https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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
 *
 * 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.cache;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.ImmutableMap;
import com.google.common.util.concurrent.ExecutionError;
import com.google.common.util.concurrent.UncheckedExecutionException;
import com.google.errorprone.annotations.CompatibleWith;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import org.checkerframework.checker.nullness.qual.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. The common way to build instances is using CacheBuilder. 
Implementations of this interface are expected to be thread-safe, and can be safely accessed
by multiple concurrent threads.

Author:Charles Fry
Since:10.0
/**
 * A semi-persistent mapping from keys to values. Cache entries are manually added using {@link
 * #get(Object, Callable)} or {@link #put(Object, Object)}, and are stored in the cache until either
 * evicted or manually invalidated. The common way to build instances is using {@link CacheBuilder}.
 *
 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
 * by multiple concurrent threads.
 *
 * @author Charles Fry
 * @since 10.0
 */
@GwtCompatible
public interface Cache<K, V> {

  
Returns the value associated with key in this cache, or null if there is no cached value for key. 
Since:11.0
/**
   * Returns the value associated with {@code key} in this cache, or {@code null} if there is no
   * cached value for {@code key}.
   *
   * @since 11.0
   */
  @Nullable
  V getIfPresent(@CompatibleWith("K") Object key);

  
Returns the value associated with key in this cache, obtaining that value from 
loader if necessary. The method improves upon the conventional "if cached, return; otherwise create, cache and return" pattern. For further improvements, use LoadingCache and its get(K) method instead of this one. 
Among the improvements that this method and LoadingCache.get(K) both provide are: 


Among the further improvements that LoadingCache can provide but this method cannot: 


Warning: For any given key, every loader used with it should compute the same value. Otherwise, a call that passes one loader may return the result of another call with a differently behaving loader. For example, a call that requests a short timeout for an RPC may wait for a similar call that requests a long timeout, or a call by an unprivileged user may return a resource accessible only to a privileged user making a similar call. To prevent this problem, create a key object that includes all values that affect the result of the query. Or use LoadingCache.get(K), which lacks the ability to refer to state other than that in the key. 
Warning: as with CacheLoader.load, loader must not return null; it may either return a non-null value or throw an exception. 
No observable state associated with this cache is modified until loading completes.

Throws:
Since:11.0
/**
   * Returns the value associated with {@code key} in this cache, obtaining that value from {@code
   * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
   * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
   * {@link LoadingCache#get(Object) get(K)} method instead of this one.
   *
   * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
   *
   * <ul>
   *   <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
   *       starting a redundant one
   *   <li>eliminating the error-prone caching boilerplate
   *   <li>tracking load {@linkplain #stats statistics}
   * </ul>
   *
   * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
   *
   * <ul>
   *   <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
   *       authoritative location}
   *   <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
   *       CacheBuilder#refreshAfterWrite automated refreshing}
   *   <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
   *       CacheLoader#loadAll bulk loading implementations}
   * </ul>
   *
   * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
   * value. Otherwise, a call that passes one {@code loader} may return the result of another call
   * with a differently behaving {@code loader}. For example, a call that requests a short timeout
   * for an RPC may wait for a similar call that requests a long timeout, or a call by an
   * unprivileged user may return a resource accessible only to a privileged user making a similar
   * call. To prevent this problem, create a key object that includes all values that affect the
   * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
   * state other than that in the key.
   *
   * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
   * {@code null}; it may either return a non-null value or throw an exception.
   *
   * <p>No observable state associated with this cache is modified until loading completes.
   *
   * @throws ExecutionException if a checked exception was thrown while loading the value
   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
   *     value
   * @throws ExecutionError if an error was thrown while loading the value
   * @since 11.0
   */
  V get(K key, Callable<? extends V> loader) throws 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. 
Since:11.0
/**
   * Returns a map of the values associated with {@code keys} in this cache. The returned map will
   * only contain entries which are already present in the cache.
   *
   * @since 11.0
   */
  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. 
Since:11.0
/**
   * Associates {@code value} with {@code key} in this cache. If the cache previously contained a
   * value associated with {@code key}, the old value is replaced by {@code value}.
   *
   * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
   * otherwise create, cache and return" pattern.
   *
   * @since 11.0
   */
  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. 
Since:12.0
/**
   * Copies all of the mappings from the specified map to the cache. The effect of this call is
   * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
   * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
   * if the specified map is modified while the operation is in progress.
   *
   * @since 12.0
   */
  void putAll(Map<? extends K, ? extends V> m);

  
Discards any cached value for key key. 
/** Discards any cached value for key {@code key}. */
  void invalidate(@CompatibleWith("K") Object key);

  
Discards any cached values for keys keys. 
Since:11.0
/**
   * Discards any cached values for keys {@code keys}.
   *
   * @since 11.0
   */
  void invalidateAll(Iterable<?> keys);

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

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

  
Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
the cache is not recording statistics. All statistics begin at zero and never decrease over the
lifetime of the cache.

Warning: this cache may not be recording statistical data. For example, a cache created using CacheBuilder only does so if the CacheBuilder.recordStats method was called. If statistics are not being recorded, a CacheStats instance with zero for all values is returned. 
/**
   * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
   * the cache is not recording statistics. All statistics begin at zero and never decrease over the
   * lifetime of the cache.
   *
   * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
   * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
   * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
   * all values is returned.
   *
   */
  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.

Iterators from the returned map are at least weakly consistent: they are safe for
concurrent use, but if the cache is modified (including by eviction) after the iterator is
created, it is undefined which of the changes (if any) will be reflected in that iterator.

/**
   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
   * the map directly affect the cache.
   *
   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
   */
  ConcurrentMap<K, V> asMap();

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

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