java - 如果我们将空键添加为对象，为什么同步对象会给出空指针异常？

Question

这个问题已经有答案了:

Why does ConcurrentHashMap prevent null keys and values? (7 个回答)

已关闭 7 年前。

这里我有哈希表和并发 HashMap 的代码示例，它们不允许空键作为对象，但两者都是同步的。

  public class Example
  {
public static void main(String[] args)
{
    ConcurrentHashMap<String,String> premiumPhone = 
                           new ConcurrentHashMap<String,String>();
    premiumPhone.put("Apple", "iPhone");
    premiumPhone.put("HTC", "HTC one");
    premiumPhone.put(null,"S5");

    Iterator iterator = premiumPhone.keySet().iterator();

    while (iterator.hasNext())
    {
        System.out.println(premiumPhone.get(iterator.next()));
        premiumPhone.put("Sony", "Xperia Z");
    }

}
}

Answer 1

来自 ConcurrentHashMap 的 Javadoc

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html

另外，看看这个:https://stackoverflow.com/a/9298113/2227788