HashMap

来源：互联网发布：淘宝差评最多的店编辑：程序博客网时间：2024/05/22 16:50

这里写图片描述

   HashMap 初始化4^2容量的table表，加载因子为0.75 ，每个table存储一个hashmapEntry（该entry含一个next引用，形成一个链表）   每次put(key,value) 判断是否需要扩容，若需要扩容，则重新hash,重新将每一个hashmapentry放置到新的table【i】中，重hash完之后，再对 新插入key hash->i,   createHashmapentry，插入table[i].//initialCapacity 容量，初始默认值为16，最大为2^30，loadFactor为加载因子，//当size>initialCapacity*loadFactor时候map在push数据时判断自动扩容为原来2//倍，init()模板方法用于子类。public HashMap(int initialCapacity, float loadFactor) {        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal initial capacity: " +                                               initialCapacity);        if (initialCapacity > MAXIMUM_CAPACITY)            initialCapacity = MAXIMUM_CAPACITY;        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new IllegalArgumentException("Illegal load factor: " +                                               loadFactor);        this.loadFactor = loadFactor;        threshold = initialCapacity;        init();    }

/**     * 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.     *     * @param key key with which the specified value is to be associated     * @param value value to be associated with the specified key     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     */public V put(K key, V value) {    //如果table表为空，则初始化表，设置极限容量threshold，设置表容量        if (table == EMPTY_TABLE) {            inflateTable(threshold);        }        //map的key可以为空，默认插入hash值为0的桶处        if (key == null)            return putForNullKey(value);        //key非空，则计算其hash值        int hash = hash(key);        //计算其在table表中的索引值下标        int i = indexFor(hash, table.length);        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            //遍历下标为i的entry链表（next引用模拟指针）            //当找到key值相同的entry时，直接将新value替换掉原来的旧value            //recordAccess(子类使用如linkedhashmap)            Object k;            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }    //如若插入值新，不存在key已之相等的，则插入entry链表中（next引用模拟指针）        modCount++;        addEntry(hash, key, value, i);        return null;    }

/**     * Inflates the table.     */    private void inflateTable(int toSize) {        // Find a power of 2 >= toSize        //map capacity容易必须为2的整数次幂         int capacity = roundUpToPowerOf2(toSize);        //计算极限容量threshold为容量*加载因子和最大容量之间比较的最小者        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        //初始化table表        table = new Entry[capacity];        initHashSeedAsNeeded(capacity);    }

    final int hash(Object k) {          int h = hashSeed;          if (0 != h && k instanceof String) {              return sun.misc.Hashing.stringHash32((String) k);          }          // 调用key的hashCode()方法得到hashCode          h ^= k.hashCode();          // 对hashCode进行一系列的位移与异或运算并把结果作为hashCode返回          h ^= (h >>> 20) ^ (h >>> 12);          return h ^ (h >>> 7) ^ (h >>> 4);      }

//将新entry插入entry链表中void addEntry(int hash, K key, V value, int bucketIndex) {    //判断是否需要扩容，当table【bucketIndex】不空且当前size>极限容量时    //自动扩容且判断是否需要重新hash        if ((size >= threshold) && (null != table[bucketIndex])) {            resize(2 * table.length);            hash = (null != key) ? hash(key) : 0;            bucketIndex = indexFor(hash, table.length);        }        createEntry(hash, key, value, bucketIndex);    }

//扩容时重新hash各entry链表void transfer(Entry[] newTable, boolean rehash) {        int newCapacity = newTable.length;        for (Entry<K,V> e : table) {            while(null != e) {            //首先将entry链表中的next entry取出来                Entry<K,V> next = e.next;                if (rehash) {                    //重新计算该entry链的hash值                    e.hash = null == e.key ? 0 : hash(e.key);                }                //计算其在新扩容表中的下标索引值                int i = indexFor(e.hash, newCapacity);                //将其next引用指向上一次取出来的entry                e.next = newTable[i];                //将其放置在 newTable[i]中                newTable[i] = e;                //继续将next依次取出来连接之前已经取出来的entry                e = next;            }        }    }

//将新map插入到entry链表中void createEntry(int hash, K key, V value, int bucketIndex) {        Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<>(hash, key, value, e);        size++;    }

0 0