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    <blockquote> <p>How do I apply this pattern for something that returns a Future?</p> </blockquote> <p>I think you have to implement your own proxy <code>Future</code> class here. Maybe something like the following that delegates to the proxy:</p> <pre><code>public class FutureProxy implements Future&lt;Integer&gt; { private Calculator calculator; private Future&lt;Integer&gt; delegate; public FutureProxy(Calculator calculator, Future&lt;Integer&gt; delegate) { this.calculator = calculator; this.delegate = delegate; } // proxy all of the methods public boolean isDone() { return delegate.isDone(); } ... // for get, you can then call back to `calculator` public Integer get() throws InterruptedException, ExecutionException { Integer result = delegate.get(); calculator.setCachedCalculation(result); return result; } // need to handle get(long, TimeUnit) as well public Integer get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; Integer result = delegate.get(timeout, unit); calculator.setCachedCalculation(result); return result; } } </code></pre> <p>Then your decorator getter would look like:</p> <pre><code>public Future&lt;Integer&gt; getCalculation() { Future&lt;Integer&gt; future = realObject.getCalculation(); return new FutureProxy(this, future); } </code></pre> <p>As you mention in the comments, if the <code>FutureProxy</code> is an inner class to the decorator, then the <code>Calculator</code> does not been to be injected. Just make sure that you use <code>volatile</code> fields if the proxy is updating fields in <code>Calculator</code> that will be accessed by other threads.</p>
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    1. POHow do I intercept and handle a Future in a decorator class?
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    1. COI definitely do NOT want the decorator to block. The background thread is an idea, but I don't want to have one thread per Future -- that doubles the amount of threading the system has to do.
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    2. COThis decorator is decorating a class that I have no control over. I am not able to change the parent's return type.
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    3. COAlso: callers will assume that they always get a Future back and that it adheres to the Future contract.
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