关于list集合存储null的问题

工作中，遇到list集合存储null的问题，不确定list能否存储null值。于是写一些demo测试list，set,table,及map存储null的问题。

1.list之arraylist

public static void main(String[] args) {
       String string= null;
       List list= new ArrayList<String>();
       System.out.println("size:"+ list.size());
       list.add(string);
       System.out.println("size:"+ list.size()+ ",list(i):"+list.get(0));
}

run：

   size:0
   size:1,list(i):null

总结：arraylist集合是能存储null的。

2.list之linkedlist

public static void main(String[] args) {
      List list= new LinkedList<String>();
      System.out.println("list.size():"+list.size());
      String string= null;
      list.add(string);
      System.out.println("list.size():"+list.size()+", list.get(0):"+list.get(0));
}

run:
   list.size():0
   list.size():1, list.get(0):null

结论：linkedlist能存储null

3.set之hashset:

public static void main(String[] args) {
        // TODO Auto-generated method stub
        Set set= new HashSet<String>();
        System.out.println("set.size():"+set.size());
        String string= null;
        String string2= null;
        set.add(string);
        for (Iterator<String> it= set.iterator() ; it.hasNext();) {
            string2 =  it.next();
        }
        System.out.println("set.size():"+set.size()+",  set.iterator.next():"+string2);
}

run:
  set.size():0
  set.size():1,  set.iterator.next():null 

结论：hashset能存储null

4.set之treeset (1):

public static void main(String[] args) {
           Set set= new TreeSet<String>();
           System.out.println("set.size():"+set.size());
           String string= null;
           String string2= null;
           set.add(string);
           for (Iterator<String> it= set.iterator() ; it.hasNext();) {
                 string2 =  it.next();
           }
           System.out.println("set.size():"+set.size()+",  set.iterator.next():"+string2);
}

run:
  set.size():0
 Exception in thread "main" java.lang.NullPointerException
 at java.util.TreeMap.compare(TreeMap.java:1188)
 at java.util.TreeMap.put(TreeMap.java:531)
 at java.util.TreeSet.add(TreeSet.java:255)
 at com.westward.Demo3.main(Demo3.java:15)

4.set之treeset (2):

public static void main(String[] args) {
           Set set= new TreeSet<String>();
           System.out.println("set.size():"+set.size());
           String string= "1";
           String string2= null;
           set.add(string);
           for (Iterator<String> it= set.iterator() ; it.hasNext();) {
                 string2 =  it.next();
           }
           System.out.println("set.size():"+set.size()+",  set.iterator.next():"+string2);
}

run:
  set.size():0
  set.size():1,  set.iterator.next():1

结论：treeset不能存储null

还有map，及table，未完待续...