gistfile1.java

public class LazyImmutableMap<K,V> extends ForwardingMap<K,V> {
  public static class AccessException extends RuntimeException {
    public AccessException(Throwable cause) {
      super(cause);
    }
  }

  private final FutureTask<Map<K, V>> task;

  public LazyImmutableMap(final Callable<Map<K,V>> eval) {    
    task = new FutureTask<Map<K,V>>(new Callable<Map<K,V>>() {
      public Map<K, V> call() throws Exception {
        return ImmutableMap.copyOf(eval.call());
      }
    });
  }

  @Override
  protected Map<K, V> delegate() {
    task.run();
    try {
      return task.get();
    } catch (InterruptedException e) {
      throw new AccessException(e);
    } catch (ExecutionException e) {
      throw new AccessException(e.getCause());
    }    
  }
}