HashMap源码分析
HashMap源码部分涉及的知识点有：
1、对象的序列化和反序列化
2、关键字transient
3、clone方法 浅拷贝 深拷贝问题
4、链表问题
5、断言
废话不多说，先看源码
package java.util;
import java.io.*;
/** @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. * 默认的初始容量 */
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. * 默认最大容量 1左移30位 一般没有人会设置这么大的容量 */
static final int MAXIMUM_CAPACITY = 1 << 30;
/** * The load factor used when none specified in constructor. * 负载因子 一般不改变，采用默认的 */
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/** * The table, resized as necessary. Length MUST Always be a power of two. * Entry数组 * transient关键字： * 当持久化对象时，可能有一个特殊的对象数据成员， * 我们不想 用serialization机制来保存它。为了在一个特定对象的一个域上关闭serialization， * 可以在这个域前加上关键字transient。 * transient是Java语言的关键字，用来表示一个域不是该对象串行化的一部分。 * 当一个对象被串行化的时候， 然而非transient型的变量是被包括进去的。 * 注意static变量也是可以串行化的 * 同时，通过反序列化得到的对象是不同的对象， * 而且得到的对象不是通过构造器得到的， * 也就是说反序列化得到的对象不执行构造器。 * 我的博文：http://blog.csdn.net/u010156024/article/details/48345257 */
transient Entry<K,V>[] 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 * 极限容量 */
int threshold;
/** * The load factor for the hash table. * 负载因子 * @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;
/** * The default threshold of map capacity above which alternative hashing is * used for String keys. Alternative hashing reduces the incidence of * collisions due to weak hash code calculation for String keys. * <p/> * This value may be overridden by defining the system property * {@code jdk.map.althashing.threshold}. A property value of {@code 1} * forces alternative hashing to be used at all times whereas * {@code -1} value ensures that alternative hashing is never used. 极限容量最大值 */
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
/** * holds values which can't be initialized until after VM is booted. */
private static class Holder {
// Unsafe mechanics
/** * Unsafe utilities */
static final sun.misc.Unsafe UNSAFE;
/** * Offset of "final" hashSeed field we must set in readObject() method. */
static final long HASHSEED_OFFSET;
/** * Table capacity above which to switch to use alternative hashing. */
static final int ALTERNATIVE_HASHING_THRESHOLD;
static {
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));
int threshold;
try {
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
// disable alternative hashing if -1
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}
if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}
ALTERNATIVE_HASHING_THRESHOLD = threshold;
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
HASHSEED_OFFSET = UNSAFE.objectFieldOffset(
HashMap.class.getDeclaredField("hashSeed"));
} catch (NoSuchFieldException | SecurityException e) {
throw new Error("Failed to record hashSeed offset", e);
}
}
}
/** * If {@code true} then perform alternative hashing of String keys to reduce * the incidence of collisions due to weak hash code calculation. */
transient boolean useAltHashing;
/** * A randomizing value associated with this instance that is applied to * hash code of keys to make hash collisions harder to find. */
transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);
/** * 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 * HashMap的最大容量 只能是MAXIMUM_CAPACITY。 * 参考初始化和resize方法中容量的设置。 */
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
/* 很多人都有这个疑问，为什么hashmap的数组初始化大小都是2的次方大小时， hashmap的效率最高，我以2的4次方举例 ，来解释一下为什么数组大小为2的幂时hashmap访问的性能最高。 左边两组是数组长度为16（2的4次方） ，右边两组是数组长度为15。两组的hashcode均为8和9， 但是很明显，当它们和1110“与”的时候，产生了相同的结果， 也就是说它们会定位到数组中的同一个位置上去，这就产生了碰撞， 8和9会被放到同一个链表上，那么查询的时候就需要遍历这个链表， 得到8或者9，这样就降低了查询的效率。同时，我们也可以发现， 当数组长度为15的时候，hashcode的值会与14（1110）进行“与”， 那么最后一位永远是0，而0001，0011，0101，1001，1011，0111， 1101这几个位置永远都不能存放元素了，空间浪费相当大， 更糟的是这种情况中，数组可以使用的位置比数组长度小了很多， 这意味着进一步增加了碰撞的几率，减慢了查询的效率！ */
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
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. * 制定HashMap中table数组长度的初始化操作。 */
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). * 默认情况 table数组长度16，负载因子0.75 */
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/** * 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);
//初始化HashMap对象是调用。该方法不会进行resize的检查。
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() {
}
/** * Retrieve object hash code and applies a supplemental hash function to the * result hash, 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. * hash值得计算。这个方法调用的是Object类的hashCode()方法，查看 * Object类的hashCode()方法，你会发现方法前有native修饰， * 说明对象的hashCode值是由底层的C、C++计算实现的。 */
final int hash(Object k) {
int h = 0;
if (useAltHashing) {
if (k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
h = hashSeed;
}
h ^= k.hashCode();
// 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. * h是hash值，length是数组容量，该值是2的n次方， * length-1得到的是除符号位为0外其他位全部为1的值。 * &操作方便直接。 */
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();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
/** * 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. * key=null时，得到对应的value */
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. * 有key值得到对应的Entry对象。 */
final Entry<K,V> getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key);
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>.) * 对外开放的put方法。 * 首先检查如果key=null，则使用 putForNullKey(value)方法 * 向第一个元素的key=null中添加元素。 * key!=null的情况，首先根据key得到hash值， * 有hash值得到数组中的index， * 如果index处不为空，直接用新的value替换原来的value。 * 如果index处为空，则使用addEntry(hash, key, value, i)方法添加。 * addEntry方法会进行resize检查。 */
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key);
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 * 向key=null的value中添加数据，value回替换原来的value值。 * addEntry(0, null, value, 0);其中的null就是key * 也就是说集合中可以添加key=null的key-value对。 * 只不过，只能添加一个key=null的值，以后所以key=null的value值 * 都会被后来添加的value替换。 * 这里有个小问题，如果循环一遍，发现没有key=null的情况，也就是说for循环 * 中的if语句没有执行，那么会执行for循环后面的addEntry方法，执行到该方法 * 说明table数组中元素都不为null，则会在addEntry方法中进行扩容，符合扩容的 * 条件。然后使用新的table数组，把原来的table中的数据全部转移到新的table数组 * 中，然后在调用createEntry把key=null的元素添加进来。 * 猜想，新的table数组容量较大，肯定会有key=null的情况，所以 * 完全可以添加进来。 */
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. * 把参数key-value 添加到集合中。 * 这个方法不会进行resize的检查操作，非常快。 * 应用在clone方法和使用集合初始化HashMap这两个地方。 */
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);
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. * clone和反序列化时候，这个循环不会发生。 * 原因在于this.table[i]还没有值呢 e==null * 其中，这里的this代表的是调用该方法的对象。 */
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;
}
}
//该方法不需要进行resize的判断。
createEntry(hash, key, value, i);
}
/**讲参数m中的元素添加到调用该方法的对象中。 * 这个方法不会进行resize的检查，相当高效 * 主要使用的地方在clone方法和利用HashMap进行初始化这两个地方。 * 原因在于clone方法和HashMap进行创建HashMap的对象的时候，元素的数量已知， * 初始化已经完成，数组已经得到初始化操作，此时只用向集合中添加数组元素即可。 * 所以此时不用resize的检查，所以高效。 */
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). * 重新分配数组长度。 * HashMap的最大容量 只能是MAXIMUM_CAPACITY。 * 参考初始化和resize方法中容量的设置。 */
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
//MAXIMUM_CAPACITY的值是1<<30的int类型值
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity];
boolean oldAltHashing = useAltHashing;
useAltHashing |= sun.misc.VM.isBooted() &&
(newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean rehash = oldAltHashing ^ useAltHashing;
transfer(newTable, rehash);
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
/** * Transfers all entries from current table to newTable. * 把当前是数组中的元素全部拷贝到新的数组中。 * 主要进行这个操作的是数据扩容之后进行。 * 由此可以看出这里是影响HashMap性能的地方。 */
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
/** * 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 * 把参数m集合中的元素全部添加到调用该方法的对象中。 * 其中参数m!=null,否则引发异常NullPointerException * 添加过程中，会重新检查容量是否够用，如果不够用 * 则按照新的数量进行扩容。 */
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. * 这里进行扩容的判断有点小复杂。 * 1、当待添加的集合中的数量不大于极限容量时，不扩容。 * 2、当待添加的集合中的数量大于极限容量时，扩容。 * 此时扩容的长度是数量/负载因子，扩容长度比实际待添加的集合的数量大。 * 最终确定的扩容的长度是newCapacity。 * 此时判断newCapacity是否大于目前数组的长度，如果大于，则扩容， * 如果小于，也不扩容。因为当前数组长度已经够用了。 * 这里面，我刚开始有一个疑惑，为什么不考虑原来的数组的元素的个数？ * 原因是：调用此方法是说明原来的数组长度并没有达到极限容量，并且 * 实际扩容的大小也比实际元素的长度大。既使最后原来数组中的数量加上 * 待添加的元素的数量超过了数组容量，也没有问题，因为 put(e.getKey(), e.getValue()); * 进行添加的，这个方法在进行添加元素时，会检查容量极限。 */
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);
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;
/** * 循环遍历Entry链 * 如果链中第一个元素就是要删除的，那就table[i]=next即可。 * 如果第一个不是要删除的元素，那就循环遍历entry链，直到 * 找到要删除的元素，然后直接让前一个元素的next指向下一个元素即可表示 * 删除当前元素。 * 即使最后没有找到要删除的元素，退出循环， * 此时e==null,最后返回e即是返回null */
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 using {@code Map.Entry.equals()} * for matching. * 该方法与上面的removeEntryForKey(Object key)方法雷同。 * 只不过removeEntryForKey(Object key)参数是key， * 而这个方法参数是Map.Entry类或者其子类。 * * 由这个方法可知，HashMap不能根据key-value对中的value进行删除集合中的数据 */
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);
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 * 浅拷贝 引用的拷贝，拷贝之后保存的引用对象是同一个对象 * 深拷贝 不仅引用的拷贝，同时引用的对象的拷贝，拷贝后引用的对象是不同的对象。 */
public Object clone() {
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
/**此处使用了断言。 * 断言是当assert后表达式为true时，无信息输出 * 当表达式 为false时，输出错误AssertionError，同时，如果 * assert 表达式1：表达式2是这种形式的话，表达式1=false，输出表达式2的信息 * Exception in thread "main" java.lang.AssertionError: 错误 * at volatileTest.main(volatileTest.java:22) */
// assert false;
}
result.table = new Entry[table.length];
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);
return result;
}
//HashMap静态内部类，集成Map.Entry接口
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
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;
}
/** * 1、如果参数不是Map.Entry类，直接返回false * 2、如果这两者的key-value对相等，则返回true */
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. */
/** * 如果size数量已经大于极限容量，并且出现了冲突，则扩容。 * 如果size数量已经大于极限容量，但是没有出现冲突，不扩容。 * 如果size数量小于极限容量，不管出不出现冲突，均不扩容。 * 此时的冲突用链表解决。 */
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
/** * 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;
}
}
/**三个子类 实现上面的HashIterator抽象类，也就是Iterator接口 * 分别遍历value key Entry * 同时，每个迭代器只用重写其中的next()方法即可。 */
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
//生成对应的迭代器的方法
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. */
/**hashmap对象进行调用，生成key的Set集合 * keySet变量的定义是在AbstractMap中定义的。 */
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 * 得到HashMap中所以Entry的Set集合。 */
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. * 序列化时的方法 */
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 (size > 0) {
for(Map.Entry<K,V> e : entrySet0()) {
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}
private static final long serialVersionUID = 362498820763181265L;
/** * Reconstitute the {@code HashMap} instance from a stream (i.e., * deserialize it). * 反序列化的方法。 */
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold (ignored), loadfactor, and any hidden stuff
s.defaultReadObject();
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
// set hashSeed (can only happen after VM boot)
Holder.UNSAFE.putIntVolatile(this, Holder.HASHSEED_OFFSET,
sun.misc.Hashing.randomHashSeed(this));
// Read in number of buckets and allocate the bucket array;
//读取数组的长度，不过忽略掉了。
s.readInt(); // ignored
// Read number of mappings
//读取数组的元素数量
int mappings = s.readInt();
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);
//计算数组的初始容量，最后初始容量比个数要大1/3
int initialCapacity = (int) Math.min(
// capacity chosen by number of mappings
// and desired load (if >= 0.25)
mappings * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY);
int capacity = 1;
// find smallest power of two which holds all mappings
//计算最终的数组初始容量。
while (capacity < initialCapacity) {
capacity <<= 1;
}
table = new Entry[capacity];
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
init(); // Give subclass a chance to do its thing.
// Read the keys and values, and put the mappings in the HashMap
for (int i=0; i<mappings; 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; }
}以上是HashMap的源码，其中英文注释，并没有去除，方便大家对照着看，几乎上面的而每个方法我都有注释进行了说明，相信大家看的时候应该会明白都是什么意思。
下面是一些总结：
1、对象的序列化其实很简单，主要使用对象输入流ObjectInputStread和对象输出流ObjectOutputStream进行对象的读取和写入操作。这部分请参考我的博文：http://blog.csdn.net/u010156024/article/details/48345257 博文中示例了对象的写入和读取操作。并介绍了关键字transient的意义。
2、clone方法，clone方法也非常简单，使用super.clone()方法得到克隆的对象，然后将得到的克隆对象进行拷贝数据。这里涉及到浅拷贝和深拷贝的问题。
总结一句话，就是，浅拷贝就是值拷贝，引用拷贝。如果是普通的值的话，没什么说的，由于拷贝得到的对象与原始对象是不同的对象，普通值得改变也就改变了，不会影响原来对象的值。但是对于引用对象而言就不同了。浅拷贝之后的对象的引用，依然指向原来对象的引用对象。通过拷贝得到对象改变引用对象的值，会影响到原来对象的引用对象的值。
深拷贝就不存在这个问题了。普通值进行了拷贝，就连引用对象也进行了拷贝，并不引用原始对象的引用对象。可以说，通过深拷贝，得到两个属于同一个类的独立的对象。
3、HashMap内部使用数组存放key-value对。利用hash值计算元素的存取位置，因此效率较高。但是正是由于利用hash值计算存放位置，所以有可能出现冲突：即存放的位置原来已经有值。
java中HashMap的解决冲突的办法是利用链表，table数组中每个元素都是HashMap类中 static class Entry< K, V > implements Map.Entry< K,V> 对象，这个Entry对象中有一个引用next,next类型就是Entry,这样table数组中的每一个对象都可以指向下一个Entry对象形成链表。
即在不冲突的情况下，添加的元素填充数组空间。在冲突的情况下，形成链表。如果数组空间接近沾满，达到极限容量，同时出现了冲突，那就扩容。
4、书中关于HashMap有这么一句话：创建HashMap是有一个默认的负载因子0.75，这是时间和空间成本上的一种折中。增加负载因子可以减少table数组所占用的内存空间，但会增加查询数据的时间开销；减小负载因子会提高数据查询的性能，但会增加table数组所占用的内存空间。
这句话理解起来有点费劲。下面是我个人的理解，如果有误， 欢迎大家批评指正。
第一种情况，增加负载因子，此时数组极限容量增大，table数组未使用的空间减少，相对来说table数组占用内存空间减少，同时由于极限容量增大，数组中存放数据增多，冲突增加，要知道的是，冲突的增加并不会立即扩容，扩容是由条件的，只有当数组数量超过极限容量且出现冲突的情况下才会扩容，那么这种情况下，查询时间开销增加。
第二种情况，减少负载因子，极限容量减少，很容易满足扩容的条件，存放的数据存放散列，查询效率必然提高，但是由于极限容量减少，扩容必然带来内存空间的开销增大。
5 几点说明
对于HashMap的集中操作，增加、删除、查询、遍历等源码中我都有注释给出。查看的时候需要前后结合来看。
需要说明的是，添加操作支持null-null这样的key-value对元素的，不过存放位置并不是table数组的第一个，而是遍历table数组中第一个key=null的元素，并将value替换成最新的value值。也就是说，hashmap最多只能存放一个key=null的元素。
6 HashMap的最大容量时多少，最大容量应该是1<<30,也就是Integer.MAX_VALUE的值的一半。2147483647这就是Integer.MAX_VALUE的大小，也就是说HashMap能存放大约十亿多条数据。为什么说最多只有这么多数据呢？请参看HashMap的初始化和resize（）方法的代码，你会发现极限容量最大就是这么多。
7、 Fail-Fast机制：
我们知道java.util.HashMap不是线程安全的，因此如果在使用迭代器的过程中有其他线程修改了map，那么将抛出ConcurrentModificationException，这就是所谓fail-fast策略。
这一策略在源码中的实现是通过modCount域，modCount顾名思义就是修改次数，对HashMap内容的修改都将增加这个值，那么在迭代器初始化过程中会将这个值赋给迭代器的expectedModCount。
HashIterator() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
}在迭代过程中，判断modCount跟expectedModCount是否相等，如果不相等就表示已经有其他线程修改了Map：
注意到modCount声明为volatile，保证线程之间修改的可见性。
final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
在HashMap的API中指出：
由所有HashMap类的“collection 视图方法”所返回的迭代器都是快速失败的：在迭代器创建之后，如果从结构上对映射进行修改，除非通过迭代器本身的 remove 方法，其他任何时间任何方式的修改，迭代器都将抛出 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，而不冒在将来不确定的时间发生任意不确定行为的风险。
注意，迭代器的快速失败行为不能得到保证，一般来说，存在非同步的并发修改时，不可能作出任何坚决的保证。快速失败迭代器尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的做法是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。（这部分摘自：http://beyond99.blog.51cto.com/1469451/429789/）
8、transfer方法详解
/** * Transfers all entries from current table to newTable. * 把当前是数组中的元素全部拷贝到新的数组中。 * 主要进行这个操作的是数据扩容之后进行。 * 由此可以看出这里是影响HashMap性能的地方。 * 程序中的三个赋值操作有点绕： * 首先使用next保存原来数组元素e的链表的下一个元素 * 然后将e.next引用新数组i中保存的元素， * 如果为null则为null,如果不为null,则将先前保存的数据向链表后移 * 然后将新数组i处的元素引用旧数组e元素， * 这样就实现了拷贝原来数组的数据到新数组的功能。 * 最后一句e=next;是进行链表的遍历， * 继续下一个元素继续复制到新数组中。直到while循环结束， * 表明e元素处的链表遍历完毕。 * 然后继续旧数组的下一个元素的遍历复制。 * */
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}- HashSet源码分析
package java.util;
/** * @param <E> the type of elements maintained by this set * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see Set * @see TreeSet * @see HashMap * @since 1.2 */
public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
{
static final long serialVersionUID = -5024744406713321676L;
/** * HashSet是基于HashMap实现的，这里的map就是HashSet进行包装的对象。 */
private transient HashMap<E,Object> map;
// Dummy value to associate with an Object in the backing Map
/** * Set是对HashMap的包装，HashMap的key的集合就是一个Set集合 * 并且key值不会重复，不需要排序 Set<K> HashMap.keySet()方法返回 * 的就是Set集合。 * 既然是HashMap的包装，value值就是下面的PRESENT对象。 * Set集合中的所有value值就是同一个PRESENT对象。 */
private static final Object PRESENT = new Object();
/** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * default initial capacity (16) and load factor (0.75). * 构造器，构造默认的HashMap集合。 */
public HashSet() {
map = new HashMap<>();
}
/** * Constructs a new set containing the elements in the specified * collection. The <tt>HashMap</tt> is created with default load factor * (0.75) and an initial capacity sufficient to contain the elements in * the specified collection. * * @param c the collection whose elements are to be placed into this set * @throws NullPointerException if the specified collection is null */
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
/** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * the specified initial capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
/** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * the specified initial capacity and default load factor (0.75). * * @param initialCapacity the initial capacity of the hash table * @throws IllegalArgumentException if the initial capacity is less * than zero */
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
/** * Constructs a new, empty linked hash set. (This package private * constructor is only used by LinkedHashSet.) The backing * HashMap instance is a LinkedHashMap with the specified initial * capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @param dummy ignored (distinguishes this * constructor from other int, float constructor.) * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive * 参数dummy 请忽略 */
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
/** * Returns an iterator over the elements in this set. The elements * are returned in no particular order. * * @return an Iterator over the elements in this set * @see ConcurrentModificationException】 * Set集合的迭代器 * 返回的是map.keySet().iterator(); * 也就是说Set的迭代器只是HashMap对象的key的集合的迭代器 * 只遍历key值。 */
public Iterator<E> iterator() {
return map.keySet().iterator();
}
/** * Returns the number of elements in this set (its cardinality). * * @return the number of elements in this set (its cardinality) */
public int size() {
return map.size();
}
/** * Returns <tt>true</tt> if this set contains no elements. * * @return <tt>true</tt> if this set contains no elements */
public boolean isEmpty() {
return map.isEmpty();
}
/** * Returns <tt>true</tt> if this set contains the specified element. * More formally, returns <tt>true</tt> if and only if this set * contains an element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this set is to be tested * @return <tt>true</tt> if this set contains the specified element */
public boolean contains(Object o) {
return map.containsKey(o);
}
/** * Adds the specified element to this set if it is not already present. * More formally, adds the specified element <tt>e</tt> to this set if * this set contains no element <tt>e2</tt> such that * <tt>(e==null ? e2==null : e.equals(e2))</tt>. * If this set already contains the element, the call leaves the set * unchanged and returns <tt>false</tt>. * * @param e element to be added to this set * @return <tt>true</tt> if this set did not already contain the specified * element * 返回值boolean值。 * 如果集合中已有key值，则返回false 说明添加失败 * 如果集合中还没有对应的key值，则返回true。说明添加成功。 * * 这里用到了HashMap类的put方法。 * 测试发现，可以向HashMap中存放null-null这样的key-value对。 * 并且第二次存放的key=null的key-value对，会覆盖前一次存放的key=null * 的key-value对。所以Set中也可以存放key=null的值，只不过，不能覆盖。 * 第二次添加key=null的值，不会覆盖前一次存放key=null的值。 */
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
/** * Removes the specified element from this set if it is present. * More formally, removes an element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>, * if this set contains such an element. Returns <tt>true</tt> if * this set contained the element (or equivalently, if this set * changed as a result of the call). (This set will not contain the * element once the call returns.) * * @param o object to be removed from this set, if present * @return <tt>true</tt> if the set contained the specified element */
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
/** * Removes all of the elements from this set. * The set will be empty after this call returns. */
public void clear() {
map.clear();
}
/** * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements * themselves are not cloned. * * @return a shallow copy of this set * 这里clone的方法与HashMap的clone的方法一样，是浅拷贝。 * 也就是说是值得拷贝-引用拷贝，引用的对象并没有拷贝。 */
public Object clone() {
try {
HashSet<E> newSet = (HashSet<E>) super.clone();
newSet.map = (HashMap<E, Object>) map.clone();
return newSet;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
/** * Save the state of this <tt>HashSet</tt> instance to a stream (that is, * serialize it). * * @serialData The capacity of the backing <tt>HashMap</tt> instance * (int), and its load factor (float) are emitted, followed by * the size of the set (the number of elements it contains) * (int), followed by all of its elements (each an Object) in * no particular order. * 序列化方法 * 关于序列化的一些说明，请参考我的博文： * http://blog.csdn.net/u010156024/article/details/48345257 */
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden serialization magic
s.defaultWriteObject();
// Write out HashMap capacity and load factor
s.writeInt(map.capacity());
s.writeFloat(map.loadFactor());
// Write out size
s.writeInt(map.size());
// Write out all elements in the proper order.
for (E e : map.keySet())
s.writeObject(e);
}
/** * Reconstitute the <tt>HashSet</tt> instance from a stream (that is, * deserialize it). * 反序列化方法 * 关于序列化的一些说明，请参考我的博文： * http://blog.csdn.net/u010156024/article/details/48345257 */
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden serialization magic
s.defaultReadObject();
// Read in HashMap capacity and load factor and create backing HashMap
int capacity = s.readInt();
float loadFactor = s.readFloat();
map = (((HashSet)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));
// Read in size
int size = s.readInt();
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
}
HashSet总结一下：
1、 HashSet就是利用HashMap实现的，HashMap的key组成了一个Set集合。为什么能够组成Set集合呢？因为HashMap的可以不重复，不排序，满足Set集合的要求。
而HashMap的value部分可能重复，因此可以组成List集合。
2、 从HashSet源码部分可以看出，比较简单，代码量也少，不过它和HashMap是由区别的。添加、删除都简单，但是查询方法却不行，HashSet集合不能查询单个元素，只能通过遍历得到。
3、既然HashMap可以存放null-null这样的key-value对，HashSet是HashMap的封装，当然也是可以的。只不过，HashSet中key=null的情况下，value不为null.当存放多次key=null的情况，和HashMap一样，不会存放多个元素，而还是只有一个key=null的元素。key部分并没有发生覆盖，而是value部分覆盖，不过HashSet中的覆盖感觉不到而已。
以上是个人对于HashMap和HashSet类的理解，如果注释部分有什么问题或不对的地方，欢迎批评指正！【握手~~】
