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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.cache;

import com.google.common.annotations.Beta;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Maps;
import com.google.common.util.concurrent.UncheckedExecutionException;

import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;

/**
 * This class provides a skeletal implementation of the {@code Cache} interface to minimize the
 * effort required to implement this interface.
 *
 * To implement a cache, the programmer needs only to extend this class and provide an
 * implementation for the {@link #get(Object)} and {@link #getIfPresent} methods.
 * {@link #getUnchecked}, {@link #get(Object, Callable)}, and {@link #getAll} are implemented in
 * terms of {@code get}; {@link #getAllPresent} is implemented in terms of {@code getIfPresent};
 * {@link #putAll} is implemented in terms of {@link #put}, {@link #invalidateAll(Iterable)} is
 * implemented in terms of {@link #invalidate}. The method {@link #cleanUp} is a no-op. All other
 * methods throw an {@link UnsupportedOperationException}.
 *
 * @author Charles Fry
 * @since 11.0
 */
@Beta
public abstract class AbstractLoadingCache
    extends AbstractCache implements LoadingCache {

  /** Constructor for use by subclasses. */
  protected AbstractLoadingCache() {}

  @Override
  public V getUnchecked(K key) {
    try {
      return get(key);
    } catch (ExecutionException e) {
      throw new UncheckedExecutionException(e.getCause());
    }
  }

  @Override
  public ImmutableMap getAll(Iterable keys) throws ExecutionException {
    Map result = Maps.newLinkedHashMap();
    for (K key : keys) {
      if (!result.containsKey(key)) {
        result.put(key, get(key));
      }
    }
    return ImmutableMap.copyOf(result);
  }

  @Override
  public final V apply(K key) {
    return getUnchecked(key);
  }

  @Override
  public void refresh(K key) {
    throw new UnsupportedOperationException();
  }
}