1 /*
2 * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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24 */
25
26 package java.util;
27
28 /**
29 * An object that maps keys to values. A map cannot contain duplicate keys;
30 * each key can map to at most one value.
31 *
32 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
33 * was a totally abstract class rather than an interface.
34 *
35 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
36 * allow a map's contents to be viewed as a set of keys, collection of values,
37 * or set of key-value mappings. The <i>order</i> of a map is defined as
38 * the order in which the iterators on the map's collection views return their
39 * elements. Some map implementations, like the <tt>TreeMap</tt> class, make
40 * specific guarantees as to their order; others, like the <tt>HashMap</tt>
41 * class, do not.
42 *
43 * <p>Note: great care must be exercised if mutable objects are used as map
44 * keys. The behavior of a map is not specified if the value of an object is
45 * changed in a manner that affects <tt>equals</tt> comparisons while the
46 * object is a key in the map. A special case of this prohibition is that it
47 * is not permissible for a map to contain itself as a key. While it is
48 * permissible for a map to contain itself as a value, extreme caution is
49 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
50 * well defined on such a map.
51 *
52 * <p>All general-purpose map implementation classes should provide two
53 * "standard" constructors: a void (no arguments) constructor which creates an
54 * empty map, and a constructor with a single argument of type <tt>Map</tt>,
55 * which creates a new map with the same key-value mappings as its argument.
56 * In effect, the latter constructor allows the user to copy any map,
57 * producing an equivalent map of the desired class. There is no way to
58 * enforce this recommendation (as interfaces cannot contain constructors) but
59 * all of the general-purpose map implementations in the JDK comply.
60 *
61 * <p>The "destructive" methods contained in this interface, that is, the
62 * methods that modify the map on which they operate, are specified to throw
63 * <tt>UnsupportedOperationException</tt> if this map does not support the
64 * operation. If this is the case, these methods may, but are not required
65 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
66 * have no effect on the map. For example, invoking the {@link #putAll(Map)}
67 * method on an unmodifiable map may, but is not required to, throw the
68 * exception if the map whose mappings are to be "superimposed" is empty.
69 *
70 * <p>Some map implementations have restrictions on the keys and values they
71 * may contain. For example, some implementations prohibit null keys and
72 * values, and some have restrictions on the types of their keys. Attempting
73 * to insert an ineligible key or value throws an unchecked exception,
74 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
75 * Attempting to query the presence of an ineligible key or value may throw an
76 * exception, or it may simply return false; some implementations will exhibit
77 * the former behavior and some will exhibit the latter. More generally,
78 * attempting an operation on an ineligible key or value whose completion
79 * would not result in the insertion of an ineligible element into the map may
80 * throw an exception or it may succeed, at the option of the implementation.
81 * Such exceptions are marked as "optional" in the specification for this
82 * interface.
83 *
84 * <p>This interface is a member of the
85 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
86 * Java Collections Framework</a>.
87 *
88 * <p>Many methods in Collections Framework interfaces are defined
89 * in terms of the {@link Object#equals(Object) equals} method. For
90 * example, the specification for the {@link #containsKey(Object)
91 * containsKey(Object key)} method says: "returns <tt>true</tt> if and
92 * only if this map contains a mapping for a key <tt>k</tt> such that
93 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
94 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
95 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
96 * be invoked for any key <tt>k</tt>. Implementations are free to
97 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
98 * for example, by first comparing the hash codes of the two keys. (The
99 * {@link Object#hashCode()} specification guarantees that two objects with
100 * unequal hash codes cannot be equal.) More generally, implementations of
101 * the various Collections Framework interfaces are free to take advantage of
102 * the specified behavior of underlying {@link Object} methods wherever the
103 * implementor deems it appropriate.
104 *
105 * @param <K> the type of keys maintained by this map
106 * @param <V> the type of mapped values
107 *
108 * @author Josh Bloch
109 * @see HashMap
110 * @see TreeMap
111 * @see Hashtable
112 * @see SortedMap
113 * @see Collection
114 * @see Set
115 * @since 1.2
116 */
117 public interface Map<K,V> {
118 // Query Operations
119
120 /**
121 * Returns the number of key-value mappings in this map. If the
122 * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
123 * <tt>Integer.MAX_VALUE</tt>.
124 *
125 * @return the number of key-value mappings in this map
126 */
127 int size();
128
129 /**
130 * Returns <tt>true</tt> if this map contains no key-value mappings.
131 *
132 * @return <tt>true</tt> if this map contains no key-value mappings
133 */
134 boolean isEmpty();
135
136 /**
137 * Returns <tt>true</tt> if this map contains a mapping for the specified
138 * key. More formally, returns <tt>true</tt> if and only if
139 * this map contains a mapping for a key <tt>k</tt> such that
140 * <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
141 * at most one such mapping.)
142 *
143 * @param key key whose presence in this map is to be tested
144 * @return <tt>true</tt> if this map contains a mapping for the specified
145 * key
146 * @throws ClassCastException if the key is of an inappropriate type for
147 * this map
148 * (<a href="Collection.html#optional-restrictions">optional</a>)
149 * @throws NullPointerException if the specified key is null and this map
150 * does not permit null keys
151 * (<a href="Collection.html#optional-restrictions">optional</a>)
152 */
153 boolean containsKey(Object key);
154
155 /**
156 * Returns <tt>true</tt> if this map maps one or more keys to the
157 * specified value. More formally, returns <tt>true</tt> if and only if
158 * this map contains at least one mapping to a value <tt>v</tt> such that
159 * <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
160 * will probably require time linear in the map size for most
161 * implementations of the <tt>Map</tt> interface.
162 *
163 * @param value value whose presence in this map is to be tested
164 * @return <tt>true</tt> if this map maps one or more keys to the
165 * specified value
166 * @throws ClassCastException if the value is of an inappropriate type for
167 * this map
168 * (<a href="Collection.html#optional-restrictions">optional</a>)
169 * @throws NullPointerException if the specified value is null and this
170 * map does not permit null values
171 * (<a href="Collection.html#optional-restrictions">optional</a>)
172 */
173 boolean containsValue(Object value);
174
175 /**
176 * Returns the value to which the specified key is mapped,
177 * or {@code null} if this map contains no mapping for the key.
178 *
179 * <p>More formally, if this map contains a mapping from a key
180 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
181 * key.equals(k))}, then this method returns {@code v}; otherwise
182 * it returns {@code null}. (There can be at most one such mapping.)
183 *
184 * <p>If this map permits null values, then a return value of
185 * {@code null} does not <i>necessarily</i> indicate that the map
186 * contains no mapping for the key; it's also possible that the map
187 * explicitly maps the key to {@code null}. The {@link #containsKey
188 * containsKey} operation may be used to distinguish these two cases.
189 *
190 * @param key the key whose associated value is to be returned
191 * @return the value to which the specified key is mapped, or
192 * {@code null} if this map contains no mapping for the key
193 * @throws ClassCastException if the key is of an inappropriate type for
194 * this map
195 * (<a href="Collection.html#optional-restrictions">optional</a>)
196 * @throws NullPointerException if the specified key is null and this map
197 * does not permit null keys
198 * (<a href="Collection.html#optional-restrictions">optional</a>)
199 */
200 V get(Object key);
201
202 // Modification Operations
203
204 /**
205 * Associates the specified value with the specified key in this map
206 * (optional operation). If the map previously contained a mapping for
207 * the key, the old value is replaced by the specified value. (A map
208 * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
209 * if {@link #containsKey(Object) m.containsKey(k)} would return
210 * <tt>true</tt>.)
211 *
212 * @param key key with which the specified value is to be associated
213 * @param value value to be associated with the specified key
214 * @return the previous value associated with <tt>key</tt>, or
215 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
216 * (A <tt>null</tt> return can also indicate that the map
217 * previously associated <tt>null</tt> with <tt>key</tt>,
218 * if the implementation supports <tt>null</tt> values.)
219 * @throws UnsupportedOperationException if the <tt>put</tt> operation
220 * is not supported by this map
221 * @throws ClassCastException if the class of the specified key or value
222 * prevents it from being stored in this map
223 * @throws NullPointerException if the specified key or value is null
224 * and this map does not permit null keys or values
225 * @throws IllegalArgumentException if some property of the specified key
226 * or value prevents it from being stored in this map
227 */
228 V put(K key, V value);
229
230 /**
231 * Removes the mapping for a key from this map if it is present
232 * (optional operation). More formally, if this map contains a mapping
233 * from key <tt>k</tt> to value <tt>v</tt> such that
234 * <code>(key==null ? k==null : key.equals(k))</code>, that mapping
235 * is removed. (The map can contain at most one such mapping.)
236 *
237 * <p>Returns the value to which this map previously associated the key,
238 * or <tt>null</tt> if the map contained no mapping for the key.
239 *
240 * <p>If this map permits null values, then a return value of
241 * <tt>null</tt> does not <i>necessarily</i> indicate that the map
242 * contained no mapping for the key; it's also possible that the map
243 * explicitly mapped the key to <tt>null</tt>.
244 *
245 * <p>The map will not contain a mapping for the specified key once the
246 * call returns.
247 *
248 * @param key key whose mapping is to be removed from the map
249 * @return the previous value associated with <tt>key</tt>, or
250 * <tt>null</tt> if there was no mapping for <tt>key</tt>.
251 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
252 * is not supported by this map
253 * @throws ClassCastException if the key is of an inappropriate type for
254 * this map
255 * (<a href="Collection.html#optional-restrictions">optional</a>)
256 * @throws NullPointerException if the specified key is null and this
257 * map does not permit null keys
258 * (<a href="Collection.html#optional-restrictions">optional</a>)
259 */
260 V remove(Object key);
261
262
263 // Bulk Operations
264
265 /**
266 * Copies all of the mappings from the specified map to this map
267 * (optional operation). The behavior of this operation is undefined
268 * if the specified map is modified while the operation is in progress.
269 *
270 * @param m mappings to be stored in this map
271 * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
272 * is not supported by this map
273 * @throws ClassCastException if the class of a key or value in the
274 * specified map prevents it from being stored in this map
275 * @throws NullPointerException if the specified map is null, or if
276 * this map does not permit null keys or values, and the
277 * specified map contains null keys or values
278 * @throws IllegalArgumentException if some property of a key or value in
279 * the specified map prevents it from being stored in this map
280 */
281 void putAll(Map<? extends K, ? extends V> m);
282
283 /**
284 * Removes all of the mappings from this map (optional operation).
285 * The map will be empty after this call returns.
286 *
287 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
288 * is not supported by this map
289 */
290 void clear();
291
292
293 // Views
294
295 /**
296 * Returns a {@link Set} view of the keys contained in this map.
297 * The set is backed by the map, so changes to the map are
298 * reflected in the set, and vice-versa. If the map is modified
299 * while an iteration over the set is in progress (except through
300 * the iterator's own <tt>remove</tt> operation), the results of
301 * the iteration are undefined. The set supports element removal,
302 * which removes the corresponding mapping from the map, via the
303 * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
304 * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
305 * operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
306 * operations.
307 *
308 * @return a set view of the keys contained in this map
309 */
310 Set<K> keySet();
311
312 /**
313 * Returns a {@link Collection} view of the values contained in this map.
314 * The collection is backed by the map, so changes to the map are
315 * reflected in the collection, and vice-versa. If the map is
316 * modified while an iteration over the collection is in progress
317 * (except through the iterator's own <tt>remove</tt> operation),
318 * the results of the iteration are undefined. The collection
319 * supports element removal, which removes the corresponding
320 * mapping from the map, via the <tt>Iterator.remove</tt>,
321 * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
322 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
323 * support the <tt>add</tt> or <tt>addAll</tt> operations.
324 *
325 * @return a collection view of the values contained in this map
326 */
327 Collection<V> values();
328
329 /**
330 * Returns a {@link Set} view of the mappings contained in this map.
331 * The set is backed by the map, so changes to the map are
332 * reflected in the set, and vice-versa. If the map is modified
333 * while an iteration over the set is in progress (except through
334 * the iterator's own <tt>remove</tt> operation, or through the
335 * <tt>setValue</tt> operation on a map entry returned by the
336 * iterator) the results of the iteration are undefined. The set
337 * supports element removal, which removes the corresponding
338 * mapping from the map, via the <tt>Iterator.remove</tt>,
339 * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
340 * <tt>clear</tt> operations. It does not support the
341 * <tt>add</tt> or <tt>addAll</tt> operations.
342 *
343 * @return a set view of the mappings contained in this map
344 */
345 Set<Map.Entry<K, V>> entrySet();
346
347 /**
348 * A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
349 * a collection-view of the map, whose elements are of this class. The
350 * <i>only</i> way to obtain a reference to a map entry is from the
351 * iterator of this collection-view. These <tt>Map.Entry</tt> objects are
352 * valid <i>only</i> for the duration of the iteration; more formally,
353 * the behavior of a map entry is undefined if the backing map has been
354 * modified after the entry was returned by the iterator, except through
355 * the <tt>setValue</tt> operation on the map entry.
356 *
357 * @see Map#entrySet()
358 * @since 1.2
359 */
360 interface Entry<K,V> {
361 /**
362 * Returns the key corresponding to this entry.
363 *
364 * @return the key corresponding to this entry
365 * @throws IllegalStateException implementations may, but are not
366 * required to, throw this exception if the entry has been
367 * removed from the backing map.
368 */
369 K getKey();
370
371 /**
372 * Returns the value corresponding to this entry. If the mapping
373 * has been removed from the backing map (by the iterator's
374 * <tt>remove</tt> operation), the results of this call are undefined.
375 *
376 * @return the value corresponding to this entry
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 getValue();
382
383 /**
384 * Replaces the value corresponding to this entry with the specified
385 * value (optional operation). (Writes through to the map.) The
386 * behavior of this call is undefined if the mapping has already been
387 * removed from the map (by the iterator's <tt>remove</tt> operation).
388 *
389 * @param value new value to be stored in this entry
390 * @return old value corresponding to the entry
391 * @throws UnsupportedOperationException if the <tt>put</tt> operation
392 * is not supported by the backing map
393 * @throws ClassCastException if the class of the specified value
394 * prevents it from being stored in the backing map
395 * @throws NullPointerException if the backing map does not permit
396 * null values, and the specified value is null
397 * @throws IllegalArgumentException if some property of this value
398 * prevents it from being stored in the backing map
399 * @throws IllegalStateException implementations may, but are not
400 * required to, throw this exception if the entry has been
401 * removed from the backing map.
402 */
403 V setValue(V value);
404
405 /**
406 * Compares the specified object with this entry for equality.
407 * Returns <tt>true</tt> if the given object is also a map entry and
408 * the two entries represent the same mapping. More formally, two
409 * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
410 * if<pre>
411 * (e1.getKey()==null ?
412 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
413 * (e1.getValue()==null ?
414 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
415 * </pre>
416 * This ensures that the <tt>equals</tt> method works properly across
417 * different implementations of the <tt>Map.Entry</tt> interface.
418 *
419 * @param o object to be compared for equality with this map entry
420 * @return <tt>true</tt> if the specified object is equal to this map
421 * entry
422 */
423 boolean equals(Object o);
424
425 /**
426 * Returns the hash code value for this map entry. The hash code
427 * of a map entry <tt>e</tt> is defined to be: <pre>
428 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
429 * (e.getValue()==null ? 0 : e.getValue().hashCode())
430 * </pre>
431 * This ensures that <tt>e1.equals(e2)</tt> implies that
432 * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
433 * <tt>e1</tt> and <tt>e2</tt>, as required by the general
434 * contract of <tt>Object.hashCode</tt>.
435 *
436 * @return the hash code value for this map entry
437 * @see Object#hashCode()
438 * @see Object#equals(Object)
439 * @see #equals(Object)
440 */
441 int hashCode();
442 }
443
444 // Comparison and hashing
445
446 /**
447 * Compares the specified object with this map for equality. Returns
448 * <tt>true</tt> if the given object is also a map and the two maps
449 * represent the same mappings. More formally, two maps <tt>m1</tt> and
450 * <tt>m2</tt> represent the same mappings if
451 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
452 * <tt>equals</tt> method works properly across different implementations
453 * of the <tt>Map</tt> interface.
454 *
455 * @param o object to be compared for equality with this map
456 * @return <tt>true</tt> if the specified object is equal to this map
457 */
458 boolean equals(Object o);
459
460 /**
461 * Returns the hash code value for this map. The hash code of a map is
462 * defined to be the sum of the hash codes of each entry in the map's
463 * <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
464 * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
465 * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
466 * {@link Object#hashCode}.
467 *
468 * @return the hash code value for this map
469 * @see Map.Entry#hashCode()
470 * @see Object#equals(Object)
471 * @see #equals(Object)
472 */
473 int hashCode();
474
475 }