/* * @(#)Map.java 1.48 04/06/28 * * Copyright 2004 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; /** * An object that maps keys to values. A map cannot contain duplicate keys; * each key can map to at most one value. * *

This interface takes the place of the Dictionary class, which * was a totally abstract class rather than an interface. * *

The Map interface provides three collection views, which * allow a map's contents to be viewed as a set of keys, collection of values, * or set of key-value mappings. The order of a map is defined as * the order in which the iterators on the map's collection views return their * elements. Some map implementations, like the TreeMap class, make * specific guarantees as to their order; others, like the HashMap * class, do not. * *

Note: great care must be exercised if mutable objects are used as map * keys. The behavior of a map is not specified if the value of an object is * changed in a manner that affects equals comparisons while the object is a * key in the map. A special case of this prohibition is that it is not * permissible for a map to contain itself as a key. While it is permissible * for a map to contain itself as a value, extreme caution is advised: the * equals and hashCode methods are no longer well defined on a such a map. * *

All general-purpose map implementation classes should provide two * "standard" constructors: a void (no arguments) constructor which creates an * empty map, and a constructor with a single argument of type Map, * which creates a new map with the same key-value mappings as its argument. * In effect, the latter constructor allows the user to copy any map, * producing an equivalent map of the desired class. There is no way to * enforce this recommendation (as interfaces cannot contain constructors) but * all of the general-purpose map implementations in the JDK comply. * *

The "destructive" methods contained in this interface, that is, the * methods that modify the map on which they operate, are specified to throw * UnsupportedOperationException if this map does not support the * operation. If this is the case, these methods may, but are not required * to, throw an UnsupportedOperationException if the invocation would * have no effect on the map. For example, invoking the {@link #putAll(Map)} * method on an unmodifiable map may, but is not required to, throw the * exception if the map whose mappings are to be "superimposed" is empty. * *

Some map implementations have restrictions on the keys and values they * may contain. For example, some implementations prohibit null keys and * values, and some have restrictions on the types of their keys. Attempting * to insert an ineligible key or value throws an unchecked exception, * typically NullPointerException or ClassCastException. * Attempting to query the presence of an ineligible key or value may throw an * exception, or it may simply return false; some implementations will exhibit * the former behavior and some will exhibit the latter. More generally, * attempting an operation on an ineligible key or value whose completion * would not result in the insertion of an ineligible element into the map may * throw an exception or it may succeed, at the option of the implementation. * Such exceptions are marked as "optional" in the specification for this * interface. * *

This interface is a member of the * * Java Collections Framework. * *

Many methods in Collections Framework interfaces are defined * in terms of the {@link Object#equals(Object) equals} method. For * example, the specification for the {@link #containsKey(Object) * contains(Object key)} method says: "returns true if and * only if this map contain a mapping for a key k such that * (key==null ? k==null : key.equals(k))." This specification should * not be construed to imply that invoking Map.containsKey * with a non-null argument key will cause key.equals(k) to * be invoked for any key k. Implementations are free to * implement optimizations whereby the equals invocation is avoided, * for example, by first comparing the hash codes of the two keys. (The * {@link Object#hashCode()} specification guarantees that two objects with * unequal hash codes cannot be equal.) More generally, implementations of * the various Collections Framework interfaces are free to take advantage of * the specified behavior of underlying {@link Object} methods wherever the * implementor deems it appropriate. * * @author Josh Bloch * @version 1.48, 06/28/04 * @see HashMap * @see TreeMap * @see Hashtable * @see SortedMap * @see Collection * @see Set * @since 1.2 */ public interface Map { // Query Operations /** * Returns the number of key-value mappings in this map. If the * map contains more than Integer.MAX_VALUE elements, returns * Integer.MAX_VALUE. * * @return the number of key-value mappings in this map. */ int size(); /** * Returns true if this map contains no key-value mappings. * * @return true if this map contains no key-value mappings. */ boolean isEmpty(); /** * Returns true if this map contains a mapping for the specified * key. More formally, returns true if and only if * this map contains a mapping for a key k such that * (key==null ? k==null : key.equals(k)). (There can be * at most one such mapping.) * * @param key key whose presence in this map is to be tested. * @return true if this map contains a mapping for the specified * key. * * @throws ClassCastException if the key is of an inappropriate type for * this map (optional). * @throws NullPointerException if the key is null and this map * does not permit null keys (optional). */ boolean containsKey(Object key); /** * Returns true if this map maps one or more keys to the * specified value. More formally, returns true if and only if * this map contains at least one mapping to a value v such that * (value==null ? v==null : value.equals(v)). This operation * will probably require time linear in the map size for most * implementations of the Map interface. * * @param value value whose presence in this map is to be tested. * @return true if this map maps one or more keys to the * specified value. * @throws ClassCastException if the value is of an inappropriate type for * this map (optional). * @throws NullPointerException if the value is null and this map * does not permit null values (optional). */ boolean containsValue(Object value); /** * Returns the value to which this map maps the specified key. Returns * null if the map contains no mapping for this key. A return * value of null does not necessarily indicate that the * map contains no mapping for the key; it's also possible that the map * explicitly maps the key to null. The containsKey * operation may be used to distinguish these two cases. * *

More formally, if this map contains a mapping from a key * k to a value v such that (key==null ? k==null : * key.equals(k)), then this method returns v; otherwise * it returns null. (There can be at most one such mapping.) * * @param key key whose associated value is to be returned. * @return the value to which this map maps the specified key, or * null if the map contains no mapping for this key. * * @throws ClassCastException if the key is of an inappropriate type for * this map (optional). * @throws NullPointerException if the key is null and this map * does not permit null keys (optional). * * @see #containsKey(Object) */ V get(Object key); // Modification Operations /** * Associates the specified value with the specified key in this map * (optional operation). If the map previously contained a mapping for * this key, the old value is replaced by the specified value. (A map * m is said to contain a mapping for a key k if and only * if {@link #containsKey(Object) m.containsKey(k)} would return * true.)) * * @param key key with which the specified value is to be associated. * @param value value to be associated with the specified key. * @return previous value associated with specified key, or null * if there was no mapping for key. A null return can * also indicate that the map previously associated null * with the specified key, if the implementation supports * null values. * * @throws UnsupportedOperationException if the put operation is * not supported by this map. * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map. * @throws IllegalArgumentException if some aspect of this key or value * prevents it from being stored in this map. * @throws NullPointerException if this map does not permit null * keys or values, and the specified key or value is * null. */ V put(K key, V value); /** * Removes the mapping for this key from this map if it is present * (optional operation). More formally, if this map contains a mapping * from key k to value v such that * (key==null ? k==null : key.equals(k)), that mapping * is removed. (The map can contain at most one such mapping.) * *

Returns the value to which the map previously associated the key, or * null if the map contained no mapping for this key. (A * null return can also indicate that the map previously * associated null with the specified key if the implementation * supports null values.) The map will not contain a mapping for * the specified key once the call returns. * * @param key key whose mapping is to be removed from the map. * @return previous value associated with specified key, or null * if there was no mapping for key. * * @throws ClassCastException if the key is of an inappropriate type for * this map (optional). * @throws NullPointerException if the key is null and this map * does not permit null keys (optional). * @throws UnsupportedOperationException if the remove method is * not supported by this map. */ V remove(Object key); // Bulk Operations /** * Copies all of the mappings from the specified map to this map * (optional operation). The effect of this call is equivalent to that * of calling {@link #put(Object,Object) put(k, v)} on this map once * for each mapping from key k to value v in the * specified map. The behavior of this operation is unspecified if the * specified map is modified while the operation is in progress. * * @param t Mappings to be stored in this map. * * @throws UnsupportedOperationException if the putAll method is * not supported by this map. * * @throws ClassCastException if the class of a key or value in the * specified map prevents it from being stored in this map. * * @throws IllegalArgumentException some aspect of a key or value in the * specified map prevents it from being stored in this map. * @throws NullPointerException if the specified map is null, or if * this map does not permit null keys or values, and the * specified map contains null keys or values. */ void putAll(Map t); /** * Removes all mappings from this map (optional operation). * * @throws UnsupportedOperationException clear is not supported by this * map. */ void clear(); // Views /** * Returns a set view of the keys contained in this map. The set is * backed by the map, so changes to the map are reflected in the set, and * vice-versa. If the map is modified while an iteration over the set is * in progress (except through the iterator's own remove * operation), the results of the iteration are undefined. The set * supports element removal, which removes the corresponding mapping from * the map, via the Iterator.remove, Set.remove, * removeAll retainAll, and clear operations. * It does not support the add or addAll operations. * * @return a set view of the keys contained in this map. */ Set keySet(); /** * Returns a collection view of the values contained in this map. The * collection is backed by the map, so changes to the map are reflected in * the collection, and vice-versa. If the map is modified while an * iteration over the collection is in progress (except through the * iterator's own remove operation), the results of the * iteration are undefined. The collection supports element removal, * which removes the corresponding mapping from the map, via the * Iterator.remove, Collection.remove, * removeAll, retainAll and clear operations. * It does not support the add or addAll operations. * * @return a collection view of the values contained in this map. */ Collection values(); /** * Returns a set view of the mappings contained in this map. Each element * in the returned set is a {@link Map.Entry}. The set is backed by the * map, so changes to the map are reflected in the set, and vice-versa. * If the map is modified while an iteration over the set is in progress * (except through the iterator's own remove operation, or through * the setValue operation on a map entry returned by the iterator) * the results of the iteration are undefined. The set supports element * removal, which removes the corresponding mapping from the map, via the * Iterator.remove, Set.remove, removeAll, * retainAll and clear operations. It does not support * the add or addAll operations. * * @return a set view of the mappings contained in this map. */ Set> entrySet(); /** * A map entry (key-value pair). The Map.entrySet method returns * a collection-view of the map, whose elements are of this class. The * only way to obtain a reference to a map entry is from the * iterator of this collection-view. These Map.Entry objects are * valid only for the duration of the iteration; more formally, * the behavior of a map entry is undefined if the backing map has been * modified after the entry was returned by the iterator, except through * the setValue operation on the map entry. * * @see Map#entrySet() * @since 1.2 */ interface Entry { /** * Returns the key corresponding to this entry. * * @return the key corresponding to this entry. * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map */ K getKey(); /** * Returns the value corresponding to this entry. If the mapping * has been removed from the backing map (by the iterator's * remove operation), the results of this call are undefined. * * @return the value corresponding to this entry. * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map */ V getValue(); /** * Replaces the value corresponding to this entry with the specified * value (optional operation). (Writes through to the map.) The * behavior of this call is undefined if the mapping has already been * removed from the map (by the iterator's remove operation). * * @param value new value to be stored in this entry. * @return old value corresponding to the entry. * * @throws UnsupportedOperationException if the put operation * is not supported by the backing map. * @throws ClassCastException if the class of the specified value * prevents it from being stored in the backing map. * @throws IllegalArgumentException if some aspect of this value * prevents it from being stored in the backing map. * @throws NullPointerException if the backing map does not permit * null values, and the specified value is * null. * @throws IllegalStateException implementations may, but are not * required to, throw this exception if the entry has been * removed from the backing map */ V setValue(V value); /** * Compares the specified object with this entry for equality. * Returns true if the given object is also a map entry and * the two entries represent the same mapping. More formally, two * entries e1 and e2 represent the same mapping * if

         *     (e1.getKey()==null ?
         *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
         *     (e1.getValue()==null ?
         *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
         *
* This ensures that the equals method works properly across * different implementations of the Map.Entry interface. * * @param o object to be compared for equality with this map entry. * @return true if the specified object is equal to this map * entry. */ boolean equals(Object o); /** * Returns the hash code value for this map entry. The hash code * of a map entry e is defined to be: 
	 *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
	 *     (e.getValue()==null ? 0 : e.getValue().hashCode())
         *
* This ensures that e1.equals(e2) implies that * e1.hashCode()==e2.hashCode() for any two Entries * e1 and e2, as required by the general * contract of Object.hashCode. * * @return the hash code value for this map entry. * @see Object#hashCode() * @see Object#equals(Object) * @see #equals(Object) */ int hashCode(); } // Comparison and hashing /** * Compares the specified object with this map for equality. Returns * true if the given object is also a map and the two Maps * represent the same mappings. More formally, two maps t1 and * t2 represent the same mappings if * t1.entrySet().equals(t2.entrySet()). This ensures that the * equals method works properly across different implementations * of the Map interface. * * @param o object to be compared for equality with this map. * @return true if the specified object is equal to this map. */ boolean equals(Object o); /** * Returns the hash code value for this map. The hash code of a map * is defined to be the sum of the hashCodes of each entry in the map's * entrySet view. This ensures that t1.equals(t2) implies * that t1.hashCode()==t2.hashCode() for any two maps * t1 and t2, as required by the general * contract of Object.hashCode. * * @return the hash code value for this map. * @see Map.Entry#hashCode() * @see Object#hashCode() * @see Object#equals(Object) * @see #equals(Object) */ int hashCode(); }