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   * Copyright (C) 2011 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
  * 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.
 import java.util.Map;
 import  javax.annotation.Nullable;

A semi-persistent mapping from keys to values. Cache entries are manually added using get(Object, Callable) or put(Object, Object), and are stored in the cache until either evicted or manually invalidated.

Implementations of this interface are expected to be thread-safe, and can be safely accessed by multiple concurrent threads.

Note that while this class is still annotated as Beta, the API is frozen from a consumer's standpoint. In other words existing methods are all considered non-Beta and won't be changed without going through an 18 month deprecation cycle; however new methods may be added at any time.

Charles Fry
 public interface Cache<K, V> {

Returns the value associated with key in this cache, or null if there is no cached value for key.

   V getIfPresent(Object key);

Returns the value associated with key in this cache, obtaining that value from valueLoader if necessary. No observable state associated with this cache is modified until loading completes. This method provides a simple substitute for the conventional "if cached, return; otherwise create, cache and return" pattern.

Warning: as with CacheLoader.load, valueLoader must not return null; it may either return a non-null value or throw an exception.

ExecutionException if a checked exception was thrown while loading the value
UncheckedExecutionException if an unchecked exception was thrown while loading the value
ExecutionError if an error was thrown while loading the value
   V get(K keyCallable<? extends V> valueLoaderthrows ExecutionException;

Returns a map of the values associated with keys in this cache. The returned map will only contain entries which are already present in the cache.

   ImmutableMap<K, V> getAllPresent(Iterable<?> keys);

Associates value with key in this cache. If the cache previously contained a value associated with key, the old value is replaced by value.

Prefer get(Object, Callable) when using the conventional "if cached, return; otherwise create, cache and return" pattern.

   void put(K key, V value);

Copies all of the mappings from the specified map to the cache. The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.

  void putAll(Map<? extends K,? extends V> m);

Discards any cached value for key key.
  void invalidate(Object key);

Discards any cached values for keys keys.

  void invalidateAll(Iterable<?> keys);

Discards all entries in the cache.
  void invalidateAll();

Returns the approximate number of entries in this cache.
  long size();

Returns a current snapshot of this cache's cumulative statistics. All stats are initialized to zero, and are monotonically increasing over the lifetime of the cache.
  CacheStats stats();

Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to the map directly affect the cache.
  ConcurrentMap<K, V> asMap();

Performs any pending maintenance operations needed by the cache. Exactly which activities are performed -- if any -- is implementation-dependent.
  void cleanUp();
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