common/collect/MapMaker.java

/*
 * Copyright (C) 2009 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.Objects.firstNonNull;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Ascii;
import com.google.common.base.Equivalence;
import com.google.common.base.Equivalences;
import com.google.common.base.Function;
import com.google.common.base.Objects;
import com.google.common.base.Ticker;
import com.google.common.collect.ComputingConcurrentHashMap.ComputingMapAdapter;
import com.google.common.collect.MapMakerInternalMap.Strength;

import java.io.Serializable;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.Collections;
import java.util.ConcurrentModificationException;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.TimeUnit;

import javax.annotation.Nullable;

/**
 * <p>A builder of {@link ConcurrentMap} instances having any combination of the following features:
 *
 * <ul>
 * <li>keys or values automatically wrapped in {@linkplain WeakReference weak} or {@linkplain
 *     SoftReference soft} references
 * <li>least-recently-used eviction when a maximum size is exceeded
 * <li>time-based expiration of entries, measured since last access or last write
 * <li>notification of evicted (or otherwise removed) entries
 * <li>on-demand computation of values for keys not already present
 * </ul>
 *
 * <p>Usage example: <pre>   {@code
 *
 *   ConcurrentMap<Key, Graph> graphs = new MapMaker()
 *       .concurrencyLevel(4)
 *       .weakKeys()
 *       .maximumSize(10000)
 *       .expireAfterWrite(10, TimeUnit.MINUTES)
 *       .makeComputingMap(
 *           new Function<Key, Graph>() {
 *             public Graph apply(Key key) {
 *               return createExpensiveGraph(key);
 *             }
 *           });}</pre>
 *
 * These features are all optional; {@code new MapMaker().makeMap()} returns a valid concurrent map
 * that behaves similarly to a {@link ConcurrentHashMap}.
 *
 * <p>The returned map is implemented as a hash table with similar performance characteristics to
 * {@link ConcurrentHashMap}. It supports all optional operations of the {@code ConcurrentMap}
 * interface. It does not permit null keys or values.
 *
 * <p><b>Note:</b> by default, the returned map uses equality comparisons (the {@link Object#equals
 * equals} method) to determine equality for keys or values. However, if {@link #weakKeys} or {@link
 * #softKeys} was specified, the map uses identity ({@code ==}) comparisons instead for keys.
 * Likewise, if {@link #weakValues} or {@link #softValues} was specified, the map uses identity
 * comparisons for values.
 *
 * <p>The view collections of the returned map have <i>weakly consistent iterators</i>. This means
 * that they are safe for concurrent use, but if other threads modify the map after the iterator is
 * created, it is undefined which of these changes, if any, are reflected in that iterator. These
 * iterators never throw {@link ConcurrentModificationException}.
 *
 * <p>If soft or weak references were requested, it is possible for a key or value present in the
 * the map to be reclaimed by the garbage collector. If this happens, the entry automatically
 * disappears from the map. A partially-reclaimed entry is never exposed to the user. Any {@link
 * java.util.Map.Entry} instance retrieved from the map's {@linkplain Map#entrySet entry set} is a
 * snapshot of that entry's state at the time of retrieval; such entries do, however, support {@link
 * java.util.Map.Entry#setValue}, which simply calls {@link Map#put} on the entry's key.
 *
 * <p>The maps produced by {@code MapMaker} are serializable, and the deserialized maps retain all
 * the configuration properties of the original map. During deserialization, if the original map had
 * used soft or weak references, the entries are reconstructed as they were, but it's not unlikely
 * they'll be quickly garbage-collected before they are ever accessed.
 *
 * <p>{@code new MapMaker().weakKeys().makeMap()} is a recommended replacement for {@link
 * java.util.WeakHashMap}, but note that it compares keys using object identity whereas {@code
 * WeakHashMap} uses {@link Object#equals}.
 *
 * @author Bob Lee
 * @author Charles Fry
 * @author Kevin Bourrillion
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible(emulated = true)
public final class MapMaker extends GenericMapMaker<Object, Object> {
  private static final int DEFAULT_INITIAL_CAPACITY = 16;
  private static final int DEFAULT_CONCURRENCY_LEVEL = 4;
  private static final int DEFAULT_EXPIRATION_NANOS = 0;

  static final int UNSET_INT = -1;

  // TODO(kevinb): dispense with this after benchmarking
  boolean useCustomMap;

  int initialCapacity = UNSET_INT;
  int concurrencyLevel = UNSET_INT;
  int maximumSize = UNSET_INT;

  Strength keyStrength;
  Strength valueStrength;

  long expireAfterWriteNanos = UNSET_INT;
  long expireAfterAccessNanos = UNSET_INT;

  RemovalCause nullRemovalCause;

  Equivalence<Object> keyEquivalence;
  Equivalence<Object> valueEquivalence;

  Ticker ticker;

  /**
   * Constructs a new {@code MapMaker} instance with default settings, including strong keys, strong
   * values, and no automatic eviction of any kind.
   */
  public MapMaker() {}

  private boolean useNullMap() {
    return (nullRemovalCause == null);
  }

  /**
   * Sets a custom {@code Equivalence} strategy for comparing keys.
   *
   * <p>By default, the map uses {@link Equivalences#identity} to determine key equality when
   * {@link #weakKeys} or {@link #softKeys} is specified, and {@link Equivalences#equals()}
   * otherwise.
   */
  @GwtIncompatible("To be supported")
  @Override
  MapMaker keyEquivalence(Equivalence<Object> equivalence) {
    checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
    keyEquivalence = checkNotNull(equivalence);
    this.useCustomMap = true;
    return this;
  }

  Equivalence<Object> getKeyEquivalence() {
    return firstNonNull(keyEquivalence, getKeyStrength().defaultEquivalence());
  }

  /**
   * Sets a custom {@code Equivalence} strategy for comparing values.
   *
   * <p>By default, the map uses {@link Equivalences#identity} to determine value equality when
   * {@link #weakValues} or {@link #softValues} is specified, and {@link Equivalences#equals()}
   * otherwise.
   */
  @GwtIncompatible("To be supported")
  @Override
  MapMaker valueEquivalence(Equivalence<Object> equivalence) {
    checkState(valueEquivalence == null,
        "value equivalence was already set to %s", valueEquivalence);
    this.valueEquivalence = checkNotNull(equivalence);
    this.useCustomMap = true;
    return this;
  }

  Equivalence<Object> getValueEquivalence() {
    return firstNonNull(valueEquivalence, getValueStrength().defaultEquivalence());
  }

  /**
   * Sets the minimum total size for the internal hash tables. For example, if the initial capacity
   * is {@code 60}, and the concurrency level is {@code 8}, then eight segments are created, each
   * having a hash table of size eight. Providing a large enough estimate at construction time
   * avoids the need for expensive resizing operations later, but setting this value unnecessarily
   * high wastes memory.
   *
   * @throws IllegalArgumentException if {@code initialCapacity} is negative
   * @throws IllegalStateException if an initial capacity was already set
   */
  @Override
  public MapMaker initialCapacity(int initialCapacity) {
    checkState(this.initialCapacity == UNSET_INT, "initial capacity was already set to %s",
        this.initialCapacity);
    checkArgument(initialCapacity >= 0);
    this.initialCapacity = initialCapacity;
    return this;
  }

  int getInitialCapacity() {
    return (initialCapacity == UNSET_INT) ? DEFAULT_INITIAL_CAPACITY : initialCapacity;
  }

  /**
   * Specifies the maximum number of entries the map may contain. Note that the map <b>may evict an
   * entry before this limit is exceeded</b>. As the map size grows close to the maximum, the map
   * evicts entries that are less likely to be used again. For example, the map may evict an entry
   * because it hasn't been used recently or very often.
   *
   * <p>When {@code size} is zero, elements can be successfully added to the map, but are evicted
   * immediately. This has the same effect as invoking {@link #expireAfterWrite
   * expireAfterWrite}{@code (0, unit)} or {@link #expireAfterAccess expireAfterAccess}{@code (0,
   * unit)}. It can be useful in testing, or to disable caching temporarily without a code change.
   *
   * <p>Caching functionality in {@code MapMaker} is being moved to
   * {@link com.google.common.cache.CacheBuilder}.
   *
   * @param size the maximum size of the map
   * @throws IllegalArgumentException if {@code size} is negative
   * @throws IllegalStateException if a maximum size was already set
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}, with {@link #maximumSize} being
   *     replaced by {@link com.google.common.cache.CacheBuilder#maximumSize}.
   */
  @Deprecated
  @Override
  MapMaker maximumSize(int size) {
    checkState(this.maximumSize == UNSET_INT, "maximum size was already set to %s",
        this.maximumSize);
    checkArgument(size >= 0, "maximum size must not be negative");
    this.maximumSize = size;
    this.useCustomMap = true;
    if (maximumSize == 0) {
      // SIZE trumps EXPIRED
      this.nullRemovalCause = RemovalCause.SIZE;
    }
    return this;
  }

  /**
   * Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The
   * table is internally partitioned to try to permit the indicated number of concurrent updates
   * without contention. Because assignment of entries to these partitions is not necessarily
   * uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
   * accommodate as many threads as will ever concurrently modify the table. Using a significantly
   * higher value than you need can waste space and time, and a significantly lower value can lead
   * to thread contention. But overestimates and underestimates within an order of magnitude do not
   * usually have much noticeable impact. A value of one permits only one thread to modify the map
   * at a time, but since read operations can proceed concurrently, this still yields higher
   * concurrency than full synchronization. Defaults to 4.
   *
   * <p><b>Note:</b> Prior to Guava release 9.0, the default was 16. It is possible the default will
   * change again in the future. If you care about this value, you should always choose it
   * explicitly.
   *
   * @throws IllegalArgumentException if {@code concurrencyLevel} is nonpositive
   * @throws IllegalStateException if a concurrency level was already set
   */
  @Override
  public MapMaker concurrencyLevel(int concurrencyLevel) {
    checkState(this.concurrencyLevel == UNSET_INT, "concurrency level was already set to %s",
        this.concurrencyLevel);
    checkArgument(concurrencyLevel > 0);
    this.concurrencyLevel = concurrencyLevel;
    return this;
  }

  int getConcurrencyLevel() {
    return (concurrencyLevel == UNSET_INT) ? DEFAULT_CONCURRENCY_LEVEL : concurrencyLevel;
  }

  /**
   * Specifies that each key (not value) stored in the map should be strongly referenced.
   *
   * @throws IllegalStateException if the key strength was already set
   */
  @Override
  MapMaker strongKeys() {
    return setKeyStrength(Strength.STRONG);
  }

  /**
   * Specifies that each key (not value) stored in the map should be wrapped in a {@link
   * WeakReference} (by default, strong references are used).
   *
   * <p><b>Warning:</b> when this method is used, the resulting map will use identity ({@code ==})
   * comparison to determine equality of keys, which is a technical violation of the {@link Map}
   * specification, and may not be what you expect.
   *
   * @throws IllegalStateException if the key strength was already set
   * @see WeakReference
   */
  @GwtIncompatible("java.lang.ref.WeakReference")
  @Override
  public MapMaker weakKeys() {
    return setKeyStrength(Strength.WEAK);
  }

  /**
   * <b>This method is broken.</b> Maps with soft keys offer no functional advantage over maps with
   * weak keys, and they waste memory by keeping unreachable elements in the map. If your goal is to
   * create a memory-sensitive map, then consider using soft values instead.
   *
   * <p>Specifies that each key (not value) stored in the map should be wrapped in a
   * {@link SoftReference} (by default, strong references are used). Softly-referenced objects will
   * be garbage-collected in a <i>globally</i> least-recently-used manner, in response to memory
   * demand.
   *
   * <p><b>Warning:</b> when this method is used, the resulting map will use identity ({@code ==})
   * comparison to determine equality of keys, which is a technical violation of the {@link Map}
   * specification, and may not be what you expect.
   *
   * @throws IllegalStateException if the key strength was already set
   * @see SoftReference
   * @deprecated use {@link #softValues} to create a memory-sensitive map, or {@link #weakKeys} to
   *     create a map that doesn't hold strong references to the keys.
   *     <b>This method is scheduled for deletion in January 2013.</b>
   */
  @Deprecated
  @GwtIncompatible("java.lang.ref.SoftReference")
  @Override
  public MapMaker softKeys() {
    return setKeyStrength(Strength.SOFT);
  }

  MapMaker setKeyStrength(Strength strength) {
    checkState(keyStrength == null, "Key strength was already set to %s", keyStrength);
    keyStrength = checkNotNull(strength);
    if (strength != Strength.STRONG) {
      // STRONG could be used during deserialization.
      useCustomMap = true;
    }
    return this;
  }

  Strength getKeyStrength() {
    return firstNonNull(keyStrength, Strength.STRONG);
  }

  /**
   * Specifies that each value (not key) stored in the map should be strongly referenced.
   *
   * @throws IllegalStateException if the value strength was already set
   */
  @Override
  MapMaker strongValues() {
    return setValueStrength(Strength.STRONG);
  }

  /**
   * Specifies that each value (not key) stored in the map should be wrapped in a
   * {@link WeakReference} (by default, strong references are used).
   *
   * <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
   * candidate for caching; consider {@link #softValues} instead.
   *
   * <p><b>Warning:</b> when this method is used, the resulting map will use identity ({@code ==})
   * comparison to determine equality of values. This technically violates the specifications of
   * the methods {@link Map#containsValue containsValue},
   * {@link ConcurrentMap#remove(Object, Object) remove(Object, Object)} and
   * {@link ConcurrentMap#replace(Object, Object, Object) replace(K, V, V)}, and may not be what you
   * expect.
   *
   * @throws IllegalStateException if the value strength was already set
   * @see WeakReference
   */
  @GwtIncompatible("java.lang.ref.WeakReference")
  @Override
  public MapMaker weakValues() {
    return setValueStrength(Strength.WEAK);
  }

  /**
   * Specifies that each value (not key) stored in the map should be wrapped in a
   * {@link SoftReference} (by default, strong references are used). Softly-referenced objects will
   * be garbage-collected in a <i>globally</i> least-recently-used manner, in response to memory
   * demand.
   *
   * <p><b>Warning:</b> in most circumstances it is better to set a per-cache {@linkplain
   * #maximumSize maximum size} instead of using soft references. You should only use this method if
   * you are well familiar with the practical consequences of soft references.
   *
   * <p><b>Warning:</b> when this method is used, the resulting map will use identity ({@code ==})
   * comparison to determine equality of values. This technically violates the specifications of
   * the methods {@link Map#containsValue containsValue},
   * {@link ConcurrentMap#remove(Object, Object) remove(Object, Object)} and
   * {@link ConcurrentMap#replace(Object, Object, Object) replace(K, V, V)}, and may not be what you
   * expect.
   *
   * @throws IllegalStateException if the value strength was already set
   * @see SoftReference
   */
  @GwtIncompatible("java.lang.ref.SoftReference")
  @Override
  public MapMaker softValues() {
    return setValueStrength(Strength.SOFT);
  }

  MapMaker setValueStrength(Strength strength) {
    checkState(valueStrength == null, "Value strength was already set to %s", valueStrength);
    valueStrength = checkNotNull(strength);
    if (strength != Strength.STRONG) {
      // STRONG could be used during deserialization.
      useCustomMap = true;
    }
    return this;
  }

  Strength getValueStrength() {
    return firstNonNull(valueStrength, Strength.STRONG);
  }

  /**
   * Old name of {@link #expireAfterWrite}.
   *
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}. Functionality equivalent to
   *     {@link MapMaker#expiration} is provided by
   *     {@link com.google.common.cache.CacheBuilder#expireAfterWrite}.
   *     <b>This method is scheduled for deletion in July 2012.</b>
   */
  @Deprecated
  @Override
  public
  MapMaker expiration(long duration, TimeUnit unit) {
    return expireAfterWrite(duration, unit);
  }

  /**
   * Specifies that each entry should be automatically removed from the map once a fixed duration
   * has elapsed after the entry's creation, or the most recent replacement of its value.
   *
   * <p>When {@code duration} is zero, elements can be successfully added to the map, but are
   * evicted immediately. This has a very similar effect to invoking {@link #maximumSize
   * maximumSize}{@code (0)}. It can be useful in testing, or to disable caching temporarily without
   * a code change.
   *
   * <p>Expired entries may be counted by {@link Map#size}, but will never be visible to read or
   * write operations. Expired entries are currently cleaned up during write operations, or during
   * occasional read operations in the absense of writes; though this behavior may change in the
   * future.
   *
   * @param duration the length of time after an entry is created that it should be automatically
   *     removed
   * @param unit the unit that {@code duration} is expressed in
   * @throws IllegalArgumentException if {@code duration} is negative
   * @throws IllegalStateException if the time to live or time to idle was already set
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}, with {@link #expireAfterWrite} being
   *     replaced by {@link com.google.common.cache.CacheBuilder#expireAfterWrite}.
   */
  @Deprecated
  @Override
  MapMaker expireAfterWrite(long duration, TimeUnit unit) {
    checkExpiration(duration, unit);
    this.expireAfterWriteNanos = unit.toNanos(duration);
    if (duration == 0 && this.nullRemovalCause == null) {
      // SIZE trumps EXPIRED
      this.nullRemovalCause = RemovalCause.EXPIRED;
    }
    useCustomMap = true;
    return this;
  }

  private void checkExpiration(long duration, TimeUnit unit) {
    checkState(expireAfterWriteNanos == UNSET_INT, "expireAfterWrite was already set to %s ns",
        expireAfterWriteNanos);
    checkState(expireAfterAccessNanos == UNSET_INT, "expireAfterAccess was already set to %s ns",
        expireAfterAccessNanos);
    checkArgument(duration >= 0, "duration cannot be negative: %s %s", duration, unit);
  }

  long getExpireAfterWriteNanos() {
    return (expireAfterWriteNanos == UNSET_INT) ? DEFAULT_EXPIRATION_NANOS : expireAfterWriteNanos;
  }

  /**
   * Specifies that each entry should be automatically removed from the map once a fixed duration
   * has elapsed after the entry's last read or write access.
   *
   * <p>When {@code duration} is zero, elements can be successfully added to the map, but are
   * evicted immediately. This has a very similar effect to invoking {@link #maximumSize
   * maximumSize}{@code (0)}. It can be useful in testing, or to disable caching temporarily without
   * a code change.
   *
   * <p>Expired entries may be counted by {@link Map#size}, but will never be visible to read or
   * write operations. Expired entries are currently cleaned up during write operations, or during
   * occasional read operations in the absense of writes; though this behavior may change in the
   * future.
   *
   * @param duration the length of time after an entry is last accessed that it should be
   *     automatically removed
   * @param unit the unit that {@code duration} is expressed in
   * @throws IllegalArgumentException if {@code duration} is negative
   * @throws IllegalStateException if the time to idle or time to live was already set
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}, with {@link #expireAfterAccess} being
   *     replaced by {@link com.google.common.cache.CacheBuilder#expireAfterAccess}.
   */
  @Deprecated
  @GwtIncompatible("To be supported")
  @Override
  MapMaker expireAfterAccess(long duration, TimeUnit unit) {
    checkExpiration(duration, unit);
    this.expireAfterAccessNanos = unit.toNanos(duration);
    if (duration == 0 && this.nullRemovalCause == null) {
      // SIZE trumps EXPIRED
      this.nullRemovalCause = RemovalCause.EXPIRED;
    }
    useCustomMap = true;
    return this;
  }

  long getExpireAfterAccessNanos() {
    return (expireAfterAccessNanos == UNSET_INT)
        ? DEFAULT_EXPIRATION_NANOS : expireAfterAccessNanos;
  }

  Ticker getTicker() {
    return firstNonNull(ticker, Ticker.systemTicker());
  }

  /**
   * Specifies a listener instance, which all maps built using this {@code MapMaker} will notify
   * each time an entry is removed from the map by any means.
   *
   * <p>Each map built by this map maker after this method is called invokes the supplied listener
   * after removing an element for any reason (see removal causes in {@link RemovalCause}). It will
   * invoke the listener during invocations of any of that map's public methods (even read-only
   * methods).
   *
   * <p><b>Important note:</b> Instead of returning <i>this</i> as a {@code MapMaker} instance,
   * this method returns {@code GenericMapMaker<K, V>}. From this point on, either the original
   * reference or the returned reference may be used to complete configuration and build the map,
   * but only the "generic" one is type-safe. That is, it will properly prevent you from building
   * maps whose key or value types are incompatible with the types accepted by the listener already
   * provided; the {@code MapMaker} type cannot do this. For best results, simply use the standard
   * method-chaining idiom, as illustrated in the documentation at top, configuring a {@code
   * MapMaker} and building your {@link Map} all in a single statement.
   *
   * <p><b>Warning:</b> if you ignore the above advice, and use this {@code MapMaker} to build a map
   * or cache whose key or value type is incompatible with the listener, you will likely experience
   * a {@link ClassCastException} at some <i>undefined</i> point in the future.
   *
   * @throws IllegalStateException if a removal listener was already set
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}, with {@link #removalListener} being
   *     replaced by {@link com.google.common.cache.CacheBuilder#removalListener}.
   */
  @Deprecated
  @GwtIncompatible("To be supported")
  <K, V> GenericMapMaker<K, V> removalListener(RemovalListener<K, V> listener) {
    checkState(this.removalListener == null);

    // safely limiting the kinds of maps this can produce
    @SuppressWarnings("unchecked")
    GenericMapMaker<K, V> me = (GenericMapMaker<K, V>) this;
    me.removalListener = checkNotNull(listener);
    useCustomMap = true;
    return me;
  }

  /**
   * Builds a thread-safe map, without on-demand computation of values. This method does not alter
   * the state of this {@code MapMaker} instance, so it can be invoked again to create multiple
   * independent maps.
   *
   * <p>The bulk operations {@code putAll}, {@code equals}, and {@code clear} are not guaranteed to
   * be performed atomically on the returned map. Additionally, {@code size} and {@code
   * containsValue} are implemented as bulk read operations, and thus may fail to observe concurrent
   * writes.
   *
   * @return a serializable concurrent map having the requested features
   */
  @Override
  public <K, V> ConcurrentMap<K, V> makeMap() {
    if (!useCustomMap) {
      return new ConcurrentHashMap<K, V>(getInitialCapacity(), 0.75f, getConcurrencyLevel());
    }
    return (nullRemovalCause == null)
        ? new MapMakerInternalMap<K, V>(this)
        : new NullConcurrentMap<K, V>(this);
  }

  /**
   * Returns a MapMakerInternalMap for the benefit of internal callers that use features of
   * that class not exposed through ConcurrentMap.
   */
  @Override
  @GwtIncompatible("MapMakerInternalMap")
  <K, V> MapMakerInternalMap<K, V> makeCustomMap() {
    return new MapMakerInternalMap<K, V>(this);
  }

  /**
   * Builds a map that supports atomic, on-demand computation of values. {@link Map#get} either
   * returns an already-computed value for the given key, atomically computes it using the supplied
   * function, or, if another thread is currently computing the value for this key, simply waits for
   * that thread to finish and returns its computed value. Note that the function may be executed
   * concurrently by multiple threads, but only for distinct keys.
   *
   * <p>New code should use {@link com.google.common.cache.CacheBuilder}, which supports
   * {@linkplain com.google.common.cache.CacheStats statistics} collection, introduces the
   * {@link com.google.common.cache.CacheLoader} interface for loading entries into the cache
   * (allowing checked exceptions to be thrown in the process), and more cleanly separates
   * computation from the cache's {@code Map} view.
   *
   * <p>If an entry's value has not finished computing yet, query methods besides {@code get} return
   * immediately as if an entry doesn't exist. In other words, an entry isn't externally visible
   * until the value's computation completes.
   *
   * <p>{@link Map#get} on the returned map will never return {@code null}. It may throw:
   *
   * <ul>
   * <li>{@link NullPointerException} if the key is null or the computing function returns a null
   *     result
   * <li>{@link ComputationException} if an exception was thrown by the computing function. If that
   * exception is already of type {@link ComputationException} it is propagated directly; otherwise
   * it is wrapped.
   * </ul>
   *
   * <p><b>Note:</b> Callers of {@code get} <i>must</i> ensure that the key argument is of type
   * {@code K}. The {@code get} method accepts {@code Object}, so the key type is not checked at
   * compile time. Passing an object of a type other than {@code K} can result in that object being
   * unsafely passed to the computing function as type {@code K}, and unsafely stored in the map.
   *
   * <p>If {@link Map#put} is called before a computation completes, other threads waiting on the
   * computation will wake up and return the stored value.
   *
   * <p>This method does not alter the state of this {@code MapMaker} instance, so it can be invoked
   * again to create multiple independent maps.
   *
   * <p>Insertion, removal, update, and access operations on the returned map safely execute
   * concurrently by multiple threads. Iterators on the returned map are weakly consistent,
   * returning elements reflecting the state of the map at some point at or since the creation of
   * the iterator. They do not throw {@link ConcurrentModificationException}, and may proceed
   * concurrently with other operations.
   *
   * <p>The bulk operations {@code putAll}, {@code equals}, and {@code clear} are not guaranteed to
   * be performed atomically on the returned map. Additionally, {@code size} and {@code
   * containsValue} are implemented as bulk read operations, and thus may fail to observe concurrent
   * writes.
   *
   * @param computingFunction the function used to compute new values
   * @return a serializable concurrent map having the requested features
   * @deprecated Caching functionality in {@code MapMaker} is being moved to
   *     {@link com.google.common.cache.CacheBuilder}, with {@link #makeComputingMap} being replaced
   *     by {@link com.google.common.cache.CacheBuilder#build}. Note that uses of
   *     {@link #makeComputingMap} with {@code AtomicLong} values can often be migrated to
   *     {@link AtomicLongMap}.
   *     <b>This method is scheduled for deletion in February 2013.</b>
   *
   */
  @Deprecated
  @Override
  public <K, V> ConcurrentMap<K, V> makeComputingMap(
      Function<? super K, ? extends V> computingFunction) {
    return useNullMap()
        ? new ComputingMapAdapter<K, V>(this, computingFunction)
        : new NullComputingConcurrentMap<K, V>(this, computingFunction);
  }

  /**
   * Returns a string representation for this MapMaker instance. The exact form of the returned
   * string is not specificed.
   */
  @Override
  public String toString() {
    Objects.ToStringHelper s = Objects.toStringHelper(this);
    if (initialCapacity != UNSET_INT) {
      s.add("initialCapacity", initialCapacity);
    }
    if (concurrencyLevel != UNSET_INT) {
      s.add("concurrencyLevel", concurrencyLevel);
    }
    if (maximumSize != UNSET_INT) {
      s.add("maximumSize", maximumSize);
    }
    if (expireAfterWriteNanos != UNSET_INT) {
      s.add("expireAfterWrite", expireAfterWriteNanos + "ns");
    }
    if (expireAfterAccessNanos != UNSET_INT) {
      s.add("expireAfterAccess", expireAfterAccessNanos + "ns");
    }
    if (keyStrength != null) {
      s.add("keyStrength", Ascii.toLowerCase(keyStrength.toString()));
    }
    if (valueStrength != null) {
      s.add("valueStrength", Ascii.toLowerCase(valueStrength.toString()));
    }
    if (keyEquivalence != null) {
      s.addValue("keyEquivalence");
    }
    if (valueEquivalence != null) {
      s.addValue("valueEquivalence");
    }
    if (removalListener != null) {
      s.addValue("removalListener");
    }
    return s.toString();
  }

  /**
   * An object that can receive a notification when an entry is removed from a map. The removal
   * resulting in notification could have occured to an entry being manually removed or replaced, or
   * due to eviction resulting from timed expiration, exceeding a maximum size, or garbage
   * collection.
   *
   * <p>An instance may be called concurrently by multiple threads to process different entries.
   * Implementations of this interface should avoid performing blocking calls or synchronizing on
   * shared resources.
   *
   * @param <K> the most general type of keys this listener can listen for; for
   *     example {@code Object} if any key is acceptable
   * @param <V> the most general type of values this listener can listen for; for
   *     example {@code Object} if any key is acceptable
   */
  interface RemovalListener<K, V> {
    /**
     * Notifies the listener that a removal occurred at some point in the past.
     */
    void onRemoval(RemovalNotification<K, V> notification);
  }

  /**
   * A notification of the removal of a single entry. The key or value may be null if it was already
   * garbage collected.
   *
   * <p>Like other {@code Map.Entry} instances associated with MapMaker, this class holds strong
   * references to the key and value, regardless of the type of references the map may be using.
   */
  static final class RemovalNotification<K, V> extends ImmutableEntry<K, V> {
    private static final long serialVersionUID = 0;

    private final RemovalCause cause;

    RemovalNotification(@Nullable K key, @Nullable V value, RemovalCause cause) {
      super(key, value);
      this.cause = cause;
    }

    /**
     * Returns the cause for which the entry was removed.
     */
    public RemovalCause getCause() {
      return cause;
    }

    /**
     * Returns {@code true} if there was an automatic removal due to eviction (the cause is neither
     * {@link RemovalCause#EXPLICIT} nor {@link RemovalCause#REPLACED}).
     */
    public boolean wasEvicted() {
      return cause.wasEvicted();
    }
  }

  /**
   * The reason why an entry was removed.
   */
  enum RemovalCause {
    /**
     * The entry was manually removed by the user. This can result from the user invoking
     * {@link Map#remove}, {@link ConcurrentMap#remove}, or {@link java.util.Iterator#remove}.
     */
    EXPLICIT {
      @Override
      boolean wasEvicted() {
        return false;
      }
    },

    /**
     * The entry itself was not actually removed, but its value was replaced by the user. This can
     * result from the user invoking {@link Map#put}, {@link Map#putAll},
     * {@link ConcurrentMap#replace(Object, Object)}, or
     * {@link ConcurrentMap#replace(Object, Object, Object)}.
     */
    REPLACED {
      @Override
      boolean wasEvicted() {
        return false;
      }
    },

    /**
     * The entry was removed automatically because its key or value was garbage-collected. This
     * can occur when using {@link #softKeys}, {@link #softValues}, {@link #weakKeys}, or {@link
     * #weakValues}.
     */
    COLLECTED {
      @Override
      boolean wasEvicted() {
        return true;
      }
    },

    /**
     * The entry's expiration timestamp has passed. This can occur when using {@link
     * #expireAfterWrite} or {@link #expireAfterAccess}.
     */
    EXPIRED {
      @Override
      boolean wasEvicted() {
        return true;
      }
    },

    /**
     * The entry was evicted due to size constraints. This can occur when using {@link
     * #maximumSize}.
     */
    SIZE {
      @Override
      boolean wasEvicted() {
        return true;
      }
    };

    /**
     * Returns {@code true} if there was an automatic removal due to eviction (the cause is neither
     * {@link #EXPLICIT} nor {@link #REPLACED}).
     */
    abstract boolean wasEvicted();
  }

  /** A map that is always empty and evicts on insertion. */
  static class NullConcurrentMap<K, V> extends AbstractMap<K, V>
      implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 0;

    private final RemovalListener<K, V> removalListener;
    private final RemovalCause removalCause;

    NullConcurrentMap(MapMaker mapMaker) {
      removalListener = mapMaker.getRemovalListener();
      removalCause = mapMaker.nullRemovalCause;
    }

    // implements ConcurrentMap

    @Override
    public boolean containsKey(@Nullable Object key) {
      return false;
    }

    @Override
    public boolean containsValue(@Nullable Object value) {
      return false;
    }

    @Override
    public V get(@Nullable Object key) {
      return null;
    }

    void notifyRemoval(K key, V value) {
      RemovalNotification<K, V> notification =
          new RemovalNotification<K, V>(key, value, removalCause);
      removalListener.onRemoval(notification);
    }

    @Override
    public V put(K key, V value) {
      checkNotNull(key);
      checkNotNull(value);
      notifyRemoval(key, value);
      return null;
    }

    @Override
    public V putIfAbsent(K key, V value) {
      return put(key, value);
    }

    @Override
    public V remove(@Nullable Object key) {
      return null;
    }

    @Override
    public boolean remove(@Nullable Object key, @Nullable Object value) {
      return false;
    }

    @Override
    public V replace(K key, V value) {
      checkNotNull(key);
      checkNotNull(value);
      return null;
    }

    @Override
    public boolean replace(K key, @Nullable V oldValue, V newValue) {
      checkNotNull(key);
      checkNotNull(newValue);
      return false;
    }

    @Override
    public Set<Entry<K, V>> entrySet() {
      return Collections.emptySet();
    }
  }

  /** Computes on retrieval and evicts the result. */
  static final class NullComputingConcurrentMap<K, V> extends NullConcurrentMap<K, V> {
    private static final long serialVersionUID = 0;

    final Function<? super K, ? extends V> computingFunction;

    NullComputingConcurrentMap(
        MapMaker mapMaker, Function<? super K, ? extends V> computingFunction) {
      super(mapMaker);
      this.computingFunction = checkNotNull(computingFunction);
    }

    @SuppressWarnings("unchecked") // unsafe, which is why Cache is preferred
    @Override
    public V get(Object k) {
      K key = (K) k;
      V value = compute(key);
      checkNotNull(value, computingFunction + " returned null for key " + key + ".");
      notifyRemoval(key, value);
      return value;
    }

    private V compute(K key) {
      checkNotNull(key);
      try {
        return computingFunction.apply(key);
      } catch (ComputationException e) {
        throw e;
      } catch (Throwable t) {
        throw new ComputationException(t);
      }
    }
  }

}