阿里-测试开发面经（六）

    /**     * The table, resized as necessary. Length MUST Always be a power of two.     */    transient Entry[] table;

// 存储时:int hash = key.hashCode(); // 这个hashCode方法这里不详述,只要理解每个key的hash是一个固定的int值int index = hash % Entry[].length;Entry[index] = value;

// 取值时:int hash = key.hashCode();int index = hash % Entry[].length;return Entry[index];

public V put(K key, V value) {        if (key == null)            return putForNullKey(value); //null总是放在数组的第一个链表中        int hash = hash(key.hashCode());        int i = indexFor(hash, table.length);        //遍历链表        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            Object k;            //如果key在链表中已存在，则替换为新value            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }

        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); //参数e, 是Entry.next    //如果size超过threshold，则扩充table大小。再散列    if (size++ >= threshold)            resize(2 * table.length);}

 public V get(Object key) {        if (key == null)            return getForNullKey();        int hash = hash(key.hashCode());        //先定位到数组元素，再遍历该元素处的链表        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;}

   private V putForNullKey(V value) {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }        modCount++;        addEntry(0, null, value, 0);        return null;    }

    private V getForNullKey() {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null)                return e.value;        }        return null;    }

   /**     * Returns index for hash code h.     */    static int indexFor(int h, int length) {        return h & (length-1);    }

  public HashMap(int initialCapacity, float loadFactor) {        .....

        // Find a power of 2 >= initialCapacity        int capacity = 1;        while (capacity < initialCapacity)            capacity <<= 1;

        this.loadFactor = loadFactor;        threshold = (int)(capacity * loadFactor);        table = new Entry[capacity];        init();    }

   /**     * Rehashes the contents of this map into a new array with a     * larger capacity.  This method is called automatically when the     * number of keys in this map reaches its threshold.     *     * If current capacity is MAXIMUM_CAPACITY, this method does not     * resize the map, but sets threshold to Integer.MAX_VALUE.     * This has the effect of preventing future calls.     *     * @param newCapacity the new capacity, MUST be a power of two;     *        must be greater than current capacity unless current     *        capacity is MAXIMUM_CAPACITY (in which case value     *        is irrelevant).     */    void resize(int newCapacity) {        Entry[] oldTable = table;        int oldCapacity = oldTable.length;        if (oldCapacity == MAXIMUM_CAPACITY) {            threshold = Integer.MAX_VALUE;            return;        }

        Entry[] newTable = new Entry[newCapacity];        transfer(newTable);        table = newTable;        threshold = (int)(newCapacity * loadFactor);    }





    /**     * Transfers all entries from current table to newTable.     */    void transfer(Entry[] newTable) {        Entry[] src = table;        int newCapacity = newTable.length;        for (int j = 0; j < src.length; j++) {            Entry<K,V> e = src[j];            if (e != null) {                src[j] = null;                do {                    Entry<K,V> next = e.next;                    //重新计算index                    int i = indexFor(e.hash, newCapacity);                    e.next = newTable[i];                    newTable[i] = e;                    e = next;                } while (e != null);            }        }    }

staticintindexFor(inth,intlength){ returnh&(length-1); }