HashMap源码分析

HashMap源码分析

•  static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
  

hasmap默认初始容量大小 16

•  static final int MAXIMUM_CAPACITY = 1 << 30;
  

hashmap数组最大容量 1<<30 2的30次方

• static final float DEFAULT_LOAD_FACTOR = 0.75f;
  

默认加载因子 若数组容量为16 当添加超过12时 要扩容 每次扩容为上次的2倍

• static final int MIN_TREEIFY_CAPACITY = 64;
  static final int TREEIFY_THRESHOLD = 8;
  

当数组长度大于64，链表长度大于8时 链表转为红黑树

• static final int UNTREEIFY_THRESHOLD = 6;
  

当红黑树元素个数小于6时，转为链表

• transient Node<K,V>[] table;
  

哈希表中的数组

• transient int size;
  

元素个数

• public HashMap() {
          this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
      //因此  刚创建HashMap对象时，  size = 0， table = null
      //目的  节省空间
      }
  

无参构造函数

1. put()方法分析：

   public V put(K key, V value) {
           return putVal(hash(key), key, value, false, true);
       }
   
   final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                      boolean evict) {
           Node<K,V>[] tab; Node<K,V> p; int n, i;
           if ((tab = table) == null || (n = tab.length) == 0)
               n = (tab = resize()).length;
           if ((p = tab[i = (n - 1) & hash]) == null)
               tab[i] = newNode(hash, key, value, null);
           else {
               Node<K,V> e; K k;
               if (p.hash == hash &&
                   ((k = p.key) == key || (key != null && key.equals(k))))
                   e = p;
               else if (p instanceof TreeNode)
                   e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
               else {
                   for (int binCount = 0; ; ++binCount) {
                       if ((e = p.next) == null) {
                           p.next = newNode(hash, key, value, null);
                           if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                               treeifyBin(tab, hash);
                           break;
                       }
                       if (e.hash == hash &&
                           ((k = e.key) == key || (key != null && key.equals(k))))
                           break;
                       p = e;
                   }
               }
               if (e != null) { // existing mapping for key
                   V oldValue = e.value;
                   if (!onlyIfAbsent || oldValue == null)
                       e.value = value;
                   afterNodeAccess(e);
                   return oldValue;
               }
           }
           ++modCount;
           if (++size > threshold)
               resize();
           afterNodeInsertion(evict);
           return null;
       }
   
   static final int hash(Object key) {
           int h;
           return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
       }
   
       final Node<K,V>[] resize() {
           Node<K,V>[] oldTab = table;
           int oldCap = (oldTab == null) ? 0 : oldTab.length;
           int oldThr = threshold;
           int newCap, newThr = 0;
           if (oldCap > 0) {
               if (oldCap >= MAXIMUM_CAPACITY) {
                   threshold = Integer.MAX_VALUE;
                   return oldTab;
               }
               else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                        oldCap >= DEFAULT_INITIAL_CAPACITY)
                   newThr = oldThr << 1; // double threshold
           }
           else if (oldThr > 0) // initial capacity was placed in threshold
               newCap = oldThr;
           else {               // zero initial threshold signifies using defaults
               newCap = DEFAULT_INITIAL_CAPACITY;
               newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
           }
           if (newThr == 0) {
               float ft = (float)newCap * loadFactor;
               newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                         (int)ft : Integer.MAX_VALUE);
           }
           threshold = newThr;
           @SuppressWarnings({"rawtypes","unchecked"})
               Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
           table = newTab;
           if (oldTab != null) {
               for (int j = 0; j < oldCap; ++j) {
                   Node<K,V> e;
                   if ((e = oldTab[j]) != null) {
                       oldTab[j] = null;
                       if (e.next == null)
                           newTab[e.hash & (newCap - 1)] = e;
                       else if (e instanceof TreeNode)
                           ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                       else { // preserve order
                           Node<K,V> loHead = null, loTail = null;
                           Node<K,V> hiHead = null, hiTail = null;
                           Node<K,V> next;
                           do {
                               next = e.next;
                               if ((e.hash & oldCap) == 0) {
                                   if (loTail == null)
                                       loHead = e;
                                   else
                                       loTail.next = e;
                                   loTail = e;
                               }
                               else {
                                   if (hiTail == null)
                                       hiHead = e;
                                   else
                                       hiTail.next = e;
                                   hiTail = e;
                               }
                           } while ((e = next) != null);
                           if (loTail != null) {
                               loTail.next = null;
                               newTab[j] = loHead;
                           }
                           if (hiTail != null) {
                               hiTail.next = null;
                               newTab[j + oldCap] = hiHead;
                           }
                       }
                   }
               }
           }
           return newTab;
       }
   

• HashSet源码

public HashSet() {
        map = new HashMap<>();
    }

//add方法
public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }