java Map集合框架之HashMap

package com.my.collection;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

/**
* 关于Map集合框架 特点：该集合存储键值对。一对一对往里存。而且要保证键的唯一性。 Map集合框架子类
* ：HashTable,HashMap,TreeMap、LinkedHashMap Map接口提供的方法有 V get(Object key);
* 根据key获取value V put(K key, V value);将数据以键值对的方式存进集合中。 void putAll(Map<? extends
* K, ? extends V> m);从指定映射中将所有映射关系复制到此映射中 int size(); 获取集合的大小 boolean
* isEmpty();判断集合是否为空 boolean containsKey(Object key);判断集合中是否包含指定key boolean
* containsValue(Object value);判断集合中是否包含指定value V remove(Object key);删除某个元素 void
* clear();清除集合 Set<K> keySet();获取key集合 Collection<V> values();value集合
* Set<Map.Entry<K, V>> entrySet(); 等
*
*
* @author wbw
*
*/
public class MyMap {

private static Map<String, Object> map = null;

/**
* 使用put()方法初始化HshMap
*/
public static void initMap() {
System.out.println("初始化Hashmap.......");
// HashMap是一个罪常用的Map,它根据键的HashCode
// 值存储数据,根据键可以直接获取它的值，具有很快的访问速度。HashMap最多只允许一条记录的键为Null;允许多条记录的值为
// Null;HashMap不支持线程的同步，即任一时刻可以有多个线程同时写HashMap;可能会导致数据的不一致。如果需要同步，可以用
// Collections的synchronizedMap方法使HashMap具有同步的能力.
map = new HashMap<String, Object>();
map.put("1", "a");
map.put("1", "c");// 不允许键重复,允许值重复,如果键重复,最后的key的值将会覆盖第一个key的值
map.put("2", "b");
map.put("3", "d");
map.put("3", null);// 键不为null,值为null
map.put(null, "e");// 键为null,值不为null

}

/**
* 遍历key和values
*/
public static void outEelem() {
System.out.println("#####遍历key和values######");
// 遍历方式一
System.out.println("#####map.keySet()遍历key和values######");
for (String key : map.keySet()) {
System.out.println("key" + key + "and value" + map.get(key));
}
// 遍历方式二：
Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();
System.out
.println("#####通过Map.Entry<String, Object>使用Iterator遍历key和values######");
while (it.hasNext()) {
Map.Entry<String, Object> entry = it.next();
System.out.println("key=   " + entry.getKey() + "   and value=   "
+ entry.getValue());
}
// 遍历方式三：
System.out
.println("##### 通过Map.Entry<String, Object> entry 遍历key和values######");
for (Map.Entry<String, Object> entry : map.entrySet()) {
System.out.println("key=   " + entry.getKey() + "  and value=   "
+ entry.getValue());
}
// 遍历方式四：
System.out.println("#####通过Map.values()遍历所有的value，但不能遍历key######");
for (Object value : map.values()) {
System.out.println("Values   " + value);
}
}

/**
* 通过containsKey()方法和containsValue()方法判断key和值是否存在
*/
public static void judegKeyAndValue(String key, String value) {
if (map.containsKey(key)) {
System.out.println(key + "存在集合中");
}
if (map.containsValue(value)) {
System.out.println(value + "存在集合中");
}

}

/**
* 通过size()方法获取map的大小
*
* @param map
* @return
*/
public static int size(Map<String, Object> map) {
if (!map.isEmpty()) {// 使用map的isEmpty()方法判断集合是否为空
return map.size();
}
return 0;

}

/**
* 通过clear()方法清空map
*
* @param map
*/
public static void clear(Map<String, Object> map) {
if (!map.isEmpty()) {// 使用map的isEmpty()方法判断集合是否为空
map.clear();
}
}

/**
* 测试
*
* @param args
*/
public static void main(String[] args) {
initMap();// 初始化map
outEelem();// 遍历map
judegKeyAndValue("1", "b");// 1存在集合中 b存在集合中
judegKeyAndValue("1", "a");// 1存在集合中 但是a 不存在集合中,因为其值已经被覆盖
System.out.println(size(map));// 集合大小
clear(map);// 清空集合之后再遍历集合
outEelem();// 遍历map
}

}

/*
* 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.
*
* <p>This interface takes the place of the <tt>Dictionary</tt> class, which
* was a totally abstract class rather than an interface.
*
* <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
* allow a map's contents to be viewed as a set of keys, collection of values,
* or set of key-value mappings.  The <i>order</i> 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 <tt>TreeMap</tt> class, make
* specific guarantees as to their order; others, like the <tt>HashMap</tt>
* class, do not.
*
* <p>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 <tt>equals</tt> 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 <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
* well defined on such a map.
*
* <p>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 <tt>Map</tt>,
* 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.
*
* <p>The "destructive" methods contained in this interface, that is, the
* methods that modify the map on which they operate, are specified to throw
* <tt>UnsupportedOperationException</tt> if this map does not support the
* operation.  If this is the case, these methods may, but are not required
* to, throw an <tt>UnsupportedOperationException</tt> 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.
*
* <p>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 <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
* 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.
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* <p>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 <tt>true</tt> if and
* only if this map contains a mapping for a key <tt>k</tt> such that
* <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
* <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
* with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
* be invoked for any key <tt>k</tt>.  Implementations are free to
* implement optimizations whereby the <tt>equals</tt> 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 <K> the type of keys maintained by this map
* @param <V> 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<K,V> {
// Query Operations

/**
* Returns the number of key-value mappings in this map.  If the
* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of key-value mappings in this map
*/
int size();

/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
boolean isEmpty();

/**
* Returns <tt>true</tt> if this map contains a mapping for the specified
* key.  More formally, returns <tt>true</tt> if and only if
* this map contains a mapping for a key <tt>k</tt> such that
* <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be
* at most one such mapping.)
*
* @param key key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
*         key
* @throws ClassCastException if the key is of an inappropriate type for
*         this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
*         does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean containsKey(Object key);

/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.  More formally, returns <tt>true</tt> if and only if
* this map contains at least one mapping to a value <tt>v</tt> such that
* <tt>(value==null ? v==null : value.equals(v))</tt>.  This operation
* will probably require time linear in the map size for most
* implementations of the <tt>Map</tt> interface.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> 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
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified value is null and this
*         map does not permit null values
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
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.
*
* <p>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.)
*
* <p>If this map permits null values, then a return value of
* {@code null} does not <i>necessarily</i> 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
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
*         does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
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
* <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
* if {@link #containsKey(Object) m.containsKey(k)} would return
* <tt>true</tt>.)
*
* @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 <tt>key</tt>, or
*         <tt>null</tt> if there was no mapping for <tt>key</tt>.
*         (A <tt>null</tt> return can also indicate that the map
*         previously associated <tt>null</tt> with <tt>key</tt>,
*         if the implementation supports <tt>null</tt> values.)
* @throws UnsupportedOperationException if the <tt>put</tt> 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 <tt>k</tt> to value <tt>v</tt> such that
* <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
* is removed.  (The map can contain at most one such mapping.)
*
* <p>Returns the value to which this map previously associated the key,
* or <tt>null</tt> if the map contained no mapping for the key.
*
* <p>If this map permits null values, then a return value of
* <tt>null</tt> does not <i>necessarily</i> indicate that the map
* contained no mapping for the key; it's also possible that the map
* explicitly mapped the key to <tt>null</tt>.
*
* <p>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 <tt>key</tt>, or
*         <tt>null</tt> if there was no mapping for <tt>key</tt>.
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
*         is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
*         this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this
*         map does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
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 <tt>k</tt> to value <tt>v</tt> 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 <tt>putAll</tt> 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<? extends K, ? extends V> m);

/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> 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 <tt>remove</tt> operation), the results of
* the iteration are undefined.  The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @return a set view of the keys contained in this map
*/
Set<K> 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 <tt>remove</tt> operation),
* the results of the iteration are undefined.  The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the values contained in this map
*/
Collection<V> 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 <tt>remove</tt> operation, or through the
* <tt>setValue</tt> 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 <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations.  It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a set view of the mappings contained in this map
*/
Set<Map.Entry<K, V>> entrySet();

/**
* A map entry (key-value pair).  The <tt>Map.entrySet</tt> method returns
* a collection-view of the map, whose elements are of this class.  The
* <i>only</i> way to obtain a reference to a map entry is from the
* iterator of this collection-view.  These <tt>Map.Entry</tt> objects are
* valid <i>only</i> 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 <tt>setValue</tt> operation on the map entry.
*
* @see Map#entrySet()
* @since 1.2
*/
interface Entry<K,V> {
/**
* 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
* <tt>remove</tt> 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 <tt>remove</tt> operation).
*
* @param value new value to be stored in this entry
* @return old value corresponding to the entry
* @throws UnsupportedOperationException if the <tt>put</tt> 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 <tt>true</tt> if the given object is also a map entry and
* the two entries represent the same mapping.  More formally, two
* entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
* if<pre>
*     (e1.getKey()==null ?
*      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
*     (e1.getValue()==null ?
*      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
* </pre>
* This ensures that the <tt>equals</tt> method works properly across
* different implementations of the <tt>Map.Entry</tt> interface.
*
* @param o object to be compared for equality with this map entry
* @return <tt>true</tt> 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 <tt>e</tt> is defined to be: <pre>
*     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
*     (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* This ensures that <tt>e1.equals(e2)</tt> implies that
* <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
* <tt>e1</tt> and <tt>e2</tt>, as required by the general
* contract of <tt>Object.hashCode</tt>.
*
* @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
* <tt>true</tt> if the given object is also a map and the two maps
* represent the same mappings.  More formally, two maps <tt>m1</tt> and
* <tt>m2</tt> represent the same mappings if
* <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the
* <tt>equals</tt> method works properly across different implementations
* of the <tt>Map</tt> interface.
*
* @param o object to be compared for equality with this map
* @return <tt>true</tt> 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
* <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
* implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
* <tt>m1</tt> and <tt>m2</tt>, 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();

}

/*
* Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/

package java.util;
import java.io.*;

/**
* Hash table based implementation of the <tt>Map</tt> interface.  This
* implementation provides all of the optional map operations, and permits
* <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, except that it is
* unsynchronized and permits nulls.)  This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
*
* <p>This implementation provides constant-time performance for the basic
* operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
* disperses the elements properly among the buckets.  Iteration over
* collection views requires time proportional to the "capacity" of the
* <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
* of key-value mappings).  Thus, it's very important not to set the initial
* capacity too high (or the load factor too low) if iteration performance is
* important.
*
* <p>An instance of <tt>HashMap</tt> has two parameters that affect its
* performance: <i>initial capacity</i> and <i>load factor</i>.  The
* <i>capacity</i> is the number of buckets in the hash table, and the initial
* capacity is simply the capacity at the time the hash table is created.  The
* <i>load factor</i> is a measure of how full the hash table is allowed to
* get before its capacity is automatically increased.  When the number of
* entries in the hash table exceeds the product of the load factor and the
* current capacity, the hash table is <i>rehashed</i> (that is, internal data
* structures are rebuilt) so that the hash table has approximately twice the
* number of buckets.
*
* <p>As a general rule, the default load factor (.75) offers a good tradeoff
* between time and space costs.  Higher values decrease the space overhead
* but increase the lookup cost (reflected in most of the operations of the
* <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>).  The
* expected number of entries in the map and its load factor should be taken
* into account when setting its initial capacity, so as to minimize the
* number of rehash operations.  If the initial capacity is greater
* than the maximum number of entries divided by the load factor, no
* rehash operations will ever occur.
*
* <p>If many mappings are to be stored in a <tt>HashMap</tt> instance,
* creating it with a sufficiently large capacity will allow the mappings to
* be stored more efficiently than letting it perform automatic rehashing as
* needed to grow the table.
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a hash map concurrently, and at least one of
* the threads modifies the map structurally, it <i>must</i> be
* synchronized externally.  (A structural modification is any operation
* that adds or deletes one or more mappings; merely changing the value
* associated with a key that an instance already contains is not a
* structural modification.)  This is typically accomplished by
* synchronizing on some object that naturally encapsulates the map.
*
* If no such object exists, the map should be "wrapped" using the
* {@link Collections#synchronizedMap Collections.synchronizedMap}
* method.  This is best done at creation time, to prevent accidental
* unsynchronized access to the map:<pre>
*   Map m = Collections.synchronizedMap(new HashMap(...));</pre>
*
* <p>The iterators returned by all of this class's "collection view methods"
* are <i>fail-fast</i>: if the map is structurally modified at any time after
* the iterator is created, in any way except through the iterator's own
* <tt>remove</tt> method, the iterator will throw a
* {@link ConcurrentModificationException}.  Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than risking
* arbitrary, non-deterministic behavior at an undetermined time in the
* future.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification.  Fail-fast iterators
* throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
*
* @author  Doug Lea
* @author  Josh Bloch
* @author  Arthur van Hoff
* @author  Neal Gafter
* @see     Object#hashCode()
* @see     Collection
* @see     Map
* @see     TreeMap
* @see     Hashtable
* @since   1.2
*/

public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
{

/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;

/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;

/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry[] table;

/**
* The number of key-value mappings contained in this map.
*/
transient int size;

/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
int threshold;

/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;

/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash).  This field is used to make iterators on Collection-views of
* the HashMap fail-fast.  (See ConcurrentModificationException).
*/
transient int modCount;

/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param  initialCapacity the initial capacity
* @param  loadFactor      the load factor
* @throws IllegalArgumentException if the initial capacity is negative
*         or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);

// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;

this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
}

/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param  initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];
init();
}

/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param   m the map whose mappings are to be placed in this map
* @throws  NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
putAllForCreate(m);
}

// internal utilities

/**
* Initialization hook for subclasses. This method is called
* in all constructors and pseudo-constructors (clone, readObject)
* after HashMap has been initialized but before any entries have
* been inserted.  (In the absence of this method, readObject would
* require explicit knowledge of subclasses.)
*/
void init() {
}

/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions.  This is critical
* because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits. Note: Null keys always map to hash 0, thus index 0.
*/
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}

/**
* Returns index for hash code h.
*/
static int indexFor(int h, int length) {
return h & (length-1);
}

/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map
*/
public int size() {
return size;
}

/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
public boolean isEmpty() {
return size == 0;
}

/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>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.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* 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.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
return e.value;
}
return null;
}

/**
* Offloaded version of get() to look up null keys.  Null keys map
* to index 0.  This null case is split out into separate methods
* for the sake of performance in the two most commonly used
* operations (get and put), but incorporated with conditionals in
* others.
*/
private V getForNullKey() {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}

/**
* Returns <tt>true</tt> if this map contains a mapping for the
* specified key.
*
* @param   key   The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return getEntry(key) != null;
}

/**
* Returns the entry associated with the specified key in the
* HashMap.  Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}

/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @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 <tt>key</tt>, or
*         <tt>null</tt> if there was no mapping for <tt>key</tt>.
*         (A <tt>null</tt> return can also indicate that the map
*         previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}

modCount++;
addEntry(hash, key, value, i);
return null;
}

/**
* Offloaded version of put for null keys
*/
private V putForNullKey(V value) {
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(0, null, value, 0);
return null;
}

/**
* This method is used instead of put by constructors and
* pseudoconstructors (clone, readObject).  It does not resize the table,
* check for comodification, etc.  It calls createEntry rather than
* addEntry.
*/
private void putForCreate(K key, V value) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);

/**
* Look for preexisting entry for key.  This will never happen for
* clone or deserialize.  It will only happen for construction if the
* input Map is a sorted map whose ordering is inconsistent w/ equals.
*/
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}

createEntry(hash, key, value, i);
}

private void putAllForCreate(Map<? extends K, ? extends V> m) {
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}

/**
* Rehashes the contents of this map into a new array with a
* larger capacity.  This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
*        must be greater than current capacity unless current
*        capacity is MAXIMUM_CAPACITY (in which case value
*        is irrelevant).
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}

Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
}

/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
src[j] = null;
do {
Entry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}

/**
* Copies all of the mappings from the specified map to this map.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map.
*
* @param m mappings to be stored in this map
* @throws NullPointerException if the specified map is null
*/
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;

/*
* Expand the map if the map if the number of mappings to be added
* is greater than or equal to threshold.  This is conservative; the
* obvious condition is (m.size() + size) >= threshold, but this
* condition could result in a map with twice the appropriate capacity,
* if the keys to be added overlap with the keys already in this map.
* By using the conservative calculation, we subject ourself
* to at most one extra resize.
*/
if (numKeysToBeAdded > threshold) {
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
newCapacity <<= 1;
if (newCapacity > table.length)
resize(newCapacity);
}

for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());
}

/**
* Removes the mapping for the specified key from this map if present.
*
* @param  key key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or
*         <tt>null</tt> if there was no mapping for <tt>key</tt>.
*         (A <tt>null</tt> return can also indicate that the map
*         previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}

/**
* Removes and returns the entry associated with the specified key
* in the HashMap.  Returns null if the HashMap contains no mapping
* for this key.
*/
final Entry<K,V> removeEntryForKey(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;

while (e != null) {
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}

/**
* Special version of remove for EntrySet.
*/
final Entry<K,V> removeMapping(Object o) {
if (!(o instanceof Map.Entry))
return null;

Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;

while (e != null) {
Entry<K,V> next = e.next;
if (e.hash == hash && e.equals(entry)) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}

/**
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
*/
public void clear() {
modCount++;
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}

/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
*         specified value
*/
public boolean containsValue(Object value) {
if (value == null)
return containsNullValue();

Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
return false;
}

/**
* Special-case code for containsValue with null argument
*/
private boolean containsNullValue() {
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value == null)
return true;
return false;
}

/**
* Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map
*/
public Object clone() {
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
result.table = new Entry[table.length];
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);

return result;
}

static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
final int hash;

/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}

public final K getKey() {
return key;
}

public final V getValue() {
return value;
}

public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}

public final boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}

public final int hashCode() {
return (key==null   ? 0 : key.hashCode()) ^
(value==null ? 0 : value.hashCode());
}

public final String toString() {
return getKey() + "=" + getValue();
}

/**
* This method is invoked whenever the value in an entry is
* overwritten by an invocation of put(k,v) for a key k that's already
* in the HashMap.
*/
void recordAccess(HashMap<K,V> m) {
}

/**
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap<K,V> m) {
}
}

/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket.  It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
}

/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization).  This version needn't worry about resizing the table.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}

private abstract class HashIterator<E> implements Iterator<E> {
Entry<K,V> next;        // next entry to return
int expectedModCount;   // For fast-fail
int index;              // current slot
Entry<K,V> current;     // current entry

HashIterator() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
}

public final boolean hasNext() {
return next != null;
}

final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();

if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
current = e;
return e;
}

public void remove() {
if (current == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Object k = current.key;
current = null;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
}

}

private final class ValueIterator extends HashIterator<V> {
public V next() {
return nextEntry().value;
}
}

private final class KeyIterator extends HashIterator<K> {
public K next() {
return nextEntry().getKey();
}
}

private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}

// Subclass overrides these to alter behavior of views' iterator() method
Iterator<K> newKeyIterator()   {
return new KeyIterator();
}
Iterator<V> newValueIterator()   {
return new ValueIterator();
}
Iterator<Map.Entry<K,V>> newEntryIterator()   {
return new EntryIterator();
}

// Views

private transient Set<Map.Entry<K,V>> entrySet = null;

/**
* 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 <tt>remove</tt> operation), the results of
* the iteration are undefined.  The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*/
public Set<K> keySet() {
Set<K> ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}

private final class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return newKeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return HashMap.this.removeEntryForKey(o) != null;
}
public void clear() {
HashMap.this.clear();
}
}

/**
* 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 <tt>remove</tt> operation),
* the results of the iteration are undefined.  The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*/
public Collection<V> values() {
Collection<V> vs = values;
return (vs != null ? vs : (values = new Values()));
}

private final class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return newValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
}

/**
* 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 <tt>remove</tt> operation, or through the
* <tt>setValue</tt> 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 <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations.  It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a set view of the mappings contained in this map
*/
public Set<Map.Entry<K,V>> entrySet() {
return entrySet0();
}

private Set<Map.Entry<K,V>> entrySet0() {
Set<Map.Entry<K,V>> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}

private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public Iterator<Map.Entry<K,V>> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<K,V> e = (Map.Entry<K,V>) o;
Entry<K,V> candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);
}
public boolean remove(Object o) {
return removeMapping(o) != null;
}
public int size() {
return size;
}
public void clear() {
HashMap.this.clear();
}
}

/**
* Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
* serialize it).
*
* @serialData The <i>capacity</i> of the HashMap (the length of the
*             bucket array) is emitted (int), followed by the
*             <i>size</i> (an int, the number of key-value
*             mappings), followed by the key (Object) and value (Object)
*             for each key-value mapping.  The key-value mappings are
*             emitted in no particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
Iterator<Map.Entry<K,V>> i =
(size > 0) ? entrySet0().iterator() : null;

// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();

// Write out number of buckets
s.writeInt(table.length);

// Write out size (number of Mappings)
s.writeInt(size);

// Write out keys and values (alternating)
if (i != null) {
while (i.hasNext()) {
Map.Entry<K,V> e = i.next();
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}

private static final long serialVersionUID = 362498820763181265L;

/**
* Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold, loadfactor, and any hidden stuff
s.defaultReadObject();

// Read in number of buckets and allocate the bucket array;
int numBuckets = s.readInt();
table = new Entry[numBuckets];

init();  // Give subclass a chance to do its thing.

// Read in size (number of Mappings)
int size = s.readInt();

// Read the keys and values, and put the mappings in the HashMap
for (int i=0; i<size; i++) {
K key = (K) s.readObject();
V value = (V) s.readObject();
putForCreate(key, value);
}
}

// These methods are used when serializing HashSets
int   capacity()     { return table.length; }
float loadFactor()   { return loadFactor;   }
}