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 static com.google.common.base.Preconditions.checkArgument;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.MoreObjects;
import com.google.common.base.Objects;
import java.util.concurrent.Callable;
import org.checkerframework.checker.nullness.qual.Nullable;


Statistics about the performance of a Cache. Instances of this class are immutable. 
Cache statistics are incremented according to the following rules:

    
  
  • When a cache lookup encounters an existing cache entry hitCount is incremented. 
  • When a cache lookup first encounters a missing cache entry, a new entry is loaded.
      
      
        
    • After successfully loading an entry missCount and loadSuccessCount are incremented, and the total loading time, in nanoseconds, is added to 
            totalLoadTime. 
    • When an exception is thrown while loading an entry, missCount and 
            loadExceptionCount are incremented, and the total loading time, in nanoseconds, is added to totalLoadTime. 
    • Cache lookups that encounter a missing cache entry that is still loading will wait for loading to complete (whether successful or not) and then increment 
            missCount. 
    
  
  • When an entry is evicted from the cache, evictionCount is incremented. 
  • No stats are modified when a cache entry is invalidated or manually removed.
  
  • No stats are modified by operations invoked on the asMap view of the cache. 


A lookup is specifically defined as an invocation of one of the methods LoadingCache.get(Object), LoadingCache.getUnchecked(Object), Cache.get(Object, Callable), or LoadingCache.getAll(Iterable). 
Author:Charles Fry
Since:10.0
/**
 * Statistics about the performance of a {@link Cache}. Instances of this class are immutable.
 *
 * <p>Cache statistics are incremented according to the following rules:
 *
 * <ul>
 *   <li>When a cache lookup encounters an existing cache entry {@code hitCount} is incremented.
 *   <li>When a cache lookup first encounters a missing cache entry, a new entry is loaded.
 *       <ul>
 *         <li>After successfully loading an entry {@code missCount} and {@code loadSuccessCount}
 *             are incremented, and the total loading time, in nanoseconds, is added to {@code
 *             totalLoadTime}.
 *         <li>When an exception is thrown while loading an entry, {@code missCount} and {@code
 *             loadExceptionCount} are incremented, and the total loading time, in nanoseconds, is
 *             added to {@code totalLoadTime}.
 *         <li>Cache lookups that encounter a missing cache entry that is still loading will wait
 *             for loading to complete (whether successful or not) and then increment {@code
 *             missCount}.
 *       </ul>
 *   <li>When an entry is evicted from the cache, {@code evictionCount} is incremented.
 *   <li>No stats are modified when a cache entry is invalidated or manually removed.
 *   <li>No stats are modified by operations invoked on the {@linkplain Cache#asMap asMap} view of
 *       the cache.
 * </ul>
 *
 * <p>A lookup is specifically defined as an invocation of one of the methods {@link
 * LoadingCache#get(Object)}, {@link LoadingCache#getUnchecked(Object)}, {@link Cache#get(Object,
 * Callable)}, or {@link LoadingCache#getAll(Iterable)}.
 *
 * @author Charles Fry
 * @since 10.0
 */
@GwtCompatible
public final class CacheStats {
  private final long hitCount;
  private final long missCount;
  private final long loadSuccessCount;
  private final long loadExceptionCount;
  private final long totalLoadTime;
  private final long evictionCount;

  
Constructs a new CacheStats instance. 
Five parameters of the same type in a row is a bad thing, but this class is not constructed
by end users and is too fine-grained for a builder.

/**
   * Constructs a new {@code CacheStats} instance.
   *
   * <p>Five parameters of the same type in a row is a bad thing, but this class is not constructed
   * by end users and is too fine-grained for a builder.
   */
  public CacheStats(
      long hitCount,
      long missCount,
      long loadSuccessCount,
      long loadExceptionCount,
      long totalLoadTime,
      long evictionCount) {
    checkArgument(hitCount >= 0);
    checkArgument(missCount >= 0);
    checkArgument(loadSuccessCount >= 0);
    checkArgument(loadExceptionCount >= 0);
    checkArgument(totalLoadTime >= 0);
    checkArgument(evictionCount >= 0);

    this.hitCount = hitCount;
    this.missCount = missCount;
    this.loadSuccessCount = loadSuccessCount;
    this.loadExceptionCount = loadExceptionCount;
    this.totalLoadTime = totalLoadTime;
    this.evictionCount = evictionCount;
  }

  
Returns the number of times Cache lookup methods have returned either a cached or uncached value. This is defined as hitCount + missCount. 
/**
   * Returns the number of times {@link Cache} lookup methods have returned either a cached or
   * uncached value. This is defined as {@code hitCount + missCount}.
   */
  public long requestCount() {
    return hitCount + missCount;
  }

  
Returns the number of times Cache lookup methods have returned a cached value. 
/** Returns the number of times {@link Cache} lookup methods have returned a cached value. */
  public long hitCount() {
    return hitCount;
  }

  
Returns the ratio of cache requests which were hits. This is defined as hitCount /
requestCount, or 1.0 when requestCount == 0. Note that hitRate +
missRate =~ 1.0. 
/**
   * Returns the ratio of cache requests which were hits. This is defined as {@code hitCount /
   * requestCount}, or {@code 1.0} when {@code requestCount == 0}. Note that {@code hitRate +
   * missRate =~ 1.0}.
   */
  public double hitRate() {
    long requestCount = requestCount();
    return (requestCount == 0) ? 1.0 : (double) hitCount / requestCount;
  }

  
Returns the number of times Cache lookup methods have returned an uncached (newly loaded) value, or null. Multiple concurrent calls to Cache lookup methods on an absent value can result in multiple misses, all returning the results of a single cache load operation. 
/**
   * Returns the number of times {@link Cache} lookup methods have returned an uncached (newly
   * loaded) value, or null. Multiple concurrent calls to {@link Cache} lookup methods on an absent
   * value can result in multiple misses, all returning the results of a single cache load
   * operation.
   */
  public long missCount() {
    return missCount;
  }

  
Returns the ratio of cache requests which were misses. This is defined as missCount /
requestCount, or 0.0 when requestCount == 0. Note that hitRate +
missRate =~ 1.0. Cache misses include all requests which weren't cache hits, including requests which resulted in either successful or failed loading attempts, and requests which waited for other threads to finish loading. It is thus the case that missCount &gt;=
loadSuccessCount + loadExceptionCount. Multiple concurrent misses for the same key will result in a single load operation. 
/**
   * Returns the ratio of cache requests which were misses. This is defined as {@code missCount /
   * requestCount}, or {@code 0.0} when {@code requestCount == 0}. Note that {@code hitRate +
   * missRate =~ 1.0}. Cache misses include all requests which weren't cache hits, including
   * requests which resulted in either successful or failed loading attempts, and requests which
   * waited for other threads to finish loading. It is thus the case that {@code missCount &gt;=
   * loadSuccessCount + loadExceptionCount}. Multiple concurrent misses for the same key will result
   * in a single load operation.
   */
  public double missRate() {
    long requestCount = requestCount();
    return (requestCount == 0) ? 0.0 : (double) missCount / requestCount;
  }

  
Returns the total number of times that Cache lookup methods attempted to load new values. This includes both successful load operations, as well as those that threw exceptions. This is defined as loadSuccessCount + loadExceptionCount. 
/**
   * Returns the total number of times that {@link Cache} lookup methods attempted to load new
   * values. This includes both successful load operations, as well as those that threw exceptions.
   * This is defined as {@code loadSuccessCount + loadExceptionCount}.
   */
  public long loadCount() {
    return loadSuccessCount + loadExceptionCount;
  }

  
Returns the number of times Cache lookup methods have successfully loaded a new value. This is usually incremented in conjunction with missCount, though missCount is also incremented when an exception is encountered during cache loading (see loadExceptionCount). Multiple concurrent misses for the same key will result in a single load operation. This may be incremented not in conjunction with missCount if the load occurs as a result of a refresh or if the cache loader returned more items than was requested. 
missCount may also be incremented not in conjunction with this (nor loadExceptionCount) on calls to getIfPresent. 
/**
   * Returns the number of times {@link Cache} lookup methods have successfully loaded a new value.
   * This is usually incremented in conjunction with {@link #missCount}, though {@code missCount} is
   * also incremented when an exception is encountered during cache loading (see {@link
   * #loadExceptionCount}). Multiple concurrent misses for the same key will result in a single load
   * operation. This may be incremented not in conjunction with {@code missCount} if the load occurs
   * as a result of a refresh or if the cache loader returned more items than was requested. {@code
   * missCount} may also be incremented not in conjunction with this (nor {@link
   * #loadExceptionCount}) on calls to {@code getIfPresent}.
   */
  public long loadSuccessCount() {
    return loadSuccessCount;
  }

  
Returns the number of times Cache lookup methods threw an exception while loading a new value. This is usually incremented in conjunction with missCount, though 
missCount is also incremented when cache loading completes successfully (see loadSuccessCount). Multiple concurrent misses for the same key will result in a single load operation. This may be incremented not in conjunction with missCount if the load occurs as a result of a refresh or if the cache loader returned more items than was requested. 
missCount may also be incremented not in conjunction with this (nor loadSuccessCount) on calls to getIfPresent. 
/**
   * Returns the number of times {@link Cache} lookup methods threw an exception while loading a new
   * value. This is usually incremented in conjunction with {@code missCount}, though {@code
   * missCount} is also incremented when cache loading completes successfully (see {@link
   * #loadSuccessCount}). Multiple concurrent misses for the same key will result in a single load
   * operation. This may be incremented not in conjunction with {@code missCount} if the load occurs
   * as a result of a refresh or if the cache loader returned more items than was requested. {@code
   * missCount} may also be incremented not in conjunction with this (nor {@link #loadSuccessCount})
   * on calls to {@code getIfPresent}.
   */
  public long loadExceptionCount() {
    return loadExceptionCount;
  }

  
Returns the ratio of cache loading attempts which threw exceptions. This is defined as 
loadExceptionCount / (loadSuccessCount + loadExceptionCount), or 0.0 when 
loadSuccessCount + loadExceptionCount == 0. 
/**
   * Returns the ratio of cache loading attempts which threw exceptions. This is defined as {@code
   * loadExceptionCount / (loadSuccessCount + loadExceptionCount)}, or {@code 0.0} when {@code
   * loadSuccessCount + loadExceptionCount == 0}.
   */
  public double loadExceptionRate() {
    long totalLoadCount = loadSuccessCount + loadExceptionCount;
    return (totalLoadCount == 0) ? 0.0 : (double) loadExceptionCount / totalLoadCount;
  }

  
Returns the total number of nanoseconds the cache has spent loading new values. This can be used to calculate the miss penalty. This value is increased every time loadSuccessCount or loadExceptionCount is incremented. 
/**
   * Returns the total number of nanoseconds the cache has spent loading new values. This can be
   * used to calculate the miss penalty. This value is increased every time {@code loadSuccessCount}
   * or {@code loadExceptionCount} is incremented.
   */
  public long totalLoadTime() {
    return totalLoadTime;
  }

  
Returns the average time spent loading new values. This is defined as totalLoadTime /
(loadSuccessCount + loadExceptionCount). 
/**
   * Returns the average time spent loading new values. This is defined as {@code totalLoadTime /
   * (loadSuccessCount + loadExceptionCount)}.
   */
  public double averageLoadPenalty() {
    long totalLoadCount = loadSuccessCount + loadExceptionCount;
    return (totalLoadCount == 0) ? 0.0 : (double) totalLoadTime / totalLoadCount;
  }

  
Returns the number of times an entry has been evicted. This count does not include manual invalidations. 
/**
   * Returns the number of times an entry has been evicted. This count does not include manual
   * {@linkplain Cache#invalidate invalidations}.
   */
  public long evictionCount() {
    return evictionCount;
  }

  
Returns a new CacheStats representing the difference between this CacheStats and other. Negative values, which aren't supported by CacheStats will be rounded up to zero. 
/**
   * Returns a new {@code CacheStats} representing the difference between this {@code CacheStats}
   * and {@code other}. Negative values, which aren't supported by {@code CacheStats} will be
   * rounded up to zero.
   */
  public CacheStats minus(CacheStats other) {
    return new CacheStats(
        Math.max(0, hitCount - other.hitCount),
        Math.max(0, missCount - other.missCount),
        Math.max(0, loadSuccessCount - other.loadSuccessCount),
        Math.max(0, loadExceptionCount - other.loadExceptionCount),
        Math.max(0, totalLoadTime - other.totalLoadTime),
        Math.max(0, evictionCount - other.evictionCount));
  }

  
Returns a new CacheStats representing the sum of this CacheStats and 
other. 
Since:11.0
/**
   * Returns a new {@code CacheStats} representing the sum of this {@code CacheStats} and {@code
   * other}.
   *
   * @since 11.0
   */
  public CacheStats plus(CacheStats other) {
    return new CacheStats(
        hitCount + other.hitCount,
        missCount + other.missCount,
        loadSuccessCount + other.loadSuccessCount,
        loadExceptionCount + other.loadExceptionCount,
        totalLoadTime + other.totalLoadTime,
        evictionCount + other.evictionCount);
  }

  @Override
  public int hashCode() {
    return Objects.hashCode(
        hitCount, missCount, loadSuccessCount, loadExceptionCount, totalLoadTime, evictionCount);
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object instanceof CacheStats) {
      CacheStats other = (CacheStats) object;
      return hitCount == other.hitCount
          && missCount == other.missCount
          && loadSuccessCount == other.loadSuccessCount
          && loadExceptionCount == other.loadExceptionCount
          && totalLoadTime == other.totalLoadTime
          && evictionCount == other.evictionCount;
    }
    return false;
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("hitCount", hitCount)
        .add("missCount", missCount)
        .add("loadSuccessCount", loadSuccessCount)
        .add("loadExceptionCount", loadExceptionCount)
        .add("totalLoadTime", totalLoadTime)
        .add("evictionCount", evictionCount)
        .toString();
  }
}