util HashMap

1.HashMap涉及的数据结构

Entry[] ;

//Entry数组:存储HashMap元素的地方.

//Entry

//1.封装了key;value;

//2.本身是一个单向链表;包含hash值;next;指针;

 static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

2.存储的过程:

通过hashcode得到index后.存储的不仅仅是该元素的key,value,hash.还有指向下一个Entry的引用.如果出现了hashcode冲突问题.则新建一个Entry;将该Entry的nex指针指向已存在的Entry.

   public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //已经存在
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
      //不存在，则插入
        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

 void addEntry(int hash, K key, V value, int bucketIndex) {
	Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        if (size++ >= threshold)
            resize(2 * table.length);
    }

3.取值的过程

还是hash-index-Entry的过程.不是直接return该index上Entry;而要检查Entry链上真正对应的那个

   public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }