HashMap 数据结构分析

测试代码

public class App {
    public static void main(String[] args) {
        System.out.println("123");
        Map<Object, Object> hashMap = new HashMap<>(4);

        for (int i = 0; i < 64; i++) {
            hashMap.put(i, i);
        }

        for (int i = 0; i < 9; i++) {
            hashMap.put(new User(), i);
        }


        hashMap.get("hi");
    }

    private static class User {
        private int hash;
        @Override
        public int hashCode() {
            hash = 123;
            return 123;
        }
    }

}

数据结构分析

HashMap{
  // 数组，若没有给定初始值，第一次 put 时才会初始化，
  /**
   	 resize() {
	   int oldThr = threshold;
	   ...
	   // 给初始化容量大小
	   else if (oldThr > 0) 
     	newCap = oldThr;
     // 不给则默认为 16 DEFAULT_INITIAL_CAPACITY = 1 << 4;
     else {          
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
     }
     ...
		 Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
   }
   **/
  transient Node<K,V>[] table;
	// 单向链表
  Node.next
}

逻辑分析

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)
          // 扩容
            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;
          // hash 相等 Key 相等则覆盖
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
          // 变红黑树之后，即在 treeifyBin(tab, hash) 方法中的                TreeNode<K,V> p = replacementTreeNode(e, null); 执行之后；
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
          // hash 冲突
            else {
              // 某个键出现 hash 冲突时，会使用(Node.next属性)进行链接(链表)。当长度 >=8 ，并且数组长度大于 MIN_TREEIFY_CAPACITY = 64时，此时链表转为红黑树，即 treeifyBin 方法中的replacementTreeNode()；
                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;
    }
=========================函数分割线=========================
final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
  // 
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

红黑树成果图

验证

链表超过8个，但是元素不足64，不会使用红黑树

只会 resize

preserve order

换了个位置