, in an anonymous class:

IResponseListener listener = new IResponseListener<Float> {

    public void response(Float response) {
        float f = response;
    }
}

answered Oct 1, 2012 at 20:11

ndeverge

21.6k4 gold badges58 silver badges86 bronze badges

2 Comments

Dave Over a year ago

The listener still has to know the type of the response; see the last paragraph of the question.

noahlz Over a year ago

You will need to define an IResponseListener for each datatype you want to support,and have a parent dispatch method that makes a call relying on runtime type resolution / overloading to reach the correct listener (roughly something like this: community.jboss.org/wiki/ProxyVisitorPattern?_sscc=t). Alternatively, you can do a a big ugly if/else chain using instanceof to resolve and downcast the type of the Response object (but don't tell anyone here).

midhunhk · Accepted Answer · 2012-10-01 20:17:37Z

1

I don't know whether this is correct, but if you are going to handle the return value differently, why not overload the response method with different type of objects that you would expect. Just a suggestion..

interface InterfaceName{
    void response(float responseVal);
    void response(boolean responseVal);
    ...
}

answered Oct 1, 2012 at 20:17

midhunhk

5,5487 gold badges55 silver badges85 bronze badges

Comments

nekman · Accepted Answer · 2012-10-01 20:40:47Z

0

I would have done as @nico_ekito says...Or use your existing solution. It is a problem that you don't know the result type.

Anyway, you could do some adjustments and let the ResponseEvent class do the casting for you.

ResponseListener.java

interface IResponseListener {
    void response(ResponseEvent event);
}

ResponseEvent.java

public class ResponseEvent {

   private Object response;

   @SuppressWarnings("unchecked")
   public <T> T getResponse() {
       return (T)response;
   }

   public <T> void setResponse(T response) {
       this.response = response;
   }
}

Usage:

IResponseListener listener = new IResponseListener() {
    public void response(ResponseEvent event) {
        float f = event.getResponse();
    } 
};

Please note that you will get a ClassCastException if your type is something other than what you expect it to be.

answered Oct 1, 2012 at 20:40

nekman

1,9192 gold badges15 silver badges26 bronze badges

1 Comment

noahlz Over a year ago

This is called "type capture," no? Also, the OP is hoping to avoid the ClassCastException, so I think using a visitor pattern approach will work best here.

Collectives™ on Stack Overflow

Java - asynchronous methods with return value

4 Answers 4

3 Comments

2 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

2 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related