java.util.concurrent.ConcurrentHashMap.put(K, V)
/** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null */ public V put(K key, V value) { if (value == null) throw new NullPointerException(); int hash = hash(key.hashCode()); return segmentFor(hash).put(key, hash, value, false); }
hashMap和linkedhashMap都允许null key和null value,
treeMap不允许null key,但允许null value
http://www.trinea.cn/android/android-source-code-analysis/concurrenthashmap-nullpointerexception-null-key/