| Map.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util;
9
10 /**
11 * An object that maps keys to values. A map cannot contain duplicate keys;
12 * each key can map to at most one value.
13 *
14 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
15 * was a totally abstract class rather than an interface.
16 *
17 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
18 * allow a map's contents to be viewed as a set of keys, collection of values,
19 * or set of key-value mappings. The <i>order</i> of a map is defined as
20 * the order in which the iterators on the map's collection views return their
21 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
22 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
23 * class, do not.
24 *
25 * <p>Note: great care must be exercised if mutable objects are used as map
26 * keys. The behavior of a map is not specified if the value of an object is
27 * changed in a manner that affects <tt>equals</tt> comparisons while the
28 * object is a key in the map. A special case of this prohibition is that it
29 * is not permissible for a map to contain itself as a key. While it is
30 * permissible for a map to contain itself as a value, extreme caution is
31 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
32 * well defined on such a map.
33 *
34 * <p>All general-purpose map implementation classes should provide two
35 * "standard" constructors: a void (no arguments) constructor which creates an
36 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
37 * which creates a new map with the same key-value mappings as its argument.
38 * In effect, the latter constructor allows the user to copy any map,
39 * producing an equivalent map of the desired class. There is no way to
40 * enforce this recommendation (as interfaces cannot contain constructors) but
41 * all of the general-purpose map implementations in the JDK comply.
42 *
43 * <p>The "destructive" methods contained in this interface, that is, the
44 * methods that modify the map on which they operate, are specified to throw
45 * <tt>UnsupportedOperationException</tt> if this map does not support the
46 * operation. If this is the case, these methods may, but are not required
47 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
48 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
49 * method on an unmodifiable map may, but is not required to, throw the
50 * exception if the map whose mappings are to be "superimposed" is empty.
51 *
52 * <p>Some map implementations have restrictions on the keys and values they
53 * may contain. For example, some implementations prohibit null keys and
54 * values, and some have restrictions on the types of their keys. Attempting
55 * to insert an ineligible key or value throws an unchecked exception,
56 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
57 * Attempting to query the presence of an ineligible key or value may throw an
58 * exception, or it may simply return false; some implementations will exhibit
59 * the former behavior and some will exhibit the latter. More generally,
60 * attempting an operation on an ineligible key or value whose completion
61 * would not result in the insertion of an ineligible element into the map may
62 * throw an exception or it may succeed, at the option of the implementation.
63 * Such exceptions are marked as "optional" in the specification for this
64 * interface.
65 *
66 * <p>This interface is a member of the
67 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
68 * Java Collections Framework</a>.
69 *
70 * <p>Many methods in Collections Framework interfaces are defined
71 * in terms of the {@link Object#equals(Object) equals} method. For
72 * example, the specification for the {@link #containsKey(Object)
73 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
74 * only if this map contains a mapping for a key <tt>k</tt> such that
75 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
76 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
77 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
78 * be invoked for any key <tt>k</tt>. Implementations are free to
79 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
80 * for example, by first comparing the hash codes of the two keys. (The
81 * {@link Object#hashCode()} specification guarantees that two objects with
82 * unequal hash codes cannot be equal.) More generally, implementations of
83 * the various Collections Framework interfaces are free to take advantage of
84 * the specified behavior of underlying {@link Object} methods wherever the
85 * implementor deems it appropriate.
86 *
87 * @param <K> the type of keys maintained by this map
88 * @param <V> the type of mapped values
89 *
90 * @author Josh Bloch
91 * @version %I%, %G%
92 * @see HashMap
93 * @see TreeMap
94 * @see Hashtable
95 * @see SortedMap
96 * @see Collection
97 * @see Set
98 * @since 1.2
99 */
100 public interface Map<K,V> {
101 // Query Operations
102
103 /**
104 * Returns the number of key-value mappings in this map. If the
105 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
106 * <tt>Integer.MAX_VALUE</tt>.
107 *
108 * @return the number of key-value mappings in this map
109 */
110 int size();
111
112 /**
113 * Returns <tt>true</tt> if this map contains no key-value mappings.
114 *
115 * @return <tt>true</tt> if this map contains no key-value mappings
116 */
117 boolean isEmpty();
118
119 /**
120 * Returns <tt>true</tt> if this map contains a mapping for the specified
121 * key. More formally, returns <tt>true</tt> if and only if
122 * this map contains a mapping for a key <tt>k</tt> such that
123 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
124 * at most one such mapping.)
125 *
126 * @param key key whose presence in this map is to be tested
127 * @return <tt>true</tt> if this map contains a mapping for the specified
128 * key
129 * @throws ClassCastException if the key is of an inappropriate type for
130 * this map (optional)
131 * @throws NullPointerException if the specified key is null and this map
132 * does not permit null keys (optional)
133 */
134 boolean containsKey(Object key);
135
136 /**
137 * Returns <tt>true</tt> if this map maps one or more keys to the
138 * specified value. More formally, returns <tt>true</tt> if and only if
139 * this map contains at least one mapping to a value <tt>v</tt> such that
140 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
141 * will probably require time linear in the map size for most
142 * implementations of the <tt>Map</tt> interface.
143 *
144 * @param value value whose presence in this map is to be tested
145 * @return <tt>true</tt> if this map maps one or more keys to the
146 * specified value
147 * @throws ClassCastException if the value is of an inappropriate type for
148 * this map (optional)
149 * @throws NullPointerException if the specified value is null and this
150 * map does not permit null values (optional)
151 */
152 boolean containsValue(Object value);
153
154 /**
155 * Returns the value to which the specified key is mapped,
156 * or {@code null} if this map contains no mapping for the key.
157 *
158 * <p>More formally, if this map contains a mapping from a key
159 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
160 * key.equals(k))}, then this method returns {@code v}; otherwise
161 * it returns {@code null}. (There can be at most one such mapping.)
162 *
163 * <p>If this map permits null values, then a return value of
164 * {@code null} does not <i>necessarily</i> indicate that the map
165 * contains no mapping for the key; it's also possible that the map
166 * explicitly maps the key to {@code null}. The {@link #containsKey
167 * containsKey} operation may be used to distinguish these two cases.
168 *
169 * @param key the key whose associated value is to be returned
170 * @return the value to which the specified key is mapped, or
171 * {@code null} if this map contains no mapping for the key
172 * @throws ClassCastException if the key is of an inappropriate type for
173 * this map (optional)
174 * @throws NullPointerException if the specified key is null and this map
175 * does not permit null keys (optional)
176 */
177 V get(Object key);
178
179 // Modification Operations
180
181 /**
182 * Associates the specified value with the specified key in this map
183 * (optional operation). If the map previously contained a mapping for
184 * the key, the old value is replaced by the specified value. (A map
185 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
186 * if {@link #containsKey(Object) m.containsKey(k)} would return
187 * <tt>true</tt>.)
188 *
189 * @param key key with which the specified value is to be associated
190 * @param value value to be associated with the specified key
191 * @return the previous value associated with <tt>key</tt>, or
192 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
193 * (A <tt>null</tt> return can also indicate that the map
194 * previously associated <tt>null</tt> with <tt>key</tt>,
195 * if the implementation supports <tt>null</tt> values.)
196 * @throws UnsupportedOperationException if the <tt>put</tt> operation
197 * is not supported by this map
198 * @throws ClassCastException if the class of the specified key or value
199 * prevents it from being stored in this map
200 * @throws NullPointerException if the specified key or value is null
201 * and this map does not permit null keys or values
202 * @throws IllegalArgumentException if some property of the specified key
203 * or value prevents it from being stored in this map
204 */
205 V put(K key, V value);
206
207 /**
208 * Removes the mapping for a key from this map if it is present
209 * (optional operation). More formally, if this map contains a mapping
210 * from key <tt>k</tt> to value <tt>v</tt> such that
211 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
212 * is removed. (The map can contain at most one such mapping.)
213 *
214 * <p>Returns the value to which this map previously associated the key,
215 * or <tt>null</tt> if the map contained no mapping for the key.
216 *
217 * <p>If this map permits null values, then a return value of
218 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
219 * contained no mapping for the key; it's also possible that the map
220 * explicitly mapped the key to <tt>null</tt>.
221 *
222 * <p>The map will not contain a mapping for the specified key once the
223 * call returns.
224 *
225 * @param key key whose mapping is to be removed from the map
226 * @return the previous value associated with <tt>key</tt>, or
227 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
228 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
229 * is not supported by this map
230 * @throws ClassCastException if the key is of an inappropriate type for
231 * this map (optional)
232 * @throws NullPointerException if the specified key is null and this
233 * map does not permit null keys (optional)
234 */
235 V remove(Object key);
236
237
238 // Bulk Operations
239
240 /**
241 * Copies all of the mappings from the specified map to this map
242 * (optional operation). The effect of this call is equivalent to that
243 * of calling {@link #put(Object,Object) put(k, v)} on this map once
244 * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
245 * specified map. The behavior of this operation is undefined if the
246 * specified map is modified while the operation is in progress.
247 *
248 * @param m mappings to be stored in this map
249 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
250 * is not supported by this map
251 * @throws ClassCastException if the class of a key or value in the
252 * specified map prevents it from being stored in this map
253 * @throws NullPointerException if the specified map is null, or if
254 * this map does not permit null keys or values, and the
255 * specified map contains null keys or values
256 * @throws IllegalArgumentException if some property of a key or value in
257 * the specified map prevents it from being stored in this map
258 */
259 void putAll(Map<? extends K, ? extends V> m);
260
261 /**
262 * Removes all of the mappings from this map (optional operation).
263 * The map will be empty after this call returns.
264 *
265 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
266 * is not supported by this map
267 */
268 void clear();
269
270
271 // Views
272
273 /**
274 * Returns a {@link Set} view of the keys contained in this map.
275 * The set is backed by the map, so changes to the map are
276 * reflected in the set, and vice-versa. If the map is modified
277 * while an iteration over the set is in progress (except through
278 * the iterator's own <tt>remove</tt> operation), the results of
279 * the iteration are undefined. The set supports element removal,
280 * which removes the corresponding mapping from the map, via the
281 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
282 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
283 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
284 * operations.
285 *
286 * @return a set view of the keys contained in this map
287 */
288 Set<K> keySet();
289
290 /**
291 * Returns a {@link Collection} view of the values contained in this map.
292 * The collection is backed by the map, so changes to the map are
293 * reflected in the collection, and vice-versa. If the map is
294 * modified while an iteration over the collection is in progress
295 * (except through the iterator's own <tt>remove</tt> operation),
296 * the results of the iteration are undefined. The collection
297 * supports element removal, which removes the corresponding
298 * mapping from the map, via the <tt>Iterator.remove</tt>,
299 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
300 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
301 * support the <tt>add</tt> or <tt>addAll</tt> operations.
302 *
303 * @return a collection view of the values contained in this map
304 */
305 Collection<V> values();
306
307 /**
308 * Returns a {@link Set} view of the mappings contained in this map.
309 * The set is backed by the map, so changes to the map are
310 * reflected in the set, and vice-versa. If the map is modified
311 * while an iteration over the set is in progress (except through
312 * the iterator's own <tt>remove</tt> operation, or through the
313 * <tt>setValue</tt> operation on a map entry returned by the
314 * iterator) the results of the iteration are undefined. The set
315 * supports element removal, which removes the corresponding
316 * mapping from the map, via the <tt>Iterator.remove</tt>,
317 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
318 * <tt>clear</tt> operations. It does not support the
319 * <tt>add</tt> or <tt>addAll</tt> operations.
320 *
321 * @return a set view of the mappings contained in this map
322 */
323 Set<Map.Entry<K, V>> entrySet();
324
325 /**
326 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
327 * a collection-view of the map, whose elements are of this class. The
328 * <i>only</i> way to obtain a reference to a map entry is from the
329 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
330 * valid <i>only</i> for the duration of the iteration; more formally,
331 * the behavior of a map entry is undefined if the backing map has been
332 * modified after the entry was returned by the iterator, except through
333 * the <tt>setValue</tt> operation on the map entry.
334 *
335 * @see Map#entrySet()
336 * @since 1.2
337 */
338 interface Entry<K,V> {
339 /**
340 * Returns the key corresponding to this entry.
341 *
342 * @return the key corresponding to this entry
343 * @throws IllegalStateException implementations may, but are not
344 * required to, throw this exception if the entry has been
345 * removed from the backing map.
346 */
347 K getKey();
348
349 /**
350 * Returns the value corresponding to this entry. If the mapping
351 * has been removed from the backing map (by the iterator's
352 * <tt>remove</tt> operation), the results of this call are undefined.
353 *
354 * @return the value corresponding to this entry
355 * @throws IllegalStateException implementations may, but are not
356 * required to, throw this exception if the entry has been
357 * removed from the backing map.
358 */
359 V getValue();
360
361 /**
362 * Replaces the value corresponding to this entry with the specified
363 * value (optional operation). (Writes through to the map.) The
364 * behavior of this call is undefined if the mapping has already been
365 * removed from the map (by the iterator's <tt>remove</tt> operation).
366 *
367 * @param value new value to be stored in this entry
368 * @return old value corresponding to the entry
369 * @throws UnsupportedOperationException if the <tt>put</tt> operation
370 * is not supported by the backing map
371 * @throws ClassCastException if the class of the specified value
372 * prevents it from being stored in the backing map
373 * @throws NullPointerException if the backing map does not permit
374 * null values, and the specified value is null
375 * @throws IllegalArgumentException if some property of this value
376 * prevents it from being stored in the backing map
377 * @throws IllegalStateException implementations may, but are not
378 * required to, throw this exception if the entry has been
379 * removed from the backing map.
380 */
381 V setValue(V value);
382
383 /**
384 * Compares the specified object with this entry for equality.
385 * Returns <tt>true</tt> if the given object is also a map entry and
386 * the two entries represent the same mapping. More formally, two
387 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
388 * if<pre>
389 * (e1.getKey()==null ?
390 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
391 * (e1.getValue()==null ?
392 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
393 * </pre>
394 * This ensures that the <tt>equals</tt> method works properly across
395 * different implementations of the <tt>Map.Entry</tt> interface.
396 *
397 * @param o object to be compared for equality with this map entry
398 * @return <tt>true</tt> if the specified object is equal to this map
399 * entry
400 */
401 boolean equals(Object o);
402
403 /**
404 * Returns the hash code value for this map entry. The hash code
405 * of a map entry <tt>e</tt> is defined to be: <pre>
406 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
407 * (e.getValue()==null ? 0 : e.getValue().hashCode())
408 * </pre>
409 * This ensures that <tt>e1.equals(e2)</tt> implies that
410 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
411 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
412 * contract of <tt>Object.hashCode</tt>.
413 *
414 * @return the hash code value for this map entry
415 * @see Object#hashCode()
416 * @see Object#equals(Object)
417 * @see #equals(Object)
418 */
419 int hashCode();
420 }
421
422 // Comparison and hashing
423
424 /**
425 * Compares the specified object with this map for equality. Returns
426 * <tt>true</tt> if the given object is also a map and the two maps
427 * represent the same mappings. More formally, two maps <tt>m1</tt> and
428 * <tt>m2</tt> represent the same mappings if
429 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
430 * <tt>equals</tt> method works properly across different implementations
431 * of the <tt>Map</tt> interface.
432 *
433 * @param o object to be compared for equality with this map
434 * @return <tt>true</tt> if the specified object is equal to this map
435 */
436 boolean equals(Object o);
437
438 /**
439 * Returns the hash code value for this map. The hash code of a map is
440 * defined to be the sum of the hash codes of each entry in the map's
441 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
442 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
443 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
444 * {@link Object#hashCode}.
445 *
446 * @return the hash code value for this map
447 * @see Map.Entry#hashCode()
448 * @see Object#equals(Object)
449 * @see #equals(Object)
450 */
451 int hashCode();
452 }
453