JAVA中哪些集合允许key为null

Hashtable、ConcurrentHashTable，TreeMap不允许key为null，编译运行时会提示空指针异常。

HashSet 、HashMap 、 ArrayList 、 LinkedList均可接受null值

public static void main(String[] args) {/*  TreeMap<Object, Object> map = new TreeMap<>();map.put(null, 1);*//*  TreeSet<Object> objects = new TreeSet<>();objects.add(null);*//*  ConcurrentHashMap<Object, Object> a = new ConcurrentHashMap<>();a.put(null, 1);*/HashSet<Object> objects1 = new HashSet<>();objects1.add(null);System.out.println(objects1.size());HashMap<Object, Object> objectObjectHashMap = new HashMap<>();objectObjectHashMap.put(null, 1);System.out.println(objectObjectHashMap.size());ArrayList<Object> objects = new ArrayList<>();objects.add(null);System.out.println(objects.size());LinkedList<Object> objects2 = new LinkedList<>();objects2.add(null);System.out.println(objects2.size());			//长度为1System.out.println(objects2.get(0));			//值为null}