JDK源码分析之集合04HashMap

//Map中定义了Map必须支持的一些操作，abstractMap中添加了其默认实现 public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>,         Cloneable, Serializable

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 (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) { } }

// 默认初始化容量；必须为2的n次幂     static final int DEFAULT_INITIAL_CAPACITY = 16;     // HashMap的最大容量，为2的30次幂     static final int MAXIMUM_CAPACITY = 1 << 30;     // 默认因子     static final float DEFAULT_LOAD_FACTOR = 0.75f;      // Entry数组，用于保存元素；长度必须为2的n次幂     transient Entry<K, V>[] table;      // map中元素的个数     transient int size;      /**      * The next size value at which to resize (capacity * load factor).      * 改变size的阈值=capacity*factor      */     int threshold;      /**      * The load factor for the hash table.      */     final float loadFactor;     // 结构改变的次数     transient int modCount;      /**      * The default threshold of map capacity above which alternative hashing is      * used for String keys. Alternative hashing reduces the incidence of      * collisions due to weak hash code calculation for String keys.      * <p/>      * This value may be overridden by defining the system property      * {@code jdk.map.althashing.threshold}. A property value of {@code 1}      * forces alternative hashing to be used at all times whereas {@code -1}      * value ensures that alternative hashing is never used.      */     static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

// 使用 默认的capacity和factor创建空的HashMap     public HashMap(int initialCapacity, float loadFactor) {         // 检查参数的合法性         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         // 找到第一个大于initialCapacity的2的整次幂         int capacity = 1;         while (capacity < initialCapacity)             capacity <<= 1;          this.loadFactor = loadFactor;         // 确定阈值         threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);         // 初始化hash表         table = new Entry[capacity];         useAltHashing = sun.misc.VM.isBooted()                 && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);         init();     }

/**      * Returns the value to which the specified key is mapped, or {@code null}      * if this map contains no mapping for the key.      */     // 返回值为null可能有两种情况：1不存在key-value对；2、key指向null     public V get(Object key) {         // 两种情况key==null和非null         if (key == null)             return getForNullKey();         Entry<K, V> entry = getEntry(key);          return null == entry ? null : entry.getValue();     }      private V getForNullKey() {         for (Entry<K, V> e = table[0]; e != null; e = e.next) {             if (e.key == null)                 return e.value;         }         return null;     }      final Entry<K, V> getEntry(Object key) {         // 先获取key对应的hash值，若key为null，则hash值为0         int hash = (key == null) ? 0 : hash(key);         // 获取table的index=hash&table.length-1         for (Entry<K, V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) {             Object k;             // 判断是否为指定key的map对：1、hash值相等；2、key为同个对象或者值相等             if (e.hash == hash                     && ((k = e.key) == key || (key != null && key.equals(k))))                 return e;         }         return null;     }

// 添加键值对，如果之前已存在，则覆盖之前的值     public V put(K key, V value) {         if (key == null)             return putForNullKey(value);         // 先获取key对应的hash值         int hash = hash(key);         // 然后获取hash值对应的index=hash&table.lenth-1         int i = indexFor(hash, table.length);         for (Entry<K, V> e = table[i]; e != null; e = e.next) { //如果索引链表中已经存在此节点；则覆盖之前的值             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;             }         } //如果索引处的Entry对象为null，则添加一个元素         modCount++;         addEntry(hash, key, value, i);         return null;     }     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;     }     void addEntry(int hash, K key, V value, int bucketIndex) {         if ((size >= threshold) && (null != table[bucketIndex])) {             // 调整map的size             resize(2 * table.length);             hash = (null != key) ? hash(key) : 0;             bucketIndex = indexFor(hash, table.length);         }          createEntry(hash, key, value, bucketIndex);     }     void createEntry(int hash, K key, V value, int bucketIndex) {         Entry<K, V> e = table[bucketIndex]; //新添加的元素指向之前的元素         table[bucketIndex] = new Entry<>(hash, key, value, e);         size++;     }

static int indexFor(int h, int length) { return h & (length-1); }

final Entry<K,V> removeEntryForKey(Object key) { int hash = (key == null) ? 0 : hash(key); int i = indexFor(hash, table.length); Entry<K,V> prev = table[i]; Entry<K,V> e = prev;   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; }