HashMap jdk1.8 jdk1.7 put改动

Posted 2020-09-04

之前hashmap处理冲突单纯使用链表法进行链接，1.8中，当一散列值中对应的链表长度超过8个后，会将链表转化为红黑树进行存储。

 

1.8中源码

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

/** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don‘t change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */

final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<k,v>[] tab; Node<k,v> p; int n, i; //如果tab为空或长度为0，则分配内存resize() if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; //(n - 1) & hash找到put位置，如果为空,则直接put if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<k,v> e; K k; //第一节节点hash值同，且key值与插入key相同 if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k)))) e = p; else if (p instanceof TreeNode)//属于红黑树处理冲突 e = ((TreeNode<k,v>)p).putTreeVal(this, tab, hash, key, value); else { //链表处理冲突 for (int binCount = 0; ; ++binCount) { //p第一次指向表头,以后依次后移 if ((e = p.next) == null) { //e为空，表示已到表尾也没有找到key值相同节点，则新建节点 p.next = newNode(hash, key, value, null); //新增节点后如果节点个数到达阈值，则将链表转换为红黑树 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st  treeifyBin(tab, hash); break; } //容许null==null if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k)))) break; p = e;//更新p指向下一个节点  } } //更新hash值和key值均相同的节点Value值 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; }

1.7源码

/**
 * 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.
 */
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);
    //这里的循环是关键
    //当新增的key所对应的索引i，对应table[i]中已经有值时，进入循环体
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        //判断是否存在本次插入的key，如果存在用本次的value替换之前oldValue，相当于update操作
        //并返回之前的oldValue
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }
    //如果本次新增key之前不存在于HashMap中，modCount加1，说明结构改变了
    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

 

   final V putVal(int hash, K key, V value, boolean onlyIfAbsent,

                  boolean evict) {

       Node<k,v>[] tab; Node<k,v> p; int n, i;

       //如果tab为空或长度为0，则分配内存resize()

       if ((tab = table) == null || (n = tab.length) == 0)

           n = (tab = resize()).length;

       //(n - 1) & hash找到put位置，如果为空,则直接put

       if ((p = tab[i = (n - 1) & hash]) == null)

           tab[i] = newNode(hash, key, value, null);

       else {

           Node<k,v> e; K k;

           //第一节节点hash值同，且key值与插入key相同

           if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))

               e = p;

           else if (p instanceof TreeNode)//属于红黑树处理冲突

               e = ((TreeNode<k,v>)p).putTreeVal(this, tab, hash, key, value);

           else {

               //链表处理冲突

               for (int binCount = 0; ; ++binCount) {

                   //p第一次指向表头,以后依次后移

                   if ((e = p.next) == null) {

                       //e为空，表示已到表尾也没有找到key值相同节点，则新建节点

                       p.next = newNode(hash, key, value, null);

                       //新增节点后如果节点个数到达阈值，则将链表转换为红黑树

                       if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st

                           treeifyBin(tab, hash);

                       break;

                   }

                   //容许null==null

                   if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))

                       break;

                   p = e;//更新p指向下一个节点

               }

           }

           //更新hash值和key值均相同的节点Value值

           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;

   }