Java集合类---HashMap源码分析

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本文深入剖析了Java中HashMap的数据结构及其实现原理，详细介绍了其构造方法、扩容机制、键值对的增删改查操作，并展示了内部Entry类的作用。

JDK版本：1.8

package java.util;    
import java.io.*;    
   
public class HashMap<K,V>    
    extends AbstractMap<K,V>    
    implements Map<K,V>, Cloneable, Serializable    
{    
   
    // 默认的初始容量  
    static final int DEFAULT_INITIAL_CAPACITY = 16;    
   
    // 最大容量 
    static final int MAXIMUM_CAPACITY = 1 << 30;    
   
    // 加载因子0.75   
    static final float DEFAULT_LOAD_FACTOR = 0.75f;    
    
    transient Entry[] table;    
   
    transient int size;    
    
    int threshold;    
   
    final float loadFactor;     
    transient volatile int modCount;    
   
    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);    
   
        int capacity = 1;    
        while (capacity < initialCapacity)    
            capacity <<= 1;    
   
        this.loadFactor = loadFactor;      
        threshold = (int)(capacity * loadFactor);    
        table = new Entry[capacity];    
        init();    
    }    
   
   
    public HashMap(int initialCapacity) {    
        this(initialCapacity, DEFAULT_LOAD_FACTOR);    
    }    
   
    public HashMap() {    
        this.loadFactor = DEFAULT_LOAD_FACTOR;    
        // 当HashMap中存储数据的数量达到threshold时，就需要将HashMap的容量加倍。    
        threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);    
        // 创建Entry数组，用来保存数据    
        table = new Entry[DEFAULT_INITIAL_CAPACITY];    
        init();    
    }    
   
    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);    
    }    
   
    static int hash(int h) {    
        h ^= (h >>> 20) ^ (h >>> 12);    
        return h ^ (h >>> 7) ^ (h >>> 4);    
    }    

    static int indexFor(int h, int length) {    
        return h & (length-1);    
    }    
   
    public int size() {    
        return size;    
    }    
   
    public boolean isEmpty() {    
        return size == 0;    
    }    
   
    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;    
    }    
   
    private V getForNullKey() {    
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {    
            if (e.key == null)    
                return e.value;    
        }    
        return null;    
    }    
   
    public boolean containsKey(Object key) {    
        return getEntry(key) != null;    
    }    
   
    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;    
    }    
    
    public V put(K key, V value) {    
        // 若“key为null”，则将该键值对添加到table[0]中。    
        if (key == null)    
            return putForNullKey(value);    
        int hash = hash(key.hashCode());    
        int i = indexFor(hash, table.length);    
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {    
            Object k;    
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {    
                V oldValue = e.value;    
                e.value = value;    
                e.recordAccess(this);    
                return oldValue;    
            }    
        }    
   
        modCount++;  
        addEntry(hash, key, value, i);    
        return null;    
    }    
   
    // 将key==null键值对添加到table[0]位置    
    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;    
    }    
    
    private void putForCreate(K key, V value) {    
        int hash = (key == null) ? 0 : hash(key.hashCode());    
        int i = indexFor(hash, table.length);    
   
        // 若该HashMap表中存在“键值等于key”的元素，则替换该元素的value值    
        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) {    
        // 利用迭代器将元素逐个添加到HashMap中    
        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());    
        }    
    }    
   
    // 重新调整HashMap的大小，newCapacity是调整后的容量    
    void resize(int newCapacity) {    
        Entry[] oldTable = table;    
        int oldCapacity = oldTable.length;   
        if (oldCapacity == MAXIMUM_CAPACITY) {    
            threshold = Integer.MAX_VALUE;    
            return;    
        }    
   
        Entry[] newTable = new Entry[newCapacity];    
        transfer(newTable);    
        table = newTable;    
        threshold = (int)(newCapacity * loadFactor);    
    }    
    
    void transfer(Entry[] newTable) {    
        Entry[] src = table;    
        int newCapacity = newTable.length;    
        for (int j = 0; j < src.length; j++) {    
            Entry<K,V> e = src[j];    
            if (e != null) {    
                src[j] = null;    
                do {    
                    Entry<K,V> next = e.next;    
                    int i = indexFor(e.hash, newCapacity);    
                    e.next = newTable[i];    
                    newTable[i] = e;    
                    e = next;    
                } while (e != null);    
            }    
        }    
    }    
    
    public void putAll(Map<? extends K, ? extends V> m) {    
        // 有效性判断    
        int numKeysToBeAdded = m.size();    
        if (numKeysToBeAdded == 0)    
            return;    
  
        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);    
        }    
   
        // 通过迭代器，将“m”中的元素逐个添加到HashMap中。    
        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());    
        }    
    }    
   
    // 删除“键为key”元素    
    public V remove(Object key) {    
        Entry<K,V> e = removeEntryForKey(key);    
        return (e == null ? null : e.value);    
    }    
    
    final Entry<K,V> removeEntryForKey(Object key) {     
        int hash = (key == null) ? 0 : hash(key.hashCode());    
        int i = indexFor(hash, table.length);    
        Entry<K,V> prev = table[i];    
        Entry<K,V> e = prev;    
   
        while (e != null) {    
            Entry<K,V> next = e.next;    
            Object k;    
            if (e.hash == hash &&    
                ((k = e.key) == key || (key != null && key.equals(k)))) {    
                modCount++;    
                size--;    
                if (prev == e)    
                    table[i] = next;    
                else   
                    prev.next = next;    
                e.recordRemoval(this);    
                return e;    
            }    
            prev = e;    
            e = next;    
        }    
   
        return e;    
    }    
    
    final Entry<K,V> removeMapping(Object o) {    
        if (!(o instanceof Map.Entry))    
            return null;    
   
        Map.Entry<K,V> entry = (Map.Entry<K,V>) o;    
        Object key = entry.getKey();    
        int hash = (key == null) ? 0 : hash(key.hashCode());    
        int i = indexFor(hash, table.length);    
        Entry<K,V> prev = table[i];    
        Entry<K,V> e = prev;    
   
        while (e != null) {    
            Entry<K,V> next = e.next;    
            if (e.hash == hash && e.equals(entry)) {    
                modCount++;    
                size--;    
                if (prev == e)    
                    table[i] = next;    
                else   
                    prev.next = next;    
                e.recordRemoval(this);    
                return e;    
            }    
            prev = e;    
            e = next;    
        }    
   
        return e;    
    }    
    
    public void clear() {    
        modCount++;    
        Entry[] tab = table;    
        for (int i = 0; i < tab.length; i++)    
            tab[i] = null;    
        size = 0;    
    }    
   
    public boolean containsValue(Object value) {    
    // 若“value为null”，则调用containsNullValue()查找    
    if (value == null)    
            return containsNullValue();    
   
    // 若“value不为null”，则查找HashMap中是否有值为value的节点。    
    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;    
    }    
   
    // 是否包含null值    
    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;    
    }    
    
    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;    
    }    
   
    // Entry是单向链表。    
    // 它是 “HashMap链式存储法”对应的链表。    
    // 它实现了Map.Entry 接口，即实现getKey(), getValue(), setValue(V value), equals(Object o), hashCode()这些函数    
    static class Entry<K,V> implements Map.Entry<K,V> {    
        final K key;    
        V value;    
        // 指向下一个节点    
        Entry<K,V> next;    
        final int hash;    
   
        // 构造函数。    
        // 输入参数包括"哈希值(h)", "键(k)", "值(v)", "下一节点(n)"    
        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;    
        }    
   
        // 判断两个Entry是否相等    
        // 若两个Entry的“key”和“value”都相等，则返回true。    
        // 否则，返回false    
        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;    
        }    
   
        // 实现hashCode()    
        public final int hashCode() {

        return Objects.hashCode(key) ^ Objects.hashCode(value);

} public final String toString() { return getKey() + "=" + getValue(); } void recordAccess(HashMap<K,V> m) { } // 当从HashMap中删除元素时，绘调用recordRemoval()。 // 这里不做任何处理 void recordRemoval(HashMap<K,V> m) { } } void addEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<K,V>(hash, key, value, e); // 若HashMap的实际大小不小于 “阈值”，则调整HashMap的大小 if (size++ >= threshold) resize(2 * table.length); } void createEntry(int hash, K key, V value, int bucketIndex) { 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> { // 下一个元素 Entry<K,V> next; // expectedModCount用于实现fast-fail机制。 int expectedModCount; // 当前索引 int index; // 当前元素 Entry<K,V> current; 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; } // 获取下一个元素 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; } // 删除当前元素 public void remove() { if (current == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); Object k = current.key; current = null; HashMap.this.removeEntryForKey(k); expectedModCount = modCount; } } private final class ValueIterator extends HashIterator<V> { public V next() { return nextEntry().value; } } private final class KeyIterator extends HashIterator<K> { public K next() { return nextEntry().getKey(); } } private final class EntryIterator extends HashIterator<Map.Entry<K,V>> { public Map.Entry<K,V> next() { return nextEntry(); } } Iterator<K> newKeyIterator() { return new KeyIterator(); } Iterator<V> newValueIterator() { return new ValueIterator(); } Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); } private transient Set<Map.Entry<K,V>> entrySet = null; 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(); } } 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(); } } // 返回“HashMap的Entry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(); } } 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); s.writeInt(size); 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; private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Read in the threshold, loadfactor, and any hidden stuff s.defaultReadObject(); // Read in number of buckets and allocate the bucket array; int numBuckets = s.readInt(); table = new Entry[numBuckets]; init(); // Give subclass a chance to do its thing. // Read in size (number of Mappings) int size = s.readInt(); // Read the keys and values, and put the mappings in the HashMap for (int i=0; i<size; i++) { K key = (K) s.readObject(); V value = (V) s.readObject(); putForCreate(key, value); } } int capacity() { return table.length; } float loadFactor() { return loadFactor; } }