Map之HashMap源码分析

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

HashMap继承自AbstractMap，实现了Map接口，有map的基本操作；实现了Cloneable接口，使得该类是可克隆的；实现了Serializable接口，说明该类支持序列化。HashMap是线程不安全的。源码版本为JDK1.7.0_75。

HashMap的数据结构



紫色的对应Entry<K,V>[] table，绿色的是Entry<K,V>对象队列，解决hash冲突。

在put时，首先调用hashCode方法获得哈希值，在调用indexFor方法获取该key存放在table数组的下标index，新建entry插入到index所指向的链表的队首。在删除时也是通过相同的方法找到该key所在的index，然后遍历链表，将key相同、hash值相等的entry删除。

HashMap还对map存放的key-value数目进行限制（使用threshold），且要求数组大小（capacity）必须是2的幂次方（考虑indexFor方法），当容量无法将entry全部保存或者entry数目超过threshold时，根据loadfactor使用resize对容量进行扩大，其中threshold=capacity*loadfactor。

HashMap中允许存放key为null的entry，对于key为null，其index为0。

inflateTable方法是在当table==EMPTY_TABLE时才用到,用于初始化table。

成员变量

/** * 默认初始化空间——必须为2的倍数 * The default initial capacity - MUST be a power of two. */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * 最大空间，若在构造方法中没有给定，则使用该值——必须为2的倍数 * 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; /** * 若在构造时没有指出则使用该值作为加载因子 * The load factor used when none specified in constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * 当表格没有膨胀，则空表实例 * An empty table instance to share when the table is not inflated. */ static final Entry<?,?>[] EMPTY_TABLE = {}; /** * 哈希表，有必要时需resize。长度总是2的倍数 * The table, resized as necessary. Length MUST Always be a power of two. */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; /** * map中包含的key-value映射数目 * The number of key-value mappings contained in this map. */ transient int size; /** * 调整大小的值？？临界值，当实际大小超过临界值时，会进行resize,threshold=capacity*load factor * The next size value at which to resize (capacity * load factor). * @serial */ // If table == EMPTY_TABLE then this is the initial capacity at which the // table will be created when inflated. int threshold; /** * 哈希表的加载因子 * The load factor for the hash table. * * @serial */ final float loadFactor; /** * HashMap进行结构上修改的次数。用来是对HashMap的集合上的迭代器fast-fail * 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; /** * 当使用string作为键值的哈希时的map容量的默认阈值。。。 * 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. */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; /** * 随机值，用在keys的hash code上，使得哈希冲突较难发生。若该值为0，则alternative hash不可用（我理解的是二次哈希） * 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;

loadFactor为加载因子，表示hash表中元素的填满程度，加载因子越大，空间利用率高，但冲突机会就加大了，链表长度会越来越长，查找效率将降低；反之，加载因子越小，填满的元素越少，冲突机会减少，但空间浪费了。

冲突机会越大，则查找效率将降低。

因此，需要在"冲突机会"与"空间利用率"之间寻找平衡——类似"时空"平衡。

参考：

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私有静态内部类Holder

/** * 保存那些直到虚拟机启动时才能初始化的值 * holds values which can't be initialized until after VM is booted. */ private static class Holder { /** * 当转向使用alternative hashing时，table的容量 * 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; } }

构造函数

/** * 指定了初始容量和加载因子，构造一个空HashMap * 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); this.loadFactor = loadFactor; threshold = initialCapacity; 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(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); inflateTable(threshold); putAllForCreate(m); }

inflateTable，该方法在table==EMPTY_TABLE时被调用，用于初始化table，并将capacity提至2的幂次方。

private static int roundUpToPowerOf2(int number) { // assert number >= 0 : "number must be non-negative"; return number >= MAXIMUM_CAPACITY ? MAXIMUM_CAPACITY : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1; } /** * Inflates the table. */ private void inflateTable(int toSize) { // Find a power of 2 >= toSize int capacity = roundUpToPowerOf2(toSize); threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); table = new Entry[capacity]; initHashSeedAsNeeded(capacity); } // internal utilities /** * 为子类初始化hook（钩子）。该方法在构造方法和伪构造方法（clone，readObject）中初始化HashMap之后entry被插入之前调用的。没有该方法，readObject将需要子类信息。 * 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() { } /** * 初始化哈希mask值。直到真的需要用到hashseed时，才初始化它。 * Initialize the hashing mask value. We defer initialization until we * really need it. */ 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; }

hashing相关

/** * 获得对象哈希码，并将一个补充的哈希函数应用到哈希结果以防poor哈希函数。 * 注意：键为null总是映射到哈希0，因此索引为0。 * 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(); // 通过若干次移位、异或操作，把hashCode的1的位置变得"松散，均匀"， // 以免在计算index时不均匀 // 等价于操作 h ^ (h >>> 4) ^ (h >>> 7) ^ (h >>> 12) ^ (h >>> 16) ^ (h >>> 20) ^ (h >>> 24) ^ (h >>> 27); // 将hashcode用十六进制表示为…nmlkjihgfedcba，则a'=a^b^c…,b'=b^c^d…,c'=c^d^e… // 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); } /** * 返回哈希码h的索引 * 因为length为2的幂次方，因此length-1使用二进制表示所有位都是1，即111111..，h&(length-1)得到的数可以均匀分布到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..，可以看出该公式相当于h%length，注意的是针对length要为2的幂次方。 * Returns index for hash code h. */ 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); }

Get key相关

/** * 返回给定key在map上对应的value，若map中没有该key，则返回null。 * 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(); } /** * 获取key为null所对应的value。前面提到key为null对应的索引为0，因此从table[0] * 开始遍历。 * 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() { if (size == 0) { return null; } 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) { if (size == 0) { return null; } 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; }

Put 相关

/** * 根据key值将key-value对插入到map中，若key已存在，则更新value。返回插入前map * 中key所对应的value值。 * 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 (table == EMPTY_TABLE) { inflateTable(threshold); } 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; } /** * 设置key为null时的value值。 * Offloaded version of put for null keys */ private V putForNullKey(V value) { for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(0, null, value, 0); return null; } /** * 不同于put，该方法不需要resize table，构造方法或伪构造方法(clone，readObject)调用该方法。 * This method is used instead of put by constructors and * pseudoconstructors (clone, readObject). It does not resize the table, * check for comodification, etc. It calls createEntry rather than * addEntry. */ private void putForCreate(K key, V value) { int hash = 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. */ for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { e.value = value; return; } } createEntry(hash, key, value, i); } private void putAllForCreate(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) putForCreate(e.getKey(), e.getValue()); }

Resize

/** * 将原来map内容重哈希到一个更大容量的新数组上。当map中key数目达到threshold时，自动调用该方法。若当前容量为MAXIMUM_CAPACITY，则不resize map，设置threshold为Integer.MAX_VALUE。 * Rehashes the contents of this map into a new array with a * larger capacity. This method is called automatically when the * number of keys in this map reaches its threshold. * * If current capacity is MAXIMUM_CAPACITY, this method does not * resize the map, but sets threshold to Integer.MAX_VALUE. * This has the effect of preventing future calls. * * @param newCapacity the new capacity, MUST be a power of two; * must be greater than current capacity unless current * capacity is MAXIMUM_CAPACITY (in which case value * is irrelevant). */ void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; transfer(newTable, initHashSeedAsNeeded(newCapacity)); table = newTable; threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } /** * 将当前表中所有entry转换到newTable中。rehash标志转换时是否需要重哈希。 * Transfers all entries from current table to newTable. */ 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; } } }

将m中的key-value对全都复制到map中，在put之前，首先对table进行resize，若要添加的键-值对大于threshold，则对table进行resize。

/** * 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; 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<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); }

删除。。。删除特定key，首先根据key，计算hash，得到索引，获取所在的"桶"，遍历桶内的链表，删除hash相等且key相等的entry。

/** * 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); } /** * 删除并返回key所对应的entry，若map中不存在该key，返回null。 * 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) { if (size == 0) { return null; } 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; 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; } /** * 根据Map.Entry.equals方法找出与对象o相匹配的entry，将其删除。 * Special version of remove for EntrySet using {@code Map.Entry.equals()} * for matching. */ final Entry<K,V> removeMapping(Object o) { if (size == 0 || !(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; }

/** * 删除map中所有映射，使用Arrays.fill方法将table数组的值都设为null * Removes all of the mappings from this map. * The map will be empty after this call returns. */ public void clear() { modCount++; Arrays.fill(table, null); size = 0; }

containXXX类方法

/** * 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; }

clone方法

/** * 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; } 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; }

静态内部类Entry

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; } 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(); } /** * 当entry的值通过调用put方法被重写时该方法将被调用 * 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) { } /** * 当entry从table中删除时该方法将被调用 * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap<K,V> m) { } }

entry相关操作

/** * 添加一个新的entry。 * 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) { 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); } /** * 创建entry，并将其添加到table[bucketIndex]队首中。 * 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++; }

迭代。。。。（没看）

私有内部类KeySet及相关方法

// Views private transient Set<Map.Entry<K,V>> entrySet = null; /** * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * operations. */ public Set<K> keySet() { Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet())); } private final class KeySet extends AbstractSet<K> { public Iterator<K> iterator() { return newKeyIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsKey(o); } public boolean remove(Object o) { return HashMap.this.removeEntryForKey(o) != null; } public void clear() { HashMap.this.clear(); } }

私有内部类Value及其相关方法

/** * 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; // values属于AbstractMap的成员变量 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(); } }

私有内部类EntrySet及相关方法

/** * 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(); // HashMap.this值获取HashMap本身，所调用的clear即HashMap的方法 } }

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