common/collect/ImmutableMap.java

/*
 * 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.base.Preconditions.checkNotNull;
import static com.google.common.collect.Iterables.getOnlyElement;

import com.google.common.annotations.GwtCompatible;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.annotation.Nullable;

/**
 * An immutable, hash-based {@link Map} with reliable user-specified iteration
 * order. Does not permit null keys or values.
 *
 * <p>Unlike {@link Collections#unmodifiableMap}, which is a <i>view</i> of a
 * separate map which can still change, an instance of {@code ImmutableMap}
 * contains its own data and will <i>never</i> change. {@code ImmutableMap} is
 * convenient for {@code public static final} maps ("constant maps") and also
 * lets you easily make a "defensive copy" of a map provided to your class by a
 * caller.
 *
 * <p><i>Performance notes:</i> unlike {@link HashMap}, {@code ImmutableMap} is
 * not optimized for element types that have slow {@link Object#equals} or
 * {@link Object#hashCode} implementations. You can get better performance by
 * having your element type cache its own hash codes, and by making use of the
 * cached values to short-circuit a slow {@code equals} algorithm.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableMap<K, V> implements Map<K, V>, Serializable {
  /**
   * Returns the empty map. This map behaves and performs comparably to
   * {@link Collections#emptyMap}, and is preferable mainly for consistency
   * and maintainability of your code.
   */
  // Casting to any type is safe because the set will never hold any elements.
  @SuppressWarnings("unchecked")
  public static <K, V> ImmutableMap<K, V> of() {
    return (ImmutableMap<K, V>) EmptyImmutableMap.INSTANCE;
  }

  /**
   * Returns an immutable map containing a single entry. This map behaves and
   * performs comparably to {@link Collections#singletonMap} but will not accept
   * a null key or value. It is preferable mainly for consistency and
   * maintainability of your code.
   */
  public static <K, V> ImmutableMap<K, V> of(K k1, V v1) {
    return new SingletonImmutableMap<K, V>(
        checkNotNull(k1), checkNotNull(v1));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(K k1, V v1, K k2, V v2) {
    return new RegularImmutableMap<K, V>(entryOf(k1, v1), entryOf(k2, v2));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(
      K k1, V v1, K k2, V v2, K k3, V v3) {
    return new RegularImmutableMap<K, V>(
        entryOf(k1, v1), entryOf(k2, v2), entryOf(k3, v3));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(
      K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
    return new RegularImmutableMap<K, V>(
        entryOf(k1, v1), entryOf(k2, v2), entryOf(k3, v3), entryOf(k4, v4));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(
      K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
    return new RegularImmutableMap<K, V>(entryOf(k1, v1),
        entryOf(k2, v2), entryOf(k3, v3), entryOf(k4, v4), entryOf(k5, v5));
  }

  // looking for of() with > 5 entries? Use the builder instead.

  /**
   * Returns a new builder. The generated builder is equivalent to the builder
   * created by the {@link Builder} constructor.
   */
  public static <K, V> Builder<K, V> builder() {
    return new Builder<K, V>();
  }

  /**
   * Verifies that {@code key} and {@code value} are non-null, and returns a new
   * immutable entry with those values.
   *
   * <p>A call to {@link Map.Entry#setValue} on the returned entry will always
   * throw {@link UnsupportedOperationException}.
   */
  static <K, V> Entry<K, V> entryOf(K key, V value) {
    return Maps.immutableEntry(
        checkNotNull(key, "null key"),
        checkNotNull(value, "null value"));
  }

  /**
   * A builder for creating immutable map instances, especially {@code public
   * static final} maps ("constant maps"). Example: <pre>   {@code
   *
   *   static final ImmutableMap<String, Integer> WORD_TO_INT =
   *       new ImmutableMap.Builder<String, Integer>()
   *           .put("one", 1)
   *           .put("two", 2)
   *           .put("three", 3)
   *           .build();}</pre>
   *
   * For <i>small</i> immutable maps, the {@code ImmutableMap.of()} methods are
   * even more convenient.
   *
   * <p>Builder instances can be reused - it is safe to call {@link #build}
   * multiple times to build multiple maps in series. Each map is a superset of
   * the maps created before it.
   *
   * @since 2.0 (imported from Google Collections Library)
   */
  public static class Builder<K, V> {
    final ArrayList<Entry<K, V>> entries = Lists.newArrayList();

    /**
     * Creates a new builder. The returned builder is equivalent to the builder
     * generated by {@link ImmutableMap#builder}.
     */
    public Builder() {}

    /**
     * Associates {@code key} with {@code value} in the built map. Duplicate
     * keys are not allowed, and will cause {@link #build} to fail.
     */
    public Builder<K, V> put(K key, V value) {
      entries.add(entryOf(key, value));
      return this;
    }

    /**
     * Adds the given {@code entry} to the map, making it immutable if
     * necessary. Duplicate keys are not allowed, and will cause {@link #build}
     * to fail.
     *
     * @since 11.0
     */
    public Builder<K, V> put(Entry<? extends K, ? extends V> entry) {
      K key = entry.getKey();
      V value = entry.getValue();
      if (entry instanceof ImmutableEntry<?, ?>) {
        checkNotNull(key);
        checkNotNull(value);
        @SuppressWarnings("unchecked") // all supported methods are covariant
        Entry<K, V> immutableEntry = (Entry<K, V>) entry;
        entries.add(immutableEntry);
      } else {
        // Directly calling entryOf(entry.getKey(), entry.getValue()) can cause
        // compilation error in Eclipse.
        entries.add(entryOf(key, value));
      }
      return this;
    }

    /**
     * Associates all of the given map's keys and values in the built map.
     * Duplicate keys are not allowed, and will cause {@link #build} to fail.
     *
     * @throws NullPointerException if any key or value in {@code map} is null
     */
    public Builder<K, V> putAll(Map<? extends K, ? extends V> map) {
      entries.ensureCapacity(entries.size() + map.size());
      for (Entry<? extends K, ? extends V> entry : map.entrySet()) {
        put(entry.getKey(), entry.getValue());
      }
      return this;
    }

    /*
     * TODO(kevinb): Should build() and the ImmutableBiMap & ImmutableSortedMap
     * versions throw an IllegalStateException instead?
     */

    /**
     * Returns a newly-created immutable map.
     *
     * @throws IllegalArgumentException if duplicate keys were added
     */
    public ImmutableMap<K, V> build() {
      return fromEntryList(entries);
    }

    private static <K, V> ImmutableMap<K, V> fromEntryList(
        List<Entry<K, V>> entries) {
      int size = entries.size();
      switch (size) {
        case 0:
          return of();
        case 1:
          return new SingletonImmutableMap<K, V>(getOnlyElement(entries));
        default:
          Entry<?, ?>[] entryArray
              = entries.toArray(new Entry<?, ?>[entries.size()]);
          return new RegularImmutableMap<K, V>(entryArray);
      }
    }
  }

  /**
   * Returns an immutable map containing the same entries as {@code map}. If
   * {@code map} somehow contains entries with duplicate keys (for example, if
   * it is a {@code SortedMap} whose comparator is not <i>consistent with
   * equals</i>), the results of this method are undefined.
   *
   * <p>Despite the method name, this method attempts to avoid actually copying
   * the data when it is safe to do so. The exact circumstances under which a
   * copy will or will not be performed are undocumented and subject to change.
   *
   * @throws NullPointerException if any key or value in {@code map} is null
   */
  public static <K, V> ImmutableMap<K, V> copyOf(
      Map<? extends K, ? extends V> map) {
    if ((map instanceof ImmutableMap) && !(map instanceof ImmutableSortedMap)) {
      // TODO(user): Make ImmutableMap.copyOf(immutableBiMap) call copyOf()
      // on the ImmutableMap delegate(), rather than the bimap itself

      @SuppressWarnings("unchecked") // safe since map is not writable
      ImmutableMap<K, V> kvMap = (ImmutableMap<K, V>) map;
      if (!kvMap.isPartialView()) {
        return kvMap;
      }
    }

    @SuppressWarnings("unchecked") // we won't write to this array
    Entry<K, V>[] entries = map.entrySet().toArray(new Entry[0]);
    switch (entries.length) {
      case 0:
        return of();
      case 1:
        return new SingletonImmutableMap<K, V>(entryOf(
            entries[0].getKey(), entries[0].getValue()));
      default:
        for (int i = 0; i < entries.length; i++) {
          K k = entries[i].getKey();
          V v = entries[i].getValue();
          entries[i] = entryOf(k, v);
        }
        return new RegularImmutableMap<K, V>(entries);
    }
  }

  ImmutableMap() {}

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override
  public final V put(K k, V v) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override
  public final V remove(Object o) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override
  public final void putAll(Map<? extends K, ? extends V> map) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always
   */
  @Override
  public final void clear() {
    throw new UnsupportedOperationException();
  }

  @Override
  public boolean isEmpty() {
    return size() == 0;
  }

  @Override
  public boolean containsKey(@Nullable Object key) {
    return get(key) != null;
  }

  // Overriding to mark it Nullable
  @Override
  public abstract boolean containsValue(@Nullable Object value);

  // Overriding to mark it Nullable
  @Override
  public abstract V get(@Nullable Object key);

  /**
   * Returns an immutable set of the mappings in this map. The entries are in
   * the same order as the parameters used to build this map.
   */
  @Override
  public abstract ImmutableSet<Entry<K, V>> entrySet();

  /**
   * Returns an immutable set of the keys in this map. These keys are in
   * the same order as the parameters used to build this map.
   */
  @Override
  public abstract ImmutableSet<K> keySet();

  /**
   * Returns an immutable collection of the values in this map. The values are
   * in the same order as the parameters used to build this map.
   */
  @Override
  public abstract ImmutableCollection<V> values();

  @Override public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    }
    if (object instanceof Map) {
      Map<?, ?> that = (Map<?, ?>) object;
      return this.entrySet().equals(that.entrySet());
    }
    return false;
  }

  abstract boolean isPartialView();

  @Override public int hashCode() {
    // not caching hash code since it could change if map values are mutable
    // in a way that modifies their hash codes
    return entrySet().hashCode();
  }

  @Override public String toString() {
    return Maps.toStringImpl(this);
  }

  /**
   * Serialized type for all ImmutableMap instances. It captures the logical
   * contents and they are reconstructed using public factory methods. This
   * ensures that the implementation types remain as implementation details.
   */
  static class SerializedForm implements Serializable {
    private final Object[] keys;
    private final Object[] values;
    SerializedForm(ImmutableMap<?, ?> map) {
      keys = new Object[map.size()];
      values = new Object[map.size()];
      int i = 0;
      for (Entry<?, ?> entry : map.entrySet()) {
        keys[i] = entry.getKey();
        values[i] = entry.getValue();
        i++;
      }
    }
    Object readResolve() {
      Builder<Object, Object> builder = new Builder<Object, Object>();
      return createMap(builder);
    }
    Object createMap(Builder<Object, Object> builder) {
      for (int i = 0; i < keys.length; i++) {
        builder.put(keys[i], values[i]);
      }
      return builder.build();
    }
    private static final long serialVersionUID = 0;
  }

  Object writeReplace() {
    return new SerializedForm(this);
  }
}