com.google.common.collect

## Class MapMaker

- java.lang.Object
- - com.google.common.collect.MapMaker

-     @GwtCompatible(emulated=true)
      public final class MapMaker
      extends java.lang.Object

  A builder of `ConcurrentMap` instances having any combination of the following features:

  - keys or values automatically wrapped in weak or soft references
  - notification of evicted (or otherwise removed) entries

  Usage example:

         ConcurrentMap<Request, Stopwatch> timers = new MapMaker()
             .concurrencyLevel(4)
             .weakKeys()
             .makeMap();

  These features are all optional; `new MapMaker().makeMap()` returns a valid concurrent map that behaves similarly to a `ConcurrentHashMap`.

  The returned map is implemented as a hash table with similar performance characteristics to `ConcurrentHashMap`. It supports all optional operations of the `ConcurrentMap` interface. It does not permit null keys or values.

  Note: by default, the returned map uses equality comparisons (the `equals` method) to determine equality for keys or values. However, if `weakKeys()` was specified, the map uses identity (`==`) comparisons instead for keys. Likewise, if `weakValues()` or `softValues()` was specified, the map uses identity comparisons for values.

  The view collections of the returned map have weakly consistent iterators. 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 `ConcurrentModificationException`.

  If `weakKeys()`, `weakValues()`, or `softValues()` are requested, it is possible for a key or value present in the map to be reclaimed by the garbage collector. Entries with reclaimed keys or values may be removed from the map on each map modification or on occasional map accesses; such entries may be counted by `Map.size()`, but will never be visible to read or write operations. A partially-reclaimed entry is never exposed to the user. Any `Map.Entry` instance retrieved from the map's entry set is a snapshot of that entry's state at the time of retrieval; such entries do, however, support `Map.Entry.setValue(V)`, which simply calls `Map.put(K, V)` on the entry's key.

  The maps produced by `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.

  `new MapMaker().weakKeys().makeMap()` is a recommended replacement for `WeakHashMap`, but note that it compares keys using object identity whereas `WeakHashMap` uses `Object.equals(java.lang.Object)`.

  Since:
    2.0

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `MapMaker()` Constructs a new `MapMaker` instance with default settings, including strong keys, strong values, and no automatic eviction of any kind.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `MapMaker` | `concurrencyLevel(int concurrencyLevel)` Guides the allowed concurrency among update operations.
    `MapMaker` | `initialCapacity(int initialCapacity)` Sets the minimum total size for the internal hash tables.
    `<K,V> java.util.concurrent.ConcurrentMap<K,V>` | `makeMap()` Builds a thread-safe map.
    `java.lang.String` | `toString()` Returns a string representation for this MapMaker instance.
    `MapMaker` | `weakKeys()` Specifies that each key (not value) stored in the map should be wrapped in a `WeakReference` (by default, strong references are used).
    `MapMaker` | `weakValues()` Specifies that each value (not key) stored in the map should be wrapped in a `WeakReference` (by default, strong references are used).

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- - ### Constructor Detail

    - #### MapMaker

          public MapMaker()

      Constructs a new `MapMaker` instance with default settings, including strong keys, strong values, and no automatic eviction of any kind.

  - ### Method Detail

    - #### initialCapacity

          public MapMaker initialCapacity(int initialCapacity)

      Sets the minimum total size for the internal hash tables. For example, if the initial capacity is `60`, and the concurrency level is `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:
        `java.lang.IllegalArgumentException` - if `initialCapacity` is negative
        `java.lang.IllegalStateException` - if an initial capacity was already set

    - #### concurrencyLevel

          public MapMaker concurrencyLevel(int concurrencyLevel)

      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.

      Note: 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:
        `java.lang.IllegalArgumentException` - if `concurrencyLevel` is nonpositive
        `java.lang.IllegalStateException` - if a concurrency level was already set

    - #### weakKeys

          @GwtIncompatible(value="java.lang.ref.WeakReference")
          public MapMaker weakKeys()

      Specifies that each key (not value) stored in the map should be wrapped in a `WeakReference` (by default, strong references are used).

      Warning: when this method is used, the resulting map will use identity (`==`) comparison to determine equality of keys, which is a technical violation of the `Map` specification, and may not be what you expect.

      Throws:
        `java.lang.IllegalStateException` - if the key strength was already set
      See Also:
        `WeakReference`

    - #### weakValues

          @GwtIncompatible(value="java.lang.ref.WeakReference")
          public MapMaker weakValues()

      Specifies that each value (not key) stored in the map should be wrapped in a `WeakReference` (by default, strong references are used).

      Weak values will be garbage collected once they are weakly reachable. This makes them a poor candidate for caching; consider `softValues()` instead.

      Warning: when this method is used, the resulting map will use identity (`==`) comparison to determine equality of values. This technically violates the specifications of the methods `containsValue`, `remove(Object, Object)` and `replace(K, V, V)`, and may not be what you expect.

      Throws:
        `java.lang.IllegalStateException` - if the value strength was already set
      See Also:
        `WeakReference`

    - #### makeMap

          public <K,V> java.util.concurrent.ConcurrentMap<K,V> makeMap()

      Builds a thread-safe map. This method does not alter the state of this `MapMaker` instance, so it can be invoked again to create multiple independent maps.

      The bulk operations `putAll`, `equals`, and `clear` are not guaranteed to be performed atomically on the returned map. Additionally, `size` and `containsValue` are implemented as bulk read operations, and thus may fail to observe concurrent writes.

      Returns:
        a serializable concurrent map having the requested features

    - #### toString

          public java.lang.String toString()

      Returns a string representation for this MapMaker instance. The exact form of the returned string is not specificed.
      Overrides:
        `toString` in class `java.lang.Object`