/*
    * Copyright (C) 2008 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.collect;
  
  import static com.google.common.collect.CollectPreconditions.checkEntryNotNull;
  
  import com.google.common.annotations.GwtCompatible;
  
  import javax.annotation.Nullable;
  
  /**
   * Implementation of {@link ImmutableMap} with exactly one entry.
   *
   * @author Jesse Wilson
   * @author Kevin Bourrillion
   */
  @GwtCompatible(serializable = true, emulated = true)
  @SuppressWarnings("serial") // uses writeReplace(), not default serialization
  final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {
  
    final transient K singleKey;
    final transient V singleValue;
  
    SingletonImmutableBiMap(K singleKey, V singleValue) {
      checkEntryNotNull(singleKey, singleValue);
      this.singleKey = singleKey;
      this.singleValue = singleValue;
    }
  
    private SingletonImmutableBiMap(K singleKey, V singleValue,
        ImmutableBiMap<V, K> inverse) {
      this.singleKey = singleKey;
      this.singleValue = singleValue;
      this.inverse = inverse;
    }
  
    SingletonImmutableBiMap(Entry<? extends K, ? extends V> entry) {
      this(entry.getKey(), entry.getValue());
    }
  
    @Override public V get(@Nullable Object key) {
      return singleKey.equals(key) ? singleValue : null;
    }
  
    @Override
    public int size() {
      return 1;
    }
  
    @Override public boolean containsKey(@Nullable Object key) {
      return singleKey.equals(key);
    }
  
    @Override public boolean containsValue(@Nullable Object value) {
      return singleValue.equals(value);
    }
  
    @Override boolean isPartialView() {
      return false;
    }
  
    @Override
    ImmutableSet<Entry<K, V>> createEntrySet() {
      return ImmutableSet.of(Maps.immutableEntry(singleKey, singleValue));
    }
  
    @Override
    ImmutableSet<K> createKeySet() {
      return ImmutableSet.of(singleKey);
    }
  
    transient ImmutableBiMap<V, K> inverse;
  
    @Override
    public ImmutableBiMap<V, K> inverse() {
      // racy single-check idiom
      ImmutableBiMap<V, K> result = inverse;
      if (result == null) {
        return inverse = new SingletonImmutableBiMap<V, K>(
            singleValue, singleKey, this);
      } else {
        return result;
      }
    }
  }