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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.util.concurrent;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

/**
 * This interface defines an asynchronous {@link #get(long, TimeUnit)} method.
 *
 * When the return value is still being computed, invoking
 * {@link #get(long, TimeUnit)} will result in a {@link TimeoutException}.
 * The method should be invoked again and again
 * until the underlying computation is completed.
 *
 * @param  The type of the return value.
 * @param  The exception type that the underlying implementation may throw.
 */
public interface AsyncGet {
  /**
   * Get the result.
   *
   * @param timeout The maximum time period to wait.
   *                When timeout == 0, it does not wait at all.
   *                When timeout < 0, it waits indefinitely.
   * @param unit The unit of the timeout value
   * @return the result, which is possibly null.
   * @throws E an exception thrown by the underlying implementation.
   * @throws TimeoutException if it cannot return after the given time period.
   * @throws InterruptedException if the thread is interrupted.
   */
  R get(long timeout, TimeUnit unit)
      throws E, TimeoutException, InterruptedException;

  /** @return true if the underlying computation is done; false, otherwise. */
  boolean isDone();

  /** Utility */
  class Util {
    /** Use {@link #get(long, TimeUnit)} timeout parameters to wait. */
    public static void wait(Object obj, long timeout, TimeUnit unit)
        throws InterruptedException {
      if (timeout < 0) {
        obj.wait();
      } else if (timeout > 0) {
        obj.wait(unit.toMillis(timeout));
      }
    }
  }
}