Return a String obtained in an inner Callback from the outer function

Question

I have Java code that roughly goes like this:

public String render(String str) {
  byte[] b = [serialize str]
  client.sendRequest(b, new Callback<byte[]>() {
    @Override
    public void onDone(byte[] data) {
      String res = [convert data back to String]
      // TODO: return this string in the outer function
  }); 
  return "???"; 
}

I'm having issues getting the render function to return the String that I obtain from the onDone callback. I tried initializing the String res outside of the sendRequest invocation but I run into issues with the variable not being final.

I was also reading up on Java Futures here. From that I was thinking I could set a Future and call isDone() on it to make sure it was set? However this code seems verbose and I was wondering if there was a simpler solution.

There is no serialization required/relevant here. Whatever problem exists - a scoping/timing issue - will remain removing any mention of such. Ie. try to "return `data`", which won't work for the same conceptual reason. One asynchronous code is used, the model *must* be switched to an asynchronous model entirely (including Futures, which just defer the decision, or further processing in callbacks) or use a synchronous method (which can including 'synchronously waiting for an asynchronous method to complete'). — user2864740, Jun 25 '16 at 01:30

score 2 · Accepted Answer · edited May 23 '17 at 12:22

I'm having issues getting the render function to return the String that I obtain from the onDone callback. I tried initializing the String res outside of the sendRequest invocation but I run into issues with the variable not being final.

You can't change final variable of primitive or immutable type from anonymous class, and can't access non final. So you can:

Use named inner class instead of anonymous
Use final object of mutable class (StringBuilder or StringBuffer)
Create mutable class that will be just "holder" of required value, create final variable of its type, and set value to it from anonymous class; or use array of required type for the same purpose.

[see this ]

But before returning value from outer function you obviously should wait until method in anonymous class will finish its work.

Thanks, I looked at the link and tried using the solution by making a final String[] and setting index 0 but someone told me that in Java, if I'm running on separate threads, that 0th index is not guaranteed to be set. I ended up using an ArrayBlockingQueue to achieve synchronous behavior for this one. — cheng, Jun 27 '16 at 19:05

Return a String obtained in an inner Callback from the outer function

1 Answers1