guava/src/com/google/common/util/concurrent/CollectionFuture.java

/*
 * Copyright (C) 2006 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 static com.google.common.base.Preconditions.checkState;
import static com.google.common.collect.Lists.newArrayListWithCapacity;
import static java.util.Collections.unmodifiableList;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Optional;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;
import java.util.List;
import org.checkerframework.checker.nullness.qual.Nullable;

/** Aggregate future that collects (stores) results of each future. */
@GwtCompatible(emulated = true)
abstract class CollectionFuture<V, C> extends AggregateFuture<V, C> {

  abstract class CollectionFutureRunningState extends RunningState {
    private List<Optional<V>> values;

    CollectionFutureRunningState(
        ImmutableCollection<? extends ListenableFuture<? extends V>> futures,
        boolean allMustSucceed) {
      super(futures, allMustSucceed, true);

      this.values =
          futures.isEmpty()
              ? ImmutableList.<Optional<V>>of()
              : Lists.<Optional<V>>newArrayListWithCapacity(futures.size());

      // Populate the results list with null initially.
      for (int i = 0; i < futures.size(); ++i) {
        values.add(null);
      }
    }

    @Override
    final void collectOneValue(boolean allMustSucceed, int index, @Nullable V returnValue) {
      List<Optional<V>> localValues = values;

      if (localValues != null) {
        localValues.set(index, Optional.fromNullable(returnValue));
      } else {
        // Some other future failed or has been cancelled, causing this one to also be cancelled or
        // have an exception set. This should only happen if allMustSucceed is true or if the output
        // itself has been cancelled.
        checkState(
            allMustSucceed || isCancelled(), "Future was done before all dependencies completed");
      }
    }

    @Override
    final void handleAllCompleted() {
      List<Optional<V>> localValues = values;
      if (localValues != null) {
        set(combine(localValues));
      } else {
        checkState(isDone());
      }
    }

    @Override
    void releaseResourcesAfterFailure() {
      super.releaseResourcesAfterFailure();
      this.values = null;
    }

    abstract C combine(List<Optional<V>> values);
  }

  /** Used for {@link Futures#allAsList} and {@link Futures#successfulAsList}. */
  static final class ListFuture<V> extends CollectionFuture<V, List<V>> {
    ListFuture(
        ImmutableCollection<? extends ListenableFuture<? extends V>> futures,
        boolean allMustSucceed) {
      init(new ListFutureRunningState(futures, allMustSucceed));
    }

    private final class ListFutureRunningState extends CollectionFutureRunningState {
      ListFutureRunningState(
          ImmutableCollection<? extends ListenableFuture<? extends V>> futures,
          boolean allMustSucceed) {
        super(futures, allMustSucceed);
      }

      @Override
      public List<V> combine(List<Optional<V>> values) {
        List<V> result = newArrayListWithCapacity(values.size());
        for (Optional<V> element : values) {
          result.add(element != null ? element.orNull() : null);
        }
        return unmodifiableList(result);
      }
    }
  }
}