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   * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.lang.ref;
  
  // Android-changed: Javadoc change to recommend against using SoftReferences for caches.
  /**
   * Soft reference objects, which are cleared at the discretion of the garbage
   * collector in response to memory demand.
   *
   * <p> Suppose that the garbage collector determines at a certain point in time
   * that an object is <a href="package-summary.html#reachability">softly
   * reachable</a>.  At that time it may choose to clear atomically all soft
   * references to that object and all soft references to any other
   * softly-reachable objects from which that object is reachable through a chain
   * of strong references.  At the same time or at some later time it will
   * enqueue those newly-cleared soft references that are registered with
   * reference queues.
   *
   * <p> All soft references to softly-reachable objects are guaranteed to have
   * been cleared before the virtual machine throws an
   * <code>OutOfMemoryError</code>.  Otherwise no constraints are placed upon the
   * time at which a soft reference will be cleared or the order in which a set
   * of such references to different objects will be cleared.  Virtual machine
   * implementations are, however, encouraged to bias against clearing
   * recently-created or recently-used soft references.
   *
   * <h3>Avoid Soft References for Caching</h3>
   * In practice, soft references are inefficient for caching. The runtime doesn't
   * have enough information on which references to clear and which to keep. Most
   * fatally, it doesn't know what to do when given the choice between clearing a
   * soft reference and growing the heap.
   *
   * <p>The lack of information on the value to your application of each reference
   * limits the usefulness of soft references. References that are cleared too
   * early cause unnecessary work; those that are cleared too late waste memory.
   *
   * <p>Most applications should use an {@code android.util.LruCache} instead of
   * soft references. LruCache has an effective eviction policy and lets the user
   * tune how much memory is allotted.
   *
   * @author   Mark Reinhold
   * @since    1.2
   */
  
  public class SoftReference<T> extends Reference<T> {
  
      /**
       * Timestamp clock, updated by the garbage collector
       */
      static private long clock;
  
      /**
       * Timestamp updated by each invocation of the get method.  The VM may use
       * this field when selecting soft references to be cleared, but it is not
       * required to do so.
       */
      private long timestamp;
  
      /**
       * Creates a new soft reference that refers to the given object.  The new
       * reference is not registered with any queue.
       *
       * @param referent object the new soft reference will refer to
       */
      public SoftReference(T referent) {
          super(referent);
          this.timestamp = clock;
      }
  
      /**
       * Creates a new soft reference that refers to the given object and is
       * registered with the given queue.
       *
       * @param referent object the new soft reference will refer to
       * @param q the queue with which the reference is to be registered,
       *          or <tt>null</tt> if registration is not required
       *
       */
      public SoftReference(T referent, ReferenceQueue<? super T> q) {
          super(referent, q);
          this.timestamp = clock;
      }
  
      /**
       * Returns this reference object's referent.  If this reference object has
       * been cleared, either by the program or by the garbage collector, then
       * this method returns <code>null</code>.
       *
       * @return   The object to which this reference refers, or
       *           <code>null</code> if this reference object has been cleared
       */
      public T get() {
          T o = super.get();
          if (o != null && this.timestamp != clock)
              this.timestamp = clock;
          return o;
      }
  
  }
  