HashMap
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/* * @(#)HashMap.java1.73 07/03/13 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.util;import java.io.*;/** * * <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 * @version 1.73, 03/13/07 * @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;// 默认Entry[] table 的大小 /** * 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;//最大Entry[] table的大小 /** * The load factor used when none specified in constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f;// 默认装载因子 /** * 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;//再散列的大小,其值为 loadFactor*capacity /** * The load factor for the hash table. * * @serial */ final float loadFactor;// 装载因子 /** * 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 volatile 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)// 其容量只参是2的n 次方,所以如果指定的参数不是2^n,那么则要求大于initialCapacity的最小2^n 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) {// 得到 h 在 table[] 中对应的索引值,这里有一个问题,为什么要拿 h & length-1 return h & (length-1);// 在 hashMap 中 Entry [] table 的个数永远是 2^n(2的n 次方),legnth-1 表示的二进制数为11111..111,所有位都是1 }// 那么拿 h & (length-1) 我们得到的数可以均匀的分布到table[]数组上,例如:length = 8,则 1&7=1,2&7=2,3&7=3...7&7=7,8&7=0,9&7=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);// 找到 table 数组中的位置,然后遍历链表 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))) {// 如果找到Key 与原HashMap 中有完全相等的,对用现在的替换到原来的,因为HashMap中不存在两个完全相等的Key V oldValue = e.value;// Key的判断方法:hash 与 equals 都相等者为true e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i);// 如果没有相等的,就将这个Key,Value插入HashMap 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) {//如果键值是null ,则不要计算其hash值,直接放到table[0] 中去 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) {// 这个与上面的put 不同的是这个不用再resize,因为在构造里面就一次分配够了空间 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) {// 利用 entrySet 进行遍历,把得到的所有结点再插入 for (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) { Map.Entry<? extends K, ? extends V> e = i.next(); 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];//生成新的表 transfer(newTable);//进行数据的再散列 table = newTable; threshold = (int)(newCapacity * loadFactor); } /** * Transfers all entries from current table to newTable. */ void transfer(Entry[] newTable) {// Hash表的扩容,将table 表中的数都添转移到 newTable 中去 Entry[] src = table; int newCapacity = newTable.length; for (int j = 0; j < src.length; j++) { Entry<K,V> e = src[j];//得到链表矩阵中第 j 行的头结点,header if (e != null) { src[j] = null;// 将原来的引用置空 do { Entry<K,V> next = e.next;//保存eader的一个结点的引用 int i = indexFor(e.hash, newCapacity);// e 在newTable映射的新的位置 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 (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) { Map.Entry<? extends K, ? extends V> e = i.next(); 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)))) {// 只是判断两者是否相同,而没有进行hashCode 的判断?XXXXX为什么不进行hashCode 判断,因为走到这hashcode 已经是相等的,前面有indexFor操作 modCount++; size--; if (prev == e)// 如果找到的是table[i],则table[i] =table[i].next; table[i] = next; else prev.next = next;// 否则 pre.next = e.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) {// 在entryset中用来remove的,其操作的对象是Entry 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> {// Hash 表中的结点类,其实是一个链表 final K key;// key 为不可变的,所以final V value; Entry<K,V> next;// Entry next 下一个结点 final int hash;// 其 hash 值 /** * Creates new entry. */ Entry(int h, K k, V v, Entry<K,V> n) {// 构造函数 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) {// 重写 Equals方法,比较key and value 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() {// Entry 的hashCode = key.hashCode ^ value.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) {//向HashMap 添加一个Entry 对象Entry<K,V> e = table[bucketIndex];// 得到要插入table[]位置的结点,也就相当于链表的头结点 table[bucketIndex] = new Entry<K,V>(hash, key, value, e);// 将每次新的结点都插入到 table[index]的位置,而新的结点的next都指向原来的table[index],也就是把新的结点都插入到链表的头部 if (size++ >= threshold)// 如果 个数到 threshold,下面进行再散列 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) {// 与 addEntry 一样,只不过是没有散列表Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<K,V>(hash, key, value, e); size++; } private abstract class HashIterator<E> implements Iterator<E> {// Abstract HashIterator 对keyIterator valueIterator EntryIterator 的抽象, Entry<K,V> next;// next entry to return// next 还是指向返回的结点,给linkedList中的next 其实一样,current= next ; next = next.next; int expectedModCount;// For fast-fail int index;// current slot Entry<K,V> current;// current entry// 与 LinkedList 中的 lastRereturned 作用一样都是 current=next; next = next.next; HashIterator() {//构造函数 expectedModCount = modCount; if (size > 0) { // advance to first entry Entry[] t = table; while (index < t.length && (next = t[index++]) == null)// 找到第一个table[index]不为空的结点 ; } } public final boolean hasNext() { return next != null; } final Entry<K,V> nextEntry() {// 返回下一个Entry,是按照 table[] 进行数组遍历,每次找到一个不为null 的Entry,此方法是共下面的子类进行调用 的 if (modCount != expectedModCount) throw new ConcurrentModificationException(); Entry<K,V> e = next; if (e == null) throw new NoSuchElementException(); if ((next = e.next) == null) {//这里就是了。如果 next.next!=null 则表明next 后还有结点,则就按照链表来遍历,next = next.next ,否则,则要再次进行数组的遍历while(index<table.length&&table[index++]==null); Entry[] t = table; while (index < t.length && (next = t[index++]) == null) ; } current = e;// current return e; } public void remove() { if (current == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); Object k = current.key; current = null;// 删除过期的对象引用, let gc do HashMap.this.removeEntryForKey(k); expectedModCount = modCount; } } private final class ValueIterator extends HashIterator<V> { public V next() {// 重写 next 方法, 调用 nextEntry().value return nextEntry().value; } } private final class KeyIterator extends HashIterator<K> {// 重写 next 方法,调用 nextEntry().getKey() public K next() { return nextEntry().getKey(); } } private final class EntryIterator extends HashIterator<Map.Entry<K,V>> { public Map.Entry<K,V> next() {// 重写 next 方法,调用 nextEntry() return nextEntry(); } } // Subclass overrides these to alter behavior of views' iterator() method Iterator<K> newKeyIterator() {// 返回 keyIterator 迭代器 return new KeyIterator(); } Iterator<V> newValueIterator() {// 返回 ValueIteator 迭代器 return new ValueIterator(); } Iterator<Map.Entry<K,V>> newEntryIterator() {// 返回 EntryIterator 迭代器 return new EntryIterator(); } // Views private transient Set<Map.Entry<K,V>> entrySet = null;// 定义EntrySet 变量 /** * 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() {// 返回KeySet,不要每次都返回一个KeySet对象,定义一个成员来维护keySet,只是在第一次的时间创建一个,后来时间都只是维护原来的那个 Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet())); } private final class KeySet extends AbstractSet<K> {// 定义KeySet类,hashMap.keyset方法返回,所以private 不static public Iterator<K> iterator() {// 重写AbstractSet extends AbstractCollection上的 abstract iterator() 方法 return newKeyIterator(); } public int size() { //重写AbstractSet extends AbstractCollection上的 abstract size() 方法 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 stuffs.defaultWriteObject();// Write out number of bucketss.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 stuffs.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 HashMapfor (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; }}
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HashMap---------------------------------------------------------------------------------------------------------------------------------------------------------------------结构:分离链接法Entry 是一个结点,对于HashMap的存储是一个链表数组,如果产生了冲突,解决冲突的方法是将相同hashCode值的元素用一个链表连接起来1.哈希映射的方法:indexFor(int hashCode,int length) return hashCode&(length-1)在 hashMap 中 Entry [] table 的个数永远是 2^n(2的n 次方),legnth-1 表示的二进制数为11111..111,所有位都是1, 那么拿 h & (length-1) 我们得到的数可以均匀的分布到table[]数组上,例如:length = 8,则 1&7=1,2&7=2,3&7=3...7&7=7,8&7=0,9&7=1; 其效果和探测散列中hash对数组取余效果一样2. HashMap 的构造函数: HashMap(int initialCapacity,float loadFactor)对于 HashMap 中的Entry[]table其capacity 只能是2^n,所以在构造的时候,要根据构造的参数求出大于initialCapacity 的最小2^n//&&&&&&&&&&& HashMap 添加时相同元素的判断方法: e.hashcode == key.hashcode && (e==key||k.equals(key)) //&&&&&&&&&&& HashMap 查找时相同元素的决断方法: e==key || (e!=null && e.equals(key))3. V put(K key,V value) 添加方法:先indexFor(hashCode,length)得到在table中的位置,然后循环遍历链表,先检查链表 中是否有相同的Key,这里相同指的hashCode 与 equals 都返回true,如果都相等,则用后面的Value替换原来的Value这也是 HashMap.put(1,"abc"),HashMap.put(1,"def") HashMap.size()返回为 1 的原因如果没有的话,就直接 addEntry(hashCode,k,v,index)public v put(K key,V value){if(key == null) 另行处理 return ;int index = indexFor(key.hashcode(),table.length);for(Entry<K,V>e=table[i];e!=null;e=e.next){if(e.key.hashcode()==key.hashcode()&&(e.key==key||e.key.equals(k)))//这里强调的是hashCode 必需相等V oldValue = e.value;e.value = value;return oldValue;}addEntry(key.hashCode,k,v,index);return null ; 如果没有重复,就返回null ,否则返回重复的}//+-------------添加的时候有一个规则,都是如果Entry 存在则:newEntry.next = table[i]; table[i] = newEntry如果Entry 不存在,Entry newEntry = new Entry(hashCode,key,value,table[index]) ; table[index ] = newEntry;//+-------------4.addEntry(hashCode,key,value,index)插入结点向指定的位置插入一个新的结点,其每次的新结点都是插入到了table[index]中,而新结点的next 指向了原来的table[index],所以新的结点都是插在了链表的头部addEntry(hashCode,key,value,index){Entry<K,V> e = table[i];记录原来的table[i];table[i] = new Entry(hashcode,key,value,e);// 将新生成的结点放到table[i]中去,同时将其next指向e}5.addForNullKey(V value) :向 HashMap 添加健值为空的Value ,对于 null,其没有再重复的去计算其hash值,而是直接将共放在了 table[0] 中,然后其方法和 put(k key,v value)原理一样6. get(K key) 查找对于其查找同插入也是一样,先得到其映射的table[index],然后再从链表的头部开始查找V get(K key){if(key == null) 另行处理: return ;int index = indexFor(key.hashCode,table.length);for(Entry<K,V> e =table[index];e!=null;e=e.next){if(e.key.hashcode == key.hashCode&&(e.key==key||e.key.equals(key)))return e.value;}return null;}7.对于 key null 的查找V getForNullKey(){for(Entry<K,V>e=table[0];e!=null;e=e.next)if(e.key==null)return e.value;return null;}7.HashMap 的再散列,如果size的值达到threshold,则就要进行再散列,也就是开辟一个更大的newTable,然后将table里的值复制到newtable中,最后将table=newtable void transfer(Entry[] newTable) {// Hash表的扩容,将table 表中的数都添转移到 newTable 中去 Entry[] src = table; int newCapacity = newTable.length; for (int j = 0; j < src.length; j++) { Entry<K,V> e = src[j];//得到链表矩阵中第 j 行的头结点,header if (e != null) { src[j] = null;// 将原来的引用置空 do { Entry<K,V> next = e.next;//保存eader的一个结点的引用 int i = indexFor(e.hash, newCapacity);// e 在newTable映射的新的位置 e.next = newTable[i];// e.next指向newtable[i]; newTable[i] = e; e = next; } while (e != null); } } }8.HashMap 根据键值的删除 removeEntryForKey(Object key)根据Key的hashCode 查看散列到table 中的位置,然后根据链表进行遍历,找到相同的结点将其删除,如果是在table[i]的位置,那么要将table[i]进行重新赋值,table[i] = table[i].next,如果不是,则 pre.next = e.next;判断结点是否相同的方法 if(key==e.key||(key!=null&&key.equals(key))),这里之所以不进行hashcode 的判断是因为前面先执行indexFor()找到了table[]中的位置,说明它们的hashcode已经是相等的了,所以不需要再判断了Entry<K,V> removeEntryForKey(Object key){Entry<E,V> e = table[indexFor(key.hashcode)];Entry pre = e;while(e!=null){if(e.key==key||(key!=null && key.equals(e.key)))// equals{if(e==table[i])table[i] = e.next;elsepre.next = e.next;return e;}pre = e;e = e.next;}}8.数据的遍历:Iterator,HashMap 的中迭代器类有:HashIterator|--KeyIterator|--ValueIterator|--EntryIteratorHashIterator定义了下面三个类的抽象,也是主要的一个迭代器类,下面三个都是调用了父类里面的方法,下面我们来看一下HashIterator 结点 成员:Entry<K,V> next; Entry<K,V> current; int index; int expectedModCount;// fail-fast策略(速错)构造:构造的时候 next 应该引用一个不为空的变量,也就是next 已经找到了hashMap 中一个不为空的值,也就是 next = table[i]; HashIterator(){for(index<table.length&&(next = table[index++])==null);// 找到一个不为空的table[i] }hashNext() return next!=nullnext():我们来分析一下next(),next()是返回下一个结点,首先我们要遍历数组table[],而当我们找到了一个Entry时,它可能会是一个结点的头,所以我们还要遍历这个链表Entry next(){Entry e = next;if(e == null ) throw new NoSuchElementException();if(next.next==null){while(index<table.length && (next = table[index++])==null);}current = e;return e;}
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