昨天写代码的时候需要往容器中添加null，所以主要测试了几个常见的容器能否添加null的情况，如下:

public class TestNull {
    public static void main(String[] args) {  
        //HashMap  允许null-null键值对  
        Map<String,String> hashMap = new HashMap<String,String>();  
        try {
            hashMap.put("test1", "value1");  
            hashMap.put("test2", null);  
            hashMap.put(null, "value2");  
            hashMap.put(null, null);  
            System.out.println("HashMap添加null-null成功。" + "(hashMap.size() = " + hashMap.size() + ")" );  
        } catch (Exception e) {
            System.out.println("Hash添加失败。" + "(Error : " + e.toString() + ")");
        }
         
       //TreeMap  不允许(null,null)，允许value为null 会调用比较器
       TreeMap<String,String> treeMap1 = new TreeMap<String,String>();  
       try {
           treeMap1.put("test1", null);
           treeMap1.put("test2", null);
           System.out.println("TreeMap添加value为null成功。");
       } catch (NullPointerException e) {
           System.out.println("TreeMap添加value为null失败。" + "(Error : " + e.toString() + ")");
       }
       TreeMap<String,String> treeMap2 = new TreeMap<String,String>();  
       try {
           treeMap2.put(null, null);
           System.out.println("TreeMap添加（null,null）成功");
       } catch (NullPointerException e) {
           System.out.println("TreeMap添加（null,null）失败。" + "(Error : " + e.toString() + ")");
       }

       //TreeSet  不允许  跟TreeMap都是用红黑树实现的 也会调用比较器
       Set<String> treeSet = new TreeSet<String>(); 
       try {
           treeSet.add(null);   
           System.out.println("TreeSet添加null成功。");  
       }catch (NullPointerException e) {
           System.out.println("TreeSet添加null失败。" + "(Error : " + e.toString() + ")");
       }

         
         
       //HashSet  
       Set<String> hashSet = new HashSet<String>(); 
       try {
           hashSet.add(null);  
           hashSet.add("test");  
           System.out.println( "HashSet添加null成功。" + "(hashSet.size() = " + hashSet.size() + ")" );  
       }catch (NullPointerException e) {
           System.out.println("HashSet添加null失败。" + "(Error : " + e.toString() + ")");
       }
 
    

       //ArrayList  
       List<String> arrayList = new ArrayList<String>(); 
       try {
           arrayList.add(null);  
           arrayList.add("test");  
           System.out.println( "ArrayList添加null成功。" + "(arrayLisht.size() = " + arrayList.size() + ")" );    
       } catch (NullPointerException e) {
            System.out.println("ArrayList添加null失败。" + "(Error : " + e.toString() + ")");
       }

         
       //LinkedList  
       List<String> linkedList = new LinkedList<String>();  
       try {
           arrayList.add(null);  
           arrayList.add("test");  
           System.out.println( "LinkedList添加null成功。" + "(linkedList.size() = " + linkedList.size() + ")" );    
       } catch (NullPointerException e) {
            System.out.println("LinkedList添加null失败。" + "(Error : " + e.toString() + ")");
       }
       
       Deque<String> arrayDeque = new ArrayDeque<String>();
       try {
           arrayDeque.addFirst(null);
           System.out.println( "ArrayDeque添加null成功。" + "(arrayDeque.size() = " + arrayDeque.size() + ")" );    
    } catch (Exception e) {
            System.out.println("ArrayDeque添加null失败。" + "(Error : " + e.toString() + ")");
    }
  }  
}

测试结果如下:

HashMap添加null-null成功。(hashMap.size() = 3)
TreeMap添加value为null成功。
TreeMap添加（null,null）失败。(Error : java.lang.NullPointerException)
TreeSet添加null失败。(Error : java.lang.NullPointerException)
HashSet添加null成功。(hashSet.size() = 2)
ArrayList添加null成功。(arrayLisht.size() = 2)
LinkedList添加null成功。(linkedList.size() = 0)
ArrayDeque添加null失败。(Error : java.lang.NullPointerException)