关于TreeMap、HashMap、HashTable的键是否能为""和Null的问题

关于TreeMap、HashMap、HashTable的键是否能为""和Null的问题

实例代码：
Map<Object,String> treeMap_1 = new TreeMap<Object,String>();

treeMap_1.put(null, "treeMap_1成功了！");

Map<String,Object> hashMap_1 = new HashMap<String,Object>();

hashMap_1.put(null, new String("hashMap_1,ok......"));

System.out.println(treeMap_1.get(null));//运行时产生java.lang.NullPointerException异常
System.out.println(hashMap_1.get(null));//打印hashMap_1,ok......

或者：

Map<Object,String> treeMap_2 = new TreeMap<Object,String>();

treeMap_2.put(null, "treeMap_2成功了！");

Map<Object,String> hashMap_2 = new HashMap<Object,String>();

hashMap_2.put(null, "hashMap_2,ok......");

System.out.println(treeMap_2.get(null));//运行时产生java.lang.NullPointerException异常

System.out.println(hashMap_2.get(null));//打印hashMap_2,ok......

TreeMap键可以为""，HashMap键可以为""。

实例代码：
Map<Object,String> treeMap_1 = new TreeMap<Object,String>();

treeMap_1.put(new String(""), "treeMap_1成功了！");

Map<String,Object> hashMap_1 = new HashMap<String,Object>();

hashMap_1.put("", new String("hashMap_1,ok......"));

System.out.println(treeMap_1.get(""));//打印treeMap_1,ok......
System.out.println(hashMap_1.get(""));//打印hashMap_1,ok......

或者：

Map<Object,String> treeMap_2 = new TreeMap<Object,String>();

treeMap_2.put(new String(""), "treeMap_2成功了！");

Map<Object,String> hashMap_2 = new HashMap<Object,String>();

hashMap_2.put(new String(""), "hashMap_2,ok......");

System.out.println(treeMap_2.get(""));//打印treeMap_2,ok......
System.out.println(hashMap_2.get(""));//打印hashMap_2,ok......

HashTable与TreeMap相同，键不能为null，可以为""。
Map<Object,Object> table_1 = new Hashtable<Object,Object>();

table_1.put(new String(""), new String("table_1"));

System.out.println(table_1.get(""));//打印table_1

Map<String,Object> table_2 = new Hashtable<String,Object>();

table_2.put("", new String("table_2"));

System.out.println(table_2.get(""));//打印table_2

-------------------------------------------------------------
Map<Object,Object> table_1 = new Hashtable<Object,Object>();

table_1.put(null, new String("table_1"));
System.out.println(table_1.get(null));//运行时产生java.lang.NullPointerException异常

Map<String,Object> table_2 = new Hashtable<String,Object>();

table_2.put(null, new String("table_2"));

System.out.println(table_2.get(null));//运行时产生java.lang.NullPointerException异常

转http://blog.163.com/eugeneheen_chen@126/blog/static/120812157200981611345429/