HashMap之clear

• 1，当前对象不为空，遍历对象，把每个hash对应下标的对象都设为空
• 2，modCount是用来计算HashMap的修改次数

/**
 * Removes all of the mappings from this map.
 * The map will be empty after this call returns.
 */
public void clear() {
    Node<K,V>[] tab;
    modCount++;
    if ((tab = table) != null && size > 0) {
        size = 0;
        for (int i = 0; i < tab.length; ++i)
            tab[i] = null;
    }
}