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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed 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 com.google.common.util.concurrent;

import com.google.common.annotations.GwtCompatible;

import java.util.concurrent.Future;

import javax.annotation.Nullable;

/**
 * Transforms a value, possibly asynchronously. For an example usage and more
 * information, see
 * {@link Futures#transformAsync(ListenableFuture, AsyncFunction)}.
 *
 * @author Chris Povirk
 * @since 11.0
 */
@GwtCompatible
public interface AsyncFunction {
  /**
   * Returns an output {@code Future} to use in place of the given {@code
   * input}. The output {@code Future} need not be {@linkplain Future#isDone
   * done}, making {@code AsyncFunction} suitable for asynchronous derivations.
   *
   * Throwing an exception from this method is equivalent to returning a
   * failing {@code Future}.
   */
  ListenableFuture apply(@Nullable I input) throws Exception;
}