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public interface ConcurrentMap<K, V> extends Map<K, V> {
    V putIfAbsent(K key, V value); // 如果指定key没有与任何值绑定，则它与value绑定
    /* 等同于
    if (!map.containsKey(key))
       return map.put(key, value);
    else
       return map.get(key);
    */
    boolean remove(Object key, Object value); // 只有当该key与value相映射时候删除掉该元素
    /* 等同于
    if (map.containsKey(key) && map.get(key).equals(value)) {
        map.remove(key);
        return true;
    } else return false;
    */
    boolean replace(K key, V oldValue, V newValue); // 当key的值为oldValue时用newValue代替
   /* 等同于
   if (map.containsKey(key) && map.get(key).equals(oldValue)) {
       map.put(key, newValue);
       return true;
   } else return false;
   */
    V replace(K key, V value); // 当map中包含有key时用value代替
   /* 等同于
   if (map.containsKey(key)) {
       return map.put(key, value);
   } else return null;
   */
}

// 常量列表
DEFAULT_INITIAL_CAPACITY = 1 << 4;  //The capacity is the number of buckets in the hash table
MAXIMUM_CAPACITY = 1 << 30;
/*
**The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
*/
DEFAULT_LOAD_FACTOR = 0.75f;        //填充因子