code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC 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.openqa.selenium.grid.config;
  
  import static com.google.common.collect.ImmutableList.toImmutableList;
  import static com.google.common.collect.ImmutableSortedSet.toImmutableSortedSet;
  import static java.util.Comparator.naturalOrder;
  
  import com.google.common.collect.ImmutableList;
  import java.util.Collection;
  import java.util.List;
  import java.util.Optional;
  import java.util.Set;
  import org.openqa.selenium.internal.Require;
  
  public class CompoundConfig implements Config {
  
    private final List<Config> allConfigs;
  
    public CompoundConfig(Config... allConfigsInDescendingOrderOfImportance) {
      if (allConfigsInDescendingOrderOfImportance.length == 0) {
        throw new ConfigException("List of config files must be greater than 0.");
      }
  
      this.allConfigs = ImmutableList.copyOf(allConfigsInDescendingOrderOfImportance);
    }
  
    @Override
    public Optional<List<String>> getAll(String section, String option) {
      Require.nonNull("Section name", section);
      Require.nonNull("Option name", option);
  
      List<String> values =
          allConfigs.stream()
              .map(config -> config.getAll(section, option))
              .filter(Optional::isPresent)
              .map(Optional::get)
              .flatMap(Collection::stream)
              .collect(toImmutableList());
  
      return values.isEmpty() ? Optional.empty() : Optional.of(values);
    }
  
    @Override
    public Set<String> getSectionNames() {
      return allConfigs.stream()
          .map(Config::getSectionNames)
          .flatMap(Collection::stream)
          .collect(toImmutableSortedSet(naturalOrder()));
    }
  
    @Override
    public Set<String> getOptions(String section) {
      Require.nonNull("Section name to get options for", section);
  
      return allConfigs.stream()
          .map(config -> config.getOptions(section))
          .flatMap(Collection::stream)
          .collect(toImmutableSortedSet(naturalOrder()));
    }
  }
  