package cn.xlystar.version_7;
import java.io.*;
import java.util.*;
// 总结：
// 1. HashMap只允许一个为null的key。
//
// 2. HashMap的扩容：当前table数组的两倍
//
// 3. HashMap实际能存储的元素个数： capacity * loadFactor
//
// 4. HashMap在扩容的时候，会重新计算hash值，并对hash的位置进行重新排列， 因此，为了效率，尽量给HashMap指定合适的容量，避免多次扩容
public class HashMap
extends AbstractMap
implements Map, Cloneable, Serializable {
/**
* 默认初始化容量 1 << 4（16） - 初始化容量必须是 2的幂次方
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* 最大容量 1 << 30 (10 7374 1824) - 构造函数可以设定的最大容量是 1 << 30
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* 负载因子0.75
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 空的表table
*/
static final Entry[] EMPTY_TABLE = {};
/**
* 需要调整大小时使用的table - 大小必须是2的幂
*/
transient Entry[] table = (Entry[]) EMPTY_TABLE;
/**
* map中映射的数量
*/
transient int size;
/**
* 阈值 (capacity * load_factor) - 达到这个值，经行扩展table的容量
*
* @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
int threshold;
/**
* 负载因子
* @serial
*/
final float loadFactor;
/**
* 对 hash表结构和数据进行修改的次数modcount
*/
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.
*
* 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.
*/
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 {
/**
* 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;
}
}
/**
* A randomizing value associated with this instance that is applied to
* hash code of keys to make hash collisions harder to find. If 0 then
* alternative hashing is disabled.
*/
transient int hashSeed = 0;
/**
* Constructs an empty HashMap 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);
this.loadFactor = loadFactor;
threshold = initialCapacity;
init();
}
/**
* Constructs an empty HashMap 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 HashMap with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new HashMap with the same mappings as the
* specified Map. The HashMap is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified Map.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
private static int roundUpToPowerOf2(int number) {
//当临界值小于HashMap最大容量时， 返回最接近临界值的2的N次方
//Integer.highestOneBit方法的作用是用来计算指定number最临近的2的N此方的数
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}
/**
* 初始化hashmap
*/
private void inflateTable(int toSize) {
// 计算出大于等于toSize并且最临近的2的N此方的值
int capacity = roundUpToPowerOf2(toSize);
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
initHashSeedAsNeeded(capacity);
}
// 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() {
}
/**
* 初始化hashmap值，在需要使用时初始化
*/
final boolean initHashSeedAsNeeded(int capacity) {
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean switching = currentAltHashing ^ useAltHashing;
if (switching) {
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}
/**
* 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.
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
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);
}
/**
* 返回通过hash值得到索引值
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length - 1);
}
/**
* 返回mapping数量
*/
@Override
public int size() {
return size;
}
/**
* 判断mapping是否为空
*/
@Override
public boolean isEmpty() {
return size == 0;
}
/**
* 返回key对用的value值
*/
@Override
public V get(Object key) {
if (key == null)
return getForNullKey();
Entry entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
/**
* 查看null的value，如果size是0返回null，不是的话，在table[0]中查找null，返回value
*/
private V getForNullKey() {
if (size == 0) {
return null;
}
for (Entry e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
/**
* 判断是否包含key值
*/
@Override
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
* 返回key对于的value值，没有就返回null
*/
final Entry getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry 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;
}
/**
* 添加/更新方法
*
*/
@Override
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
// table中，同一个位置（也就是同一个hash）可能出现多个元素（链表实现），故此处需要循环
// 如果key已经存在，那么直接设置新
for (Entry 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;
}
/**
* key为null的添加/更新方法
*/
private V putForNullKey(V value) {
for (Entry 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;
}
/**
* 将指定Map转换为HashMap的方法
*/
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);
int i = indexFor(hash, table.length);
/**
* 替换旧的key值
*/
for (Entry 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);
}
/**
* 将指定Map转换为HashMap的方法，主要看putForCreate方法
*/
private void putAllForCreate(Map m) {
for (Map.Entry e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}
/**
* 重新调整hashmap的大小，进行扩容
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
// HashMap的容量已经扩充到最大值，那么临界值threshold设置为最大的int值MAXIMUM_CAPACITY
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
// 创建新Entry数组，将table引用指向这个新创建的数组
Entry[] newTable = new Entry[newCapacity];
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
// loadFactor设置得越小，遇到hash冲突的几率就越小
threshold = (int) Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
/**
* 扩容后，经行数据的转移
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry e : table) {
while (null != e) {
Entry 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
*/
@Override
public void putAll(Map m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
if (table == EMPTY_TABLE) {
inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
}
/*
* 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 e : m.entrySet())
put(e.getKey(), e.getValue());
}
/**
* 删除mapping
*/
@Override
public V remove(Object key) {
Entry e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
/**
* 删除mapping的核心方法
*/
final Entry removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;
while (e != null) {
Entry 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.
*/
final Entry removeMapping(Object o) {
if (size == 0 || !(o instanceof Map.Entry))
return null;
Map.Entry entry = (Map.Entry) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;
while (e != null) {
Entry 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.
*/
@Override
public void clear() {
modCount++;
Arrays.fill(table, null);
size = 0;
}
/**
* Returns true 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 true if this map maps one or more keys to the
* specified value
*/
@Override
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 HashMap instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map
*/
@Override
public Object clone() {
HashMap result = null;
try {
result = (HashMap) super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
if (result.table != EMPTY_TABLE) {
result.inflateTable(Math.min(
(int) Math.min(
size * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY),
table.length));
}
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);
return result;
}
static class Entry implements Map.Entry {
final K key;
V value;
// 保存了对下一个元素的引用，说明此处为链表
// 作用：
// 为了解决hash一致的时候的冲突，当两个或者多个hash一致的时候，
// 那么就将这两个或者多个元素存储在一个位置，用next来保存对下个元素的引用
Entry next;
int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry 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 Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}
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 m) {
}
/**
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap m) {
}
}
/**
* 添加新的entry
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
// 达到阈值时，涉及扩容
if ((size >= threshold) && (null != table[bucketIndex])) {
// table容量不够,扩容了 - 原来的两倍
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
// 真正创建entry的方法
createEntry(hash, key, value, bucketIndex);
}
/**
* 在table的指定位置新创建一个Entry
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
private abstract class HashIterator implements Iterator {
Entry next; // next entry to return
int expectedModCount; // For fast-fail
int index; // current slot
Entry 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 nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry 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 {
public V next() {
return nextEntry().value;
}
}
private final class KeyIterator extends HashIterator {
public K next() {
return nextEntry().getKey();
}
}
private final class EntryIterator extends HashIterator> {
public Map.Entry next() {
return nextEntry();
}
}
// Subclass overrides these to alter behavior of views' iterator() method
Iterator newKeyIterator() {
return new KeyIterator();
}
Iterator newValueIterator() {
return new ValueIterator();
}
Iterator> newEntryIterator() {
return new EntryIterator();
}
// Views
private transient Set> 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 remove operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* Iterator.remove, Set.remove,
* removeAll, retainAll, and clear
* operations. It does not support the add or addAll
* operations.
*/
@Override
public Set keySet() {
Set ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}
private final class KeySet extends AbstractSet {
public Iterator 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 remove operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll and clear operations. It does not
* support the add or addAll operations.
*/
@Override
public Collection values() {
Collection vs = values;
return (vs != null ? vs : (values = new Values()));
}
private final class Values extends AbstractCollection {
public Iterator 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 remove operation, or through the
* setValue operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Set.remove, removeAll, retainAll and
* clear operations. It does not support the
* add or addAll operations.
*
* @return a set view of the mappings contained in this map
*/
@Override
public Set> entrySet() {
return entrySet0();
}
private Set> entrySet0() {
Set> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}
private final class EntrySet extends AbstractSet> {
@Override
public Iterator> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry)) {
return false;
}
Map.Entry e = (Map.Entry) o;
Entry candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);
}
@Override
public boolean remove(Object o) {
return removeMapping(o) != null;
}
@Override
public int size() {
return size;
}
@Override
public void clear() {
HashMap.this.clear();
}
}
private static final long serialVersionUID = 362498820763181265L;
/**
* 序列化
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
// Write out number of buckets
if (table == EMPTY_TABLE) {
s.writeInt(roundUpToPowerOf2(threshold));
} else {
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 e : entrySet0()) {
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}
/**
* 反序列化
*/
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 other fields that need values
table = (Entry[]) EMPTY_TABLE;
// Read in number of buckets
s.readInt(); // ignored.
// Read number of mappings
int mappings = s.readInt();
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);
// capacity chosen by number of mappings and desired load (if >= 0.25)
int capacity = (int) Math.min(
mappings * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY);
// allocate the bucket array;
if (mappings > 0) {
inflateTable(capacity);
} else {
threshold = capacity;
}
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);
}
}
// table的大小
int capacity() {
return table.length;
}
float loadFactor() {
return loadFactor;
}
}
