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Google Collections Library is a suite of new collections and collection-related goodness for Java 5.0
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/*
 * Copyright (C) 2008 Google Inc.
 *
 * 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 com.google.common.annotations.GwtCompatible;

import java.util.Map;

import javax.annotation.Nullable;

/**
 * Implementation of {@link ImmutableMap} with exactly one entry.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 */
@GwtCompatible(serializable = true)
final class SingletonImmutableMap extends ImmutableMap {

  // These fields are not final so that GWT is able to derive the key and value
  // types by inspecting these fields at GWT compile time.  It also makes this
  // class GWT serializable without a custom field serializer.
  private K singleKey;
  private V singleValue;

  private transient Entry entry;

  // Only used in GWT deserialization.
  private SingletonImmutableMap() {
  }

  SingletonImmutableMap(K singleKey, V singleValue) {
    this.singleKey = singleKey;
    this.singleValue = singleValue;
  }

  SingletonImmutableMap(Entry entry) {
    this.entry = entry;
    this.singleKey = entry.getKey();
    this.singleValue = entry.getValue();
  }

  private Entry entry() {
    Entry e = entry;
    return (e == null)
        ? (entry = Maps.immutableEntry(singleKey, singleValue)) : e;
  }

  @Override public V get(Object key) {
    return singleKey.equals(key) ? singleValue : null;
  }

  public int size() {
    return 1;
  }

  public boolean isEmpty() {
    return false;
  }

  @Override public boolean containsKey(Object key) {
    return singleKey.equals(key);
  }

  @Override public boolean containsValue(Object value) {
    return singleValue.equals(value);
  }

  private transient ImmutableSet> entrySet;

  @Override public ImmutableSet> entrySet() {
    ImmutableSet> es = entrySet;
    return (es == null) ? (entrySet = ImmutableSet.of(entry())) : es;
  }

  private transient ImmutableSet keySet;

  @Override public ImmutableSet keySet() {
    ImmutableSet ks = keySet;
    return (ks == null) ? (keySet = ImmutableSet.of(singleKey)) : ks;
  }

  private transient ImmutableCollection values;

  @Override public ImmutableCollection values() {
    ImmutableCollection v = values;
    return (v == null) ? (values = new Values(singleValue)) : v;
  }

  private static class Values extends ImmutableCollection {
    final V singleValue;

    Values(V singleValue) {
      this.singleValue = singleValue;
    }

    @Override public boolean contains(Object object) {
      return singleValue.equals(object);
    }

    @Override public boolean isEmpty() {
      return false;
    }

    public int size() {
      return 1;
    }

    @Override public UnmodifiableIterator iterator() {
      return Iterators.singletonIterator(singleValue);
    }
  }

  @Override public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    }
    if (object instanceof Map) {
      Map that = (Map) object;
      if (that.size() != 1) {
        return false;
      }
      Entry entry = that.entrySet().iterator().next();
      return singleKey.equals(entry.getKey())
          && singleValue.equals(entry.getValue());
    }
    return false;
  }

  @Override public int hashCode() {
    return singleKey.hashCode() ^ singleValue.hashCode();
  }

  @Override public String toString() {
    return new StringBuilder()
        .append('{')
        .append(singleKey.toString())
        .append('=')
        .append(singleValue.toString())
        .append('}')
        .toString();
  }
}