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21 
22 
23 
24 package com.sun.star.lib.util;
25 
26 import java.lang.ref.ReferenceQueue;
27 import java.lang.ref.WeakReference;
28 import java.util.Collection;
29 import java.util.HashMap;
30 import java.util.Iterator;
31 import java.util.Map;
32 import java.util.Set;
33 
34 /**
35  * A hash map that holds values of type <code>WeakReference</code>.
36  *
37  * <p>Like <code>HashMap</code>, this implementation provides all of the
38  * optional map operations, and permits the <code>null</code> key.</p>
39  *
40  * <p>Also like <code>HashMap</code>, this implementation is not synchronized.
41  * If multiple threads share an instance, and at least one of them executes any
42  * modifying operations on the <code>WeakMap</code>, they have to use external
43  * synchronization.</p>
44  *
45  * <p>Unlike other map implementations, <code>WeakMap</code> is asymmetric in
46  * that <code>put</code> expects the given value to be a plain object that is
47  * then wrapped in a <code>WeakReference</code>, while the occurences of values
48  * in all other methods (<code>containsValue</code>, <code>entrySet</code>,
49  * <code>equals</code>, <code>get</code>, <code>hashCode</code>,
50  * <code>remove</code>, <code>values</code>, and also the return value of
51  * <code>put</code>) expect already wrapped instances of
52  * <code>WeakReference</code>.  That is, after <code>weakMap.put("key",
53  * o)</code>, <code>weakMap.get("key").equals(o)</code> does not work as
54  * na&iuml;vely expected; neither does
55  * <code>weakMap1.putAll(weakMap2)</code>.</p>
56  *
57  * <p>At an arbitrary time after the <code>WeakReference</code> value of an
58  * entry has been cleared by the garbage collector, the entry is automatically
59  * removed from the map.</p>
60  *
61  * <p>Values placed into a <code>WeakMap</code> may optionally support the
62  * <code>DisposeNotifier</code> interface.  For those that do, the associated
63  * <code>WeakReference</code> wrappers are automatically cleared as soon as the
64  * values are disposed.</p>
65  */
66 public final class WeakMap implements Map {
67     /**
68      * Constructs an empty <code>WeakMap</code>.
69      */
WeakMap()70     public WeakMap() {}
71 
72     /**
73      * Constructs a new <code>WeakMap</code> with the same mappings as the
74      * specified <code>Map</code>.
75      *
76      * @param m the map whose mappings are to be placed in this map
77      */
WeakMap(Map m)78     public WeakMap(Map m) {
79         putAll(m);
80     }
81 
82     /**
83      * Returns the number of key&ndash;value mappings in this map.
84      *
85      * <p>This is a non-modifying operation.</p>
86      *
87      * @return the number of key&ndash;value mappings in this map
88      */
size()89     public int size() {
90         return map.size();
91     }
92 
93     /**
94      * Returns <code>true</code> if this map contains no key&ndash;value
95      * mappings.
96      *
97      * <p>This is a non-modifying operation.</p>
98      *
99      * @return <code>true</code> if this map contains no key&ndash;value
100      * mappings
101      */
isEmpty()102     public boolean isEmpty() {
103         return map.isEmpty();
104     }
105 
106     /**
107      * Returns <code>true</code> if this map contains a mapping for the
108      * specified key.
109      *
110      * <p>This is a non-modifying operation.</p>
111      *
112      * @param key the key whose presence in this map is to be tested
113      * @return <code>true</code> if this map contains a mapping for the
114      * specified key
115      */
containsKey(Object key)116     public boolean containsKey(Object key) {
117         return map.containsKey(key);
118     }
119 
120     /**
121      * Returns <code>true</code> if this map maps one or more keys to the
122      * specified value.
123      *
124      * <p>This is a non-modifying operation.</p>
125      *
126      * @param value the value whose presence in this map is to be tested
127      * @return <code>true</code> if this map maps one or more keys to the
128      * specified value
129      */
containsValue(Object value)130     public boolean containsValue(Object value) {
131         return map.containsValue(value);
132     }
133 
134     /**
135      * Returns the value to which the specified key is mapped in this map, or
136      * <code>null</code> if the map contains no mapping for this key.
137      *
138      * <p>This is a non-modifying operation.</p>
139      *
140      * @param key the key whose associated value is to be returned
141      *
142      * @return the value to which this map maps the specified key, or
143      * <code>null</code> if the map contains no mapping for this key
144      */
get(Object key)145     public Object get(Object key) {
146         return map.get(key);
147     }
148 
149     /**
150      * Associates the specified value with the specified key in this map.
151      *
152      * <p>This is a modifying operation.</p>
153      *
154      * @param key the key with witch the specified value is to be associated
155      * @param value the value to be associated with the specified key.  This
156      * must be a plain object, which is then wrapped in a
157      * <code>WeakReference</code>.
158      * @return previous value associated with the specified key, or
159      * <code>null</code> if there was no mapping for the key
160      */
put(Object key, Object value)161     public Object put(Object key, Object value) {
162         cleanUp();
163         return map.put(key, new Entry(key, value, queue));
164     }
165 
166     /**
167      * Removes the mapping for this key from this map if present.
168      *
169      * <p>This is a modifying operation.</p>
170      *
171      * @param key the key whose mapping is to be removed from the map
172      * @return previous value associated with the specified key, or
173      * <code>null</code> if there was no mapping for the key
174      */
remove(Object key)175     public Object remove(Object key) {
176         cleanUp();
177         return map.remove(key);
178     }
179 
180     /**
181      * Copies all of the mappings from the specified map to this map.
182      *
183      * <p>This is a modifying operation.</p>
184      *
185      * @param m mappings to be stored in this map.  The values of those mappings
186      * must be plain objects, which are then wrapped in instances of
187      * <code>WeakReference</code>.
188      */
putAll(Map t)189     public void putAll(Map t) {
190         cleanUp();
191         for (Iterator i = t.entrySet().iterator(); i.hasNext();) {
192             Map.Entry e = (Map.Entry) i.next();
193             Object k = e.getKey();
194             map.put(k, new Entry(k, e.getValue(), queue));
195         }
196     }
197 
198     /**
199      * Removes all mappings from this map.
200      *
201      * <p>This is a modifying operation.</p>
202      */
clear()203     public void clear() {
204         cleanUp();
205         map.clear();
206     }
207 
208     /**
209      * Returns a view of the keys contained in this map.
210      *
211      * <p>This is a non-modifying operation.</p>
212      *
213      * @return a set view of the keys contained in this map
214      */
keySet()215     public Set keySet() {
216         return map.keySet();
217     }
218 
219     /**
220      * Returns a collection view of the values contained in this map.
221      *
222      * <p>This is a non-modifying operation.</p>
223      *
224      * @return a collection view of the values contained in this map
225      */
values()226     public Collection values() {
227         return map.values();
228     }
229 
230     /**
231      * Returns a collection view of the mappings contained in this map.
232      *
233      * <p>This is a non-modifying operation.</p>
234      *
235      * @return a collection view of the mappings contained in this map
236      */
entrySet()237     public Set entrySet() {
238         return map.entrySet();
239     }
240 
equals(Object o)241     public boolean equals(Object o) {
242         return map.equals(o);
243     }
244 
hashCode()245     public int hashCode() {
246         return map.hashCode();
247     }
248 
249     /**
250      * Returns the referent of a <code>WeakReference</code>, silently handling a
251      * <code>null</code> argument.
252      *
253      * <p>This static method is useful to wrap around the return values of
254      * methods like <code>get</code>.</p>
255      *
256      * @param ref must be either an instance of <code>WeakReference</code> or
257      * <code>null</code>
258      * @return the referent of the specified <code>WeakReference</code>, or
259      * <code>null</code> if <code>ref</code> is <code>null</code>
260      */
getValue(Object ref)261     public static Object getValue(Object ref) {
262         return ref == null ? null : ((WeakReference) ref).get();
263     }
264 
265     // cleanUp must only be called from within modifying methods.  Otherwise,
266     // the implementations of entrySet, keySet and values would break
267     // (specificially, iterating over the collections returned by those
268     // methods), as non-modifying methods might modify the underlying map.
cleanUp()269     private void cleanUp() {
270         for (;;) {
271             Entry e = (Entry) queue.poll();
272             if (e == null) {
273                 break;
274             }
275             // It is possible that an Entry e1 becomes weakly reachable, then
276             // another Entry e2 is added to the map for the same key, and only
277             // then e1 is enqueued.  To not erroneously remove the new e2 in
278             // that case, check whether the map still contains e1:
279             Object k = e.key;
280             if (e == map.get(k)) {
281                 map.remove(k);
282             }
283         }
284     }
285 
286     private static final class Entry extends WeakReference
287         implements DisposeListener
288     {
notifyDispose(DisposeNotifier source)289         public void notifyDispose(DisposeNotifier source) {
290             Entry.this.clear(); // qualification needed for Java 1.3
291             enqueue();
292         }
293 
Entry(Object key, Object value, ReferenceQueue queue)294         private Entry(Object key, Object value, ReferenceQueue queue) {
295             super(value, queue);
296             this.key = key;
297             if (value instanceof DisposeNotifier) {
298                 ((DisposeNotifier) value).addDisposeListener(this);
299             }
300         }
301 
302         private final Object key;
303     }
304 
305     private final HashMap map = new HashMap();
306     private final ReferenceQueue queue = new ReferenceQueue();
307 }
308