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Roles are a powerful way for designing complex behaviours while keeping good practices such as Single Responsibility, Dependency Inversion and Interface Segregation.

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/*
 * #%L
 * *********************************************************************************************************************
 *
 * These Foolish Things - Miscellaneous utilities
 * http://thesefoolishthings.tidalwave.it - git clone [email protected]:tidalwave/thesefoolishthings-src.git
 * %%
 * Copyright (C) 2009 - 2016 Tidalwave s.a.s. (http://tidalwave.it)
 * %%
 * *********************************************************************************************************************
 *
 * 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.
 *
 * *********************************************************************************************************************
 *
 * $Id$
 *
 * *********************************************************************************************************************
 * #L%
 */
package it.tidalwave.role.spi.impl;

import javax.annotation.Nonnull;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;

/***********************************************************************************************************************
 *
 * @author  Fabrizio Giudici
 * @version $Id$
 *
 **********************************************************************************************************************/
public class MultiMap extends HashMap>
  {
    private static final long serialVersionUID = 8834342771135005212L;

    public synchronized void add (final @Nonnull K key, final @Nonnull V value)
      {
        internalGetValues(key).add(value);
      }

    public synchronized void addAll (final @Nonnull K key, final @Nonnull Collection values)
      {
        if (!values.isEmpty())
          {
            internalGetValues(key).addAll(values);
          }
      }

    @Nonnull
    public synchronized Set getValues (final @Nonnull K key)
      {
        final Set values = get(key);
        return (values == null) ? Collections.emptySet() : Collections.unmodifiableSet(values);
      }

    @Nonnull
    private Set internalGetValues (final @Nonnull K key)
      {
        Set values = get(key);

        if (values == null)
          {
            values = new HashSet<>();
            put(key, values);
          }

        return values;
      }
  }