读JSE源码（四）集合之Map接口
1 Map总体结构
2 Map方法详细说明
3 Map源代码
1 Map总体结构
Map是一个泛型接口，设计用来保存键值对（key-value）的元素，不允许键重复，但允许值重复，每个键最多与一个值对应（An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.）
其接口设计如下
public interface Map
{....}
其中K是键的类型，V是值的类型，采用泛型可以不用指定类型，使代码更通用。下图右边的脸谱face是Map含有的方法，左边的body是JAVA中实现了Map接口的类
2 Map方法详细说明
Map里还有一个内部接口Entry，其作用是方便同时得到key和value的值。一般情况下，要输出Map中的key 和 value 是先得到key的集合，然后再迭代（循环）由每个key得到每个value，而利用Entry中的方法可以一次性得到key 和value的值（使用见TreeMap使用（增删查改）下代码中的4.1和4.2的区别）
Map.Entry接口的方法如下：
Map的方法描述如下
3 Map源代码
/*
* Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE 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 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)
* containsKey(Object key)} method says: "returns true if and
* only if this map contains 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.
*
* @param the type of keys maintained by this map
* @param the type of mapped values
*
* @author Josh Bloch
* @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 specified 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 specified value is null and this
* map does not permit null values
* (optional)
*/
boolean containsValue(Object value);
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
*
If this map permits null values, then a return value of
* {@code 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 {@code null}. The {@link #containsKey
* containsKey} operation may be used to distinguish these two cases.
*
* @param key the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or
* {@code null} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (optional)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (optional)
*/
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
* the 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 the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with 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 NullPointerException if the specified key or value is null
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
*/
V put(K key, V value);
/**
* Removes the mapping for a 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 this map previously associated the key,
* or null if the map contained no mapping for the key.
*
*
If this map permits null values, then a return value of
* null does not necessarily indicate that the map
* contained no mapping for the key; it's also possible that the map
* explicitly mapped the key to null.
*
*
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 the previous value associated with key, or
* null if there was no mapping for key.
* @throws UnsupportedOperationException if the remove operation
* is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (optional)
* @throws NullPointerException if the specified key is null and this
* map does not permit null keys
* (optional)
*/
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 undefined if the
* specified map is modified while the operation is in progress.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the putAll operation
* 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 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
* @throws IllegalArgumentException if some property of a key or value in
* the specified map prevents it from being stored in this map
*/
void putAll(Map m);
/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the clear operation
* is not supported by this map
*/
void clear();
// Views
/**
* Returns a {@link 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 {@link 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 {@link Set} view of the mappings 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, 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 NullPointerException if the backing map does not permit
* null values, and the specified value is null
* @throws IllegalArgumentException if some property of this value
* prevents it from being stored in the backing map
* @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 m1 and
* m2 represent the same mappings if
* m1.entrySet().equals(m2.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 hash codes of each entry in the map's
* entrySet() view. This ensures that m1.equals(m2)
* implies that m1.hashCode()==m2.hashCode() for any two maps
* m1 and m2, as required by the general contract of
* {@link Object#hashCode}.
*
* @return the hash code value for this map
* @see Map.Entry#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
}