HashMap

Posted 2020-11-02 saiqsai

结构图：

Entry的数据结构：

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

put操作：

public V put(K key, V value) {
    //初始化
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    //插入的key可以为null
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key);
    int i = indexFor(hash, table.length);
    //key值相同，put操作会把value覆盖
    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))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }

    modCount++;
    //具体的插入操作
    addEntry(hash, key, value, i);
    return null;
}

void addEntry(int hash, K key, V value, int bucketIndex) {
    //table扩容
    if ((size >= threshold) && (null != table[bucketIndex])) {
        resize(2 * table.length);
        hash = (null != key) ? hash(key) : 0;
        bucketIndex = indexFor(hash, table.length);
    }
    //创建Entry，并插入到table中
    createEntry(hash, key, value, bucketIndex);
}

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];
    //把老table里面的Entry搬到新table里
    transfer(newTable, initHashSeedAsNeeded(newCapacity));
    table = newTable;
    threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}

void transfer(Entry[] newTable, boolean rehash) {
    int newCapacity = newTable.length;
    for (Entry<K,V> e : table) {
        while(null != e) {
            Entry<K,V> next = e.next;
            if (rehash) {
                e.hash = null == e.key ? 0 : hash(e.key);
            }
            int i = indexFor(e.hash, newCapacity);
            e.next = newTable[i];
            newTable[i] = e;
            e = next;
        }
    }
}

transfer后：

查找元素：

public V get(Object key) {
    if (key == null)
        return getForNullKey();
    //根据key获取Entry
    Entry<K,V> entry = getEntry(key);

    return null == entry ? null : entry.getValue();
}

final Entry<K,V> getEntry(Object key) {
    if (size == 0) {
        return null;
    }

    int hash = (key == null) ? 0 : hash(key);
    //计算key落在teble的哪个位置
    for (Entry<K,V> e = table[indexFor(hash, table.length)];
         e != null;
         e = e.next) {
        Object k;
        //先比较hash code，过滤掉大部分数据
        if (e.hash == hash &&
            ((k = e.key) == key || (key != null && key.equals(k))))
            return e;
    }
    return null;
}