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HashMap系列-放入元素的流程

article 2024/9/23 6:31:44

1.put

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {
    //通过key生成hash后，调用putVal方法
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
}

2.putVal

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    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table为null或者长度为0，则调用resize初始化table
        if ((tab = table) == null || (n = tab.length) == 0){
            n = (tab = resize()).length;
        }
        //key -> hash -> i
        if ((p = tab[i = (n - 1) & hash]) == null){//如果table[i]的元素为null
            tab[i] = newNode(hash, key, value, null); //放入table[i]
        } else {
            Node<K,V> e; K k;
            //这时候的p是table[i]
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k)))){
                //hash 相等，key相等，要放在table[i]处
                e = p; 
            } else if (p instanceof TreeNode){//不能放在table[i]
                //是红黑树
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            } else {//是链表
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {//如果一直没有找到一个hash和key相等的
                        p.next = newNode(hash, key, value, null);//添加到最后
                        //判断链表长度是否达到了大于或者等于8
                        if (binCount >= TREEIFY_THRESHOLD - 1){ // -1 for 1st
                            treeifyBin(tab, hash); //转为树结构，或者扩容
                        }
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k)))){
                        //hash 相等，key相等
                        break;
                    }
                    p = e;//位移到下一个
                }
            }
            //如果是放在已经存在的节点上
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null){
                    e.value = value;//更新值
                }
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold){如果数据个数大于threshold
            resize(); //扩容
        }
        afterNodeInsertion(evict);
        return null;
    }
}