Java 中将地图转换为 ConcurrentMap 时出错

Question

我有一个Map<String,Object>要转换成ConcurrentMap<String,Object>

    Map<String,Object> testMap = new HashMap<String,Object>();
    testMap.put("test", null); //This null causing issues in conversion
    testMap.put("test2","123");
    testMap.put("test3",234);
    ConcurrentMap<String,Object> concMap = new ConcurrentHashMap<>(testMap);

我得到一个空指针异常。如果我复制到一个新的 HashMap<String,Object>

    Map<String,Object> testMap = new HashMap<String,Object>();
    testMap.put("test", null);
    testMap.put("test2","123");
    testMap.put("test3",234);
    Map<String,Object> concMap = new HashMap<>(testMap);

我没有收到任何错误。有没有 Map<String,Object> 到 ConcurrentMap<String,Object> 的安全方法而不需要 NullPointerException

Answer 1

它在 documentation for ConcurrentHashMap:

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

至于其他ConcurrentMaps，the only other implementation ConcurrentSkipListMap, does not allow null either。

Answer 2

问题不在于将 HashMap 转换为 ConcurrentHashMap。更多的是 ConcurrentHashMap 是空安全的，它不允许 null 值。从它的 documentation:

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

Answer 3

如果您查看 ConcurrentHashMap 的源代码，您会发现它不允许空键或空值 -

java.util.concurrent.ConcurrentHashMap#putVal

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();

Java 中将地图转换为 ConcurrentMap 时出错

Error in Converting a Map to ConcurrentMap in Java

java

java.util.concurrent