Java中,有一种key值可以重复的map,就是IdentityHashMap。在IdentityHashMap中,判断两个键值k1和 k2相等的条件是 k1 == k2 。在正常的Map 实现(如 HashMap)中,当且仅当满足下列条件时才认为两个键 k1 和 k2 相等:(k1==null ? k2==null : e1.equals(e2))。

  IdentityHashMap类利用哈希表实现 Map 接口,比较键(和值)时使用引用相等性代替对象相等性。该类不是 通用 Map 实现!此类实现 Map 接口时,它有意违反 Map 的常规协定,该协定在比较对象时强制使用 equals 方法。此类设计仅用于其中需要引用相等性语义的罕见情况。

  具体说明,详见:http://download.oracle.com/javase/6/docs/api/java/util/IdentityHashMap.html

                          http://www.cjsdn.net/Doc/JDK50/java/util/IdentityHashMap.html

  在使用IdentityHashMap有些需要注意的地方:

  例子1:

IdentityHashMap<String,Object> map =new IdentityHashMap<String,Object>();  
map.put(newString("xx"),"first");  
map.put(newString("xx"),"second");  
for (Entry<String, Object> entry : map.entrySet()) {  
    System.out.print(entry.getKey() +"    ");  
    System.out.println(entry.getValue());  
}  
System.out.println("idenMap="+map.containsKey("xx"));  
System.out.println("idenMap="+map.get("xx")); 

  输出结果是:

xx    first  
xx    second  
idenMap=false  
idenMap=null 

  例子2:

IdentityHashMap<String,Object> map =new IdentityHashMap<String,Object>();  
String fsString =newString("xx");  
map.put(fsString,"first");  
map.put(newString("xx"),"second");  
for(Entry<String, Object> entry : map.entrySet()) {  
     System.out.print(entry.getKey() +"    ");  
     System.out.println(entry.getValue());  
}  
System.out.println("idenMap="+map.containsKey(fsString));  
System.out.println("idenMap="+map.get(fsString));

  输出结果是:

xx    second  
xx    first  
idenMap=true  
idenMap=first

  例子3:

IdentityHashMap<String,Object> map =new IdentityHashMap<String,Object>();  
String fsString =newString("xx");  
map.put(fsString,"first");  
map.put(fsString,"second");  
for(Entry<String, Object> entry : map.entrySet()) {  
      System.out.print(entry.getKey() +"    ");  
      System.out.println(entry.getValue());  
}  
System.out.println("idenMap="+map.containsKey(fsString));  
System.out.println("idenMap="+map.get(fsString));

  输出结果是:

xx    second  
idenMap=true  
idenMap=second

  例子4:

IdentityHashMap<String,Object> map =new IdentityHashMap<String,Object>();  
String fsString =newString("xx");  
String secString =newString("xx");  
map.put(fsString,"first");  
map.put(secString,"second");  
for(Entry<String, Object> entry : map.entrySet()) {  
    System.out.print(entry.getKey() +"    ");  
    System.out.println(entry.getValue());  
}  
System.out.println("idenMap="+map.containsKey(fsString));  
System.out.println("idenMap="+map.get(fsString));  
      
System.out.println("idenMap="+map.containsKey(secString));  
System.out.println("idenMap="+map.get(secString));  

  输出结果是:

xx    first  
xx    second  
idenMap=true  
idenMap=first  
idenMap=true  
idenMap=second 

  例子5:

IdentityHashMap<String,Object> map =new IdentityHashMap<String,Object>();  
map.put("xx","first");  
map.put("xx","second");  
for(Entry<String, Object> entry : map.entrySet()) {  
    System.out.print(entry.getKey() +"    ");  
    System.out.println(entry.getValue());  
}  

  输出结果是:

xx    second

  可以看到,在IdentityHashMap中,是判断key是否为同一个对象,而不是普通HashMap的equals方式判断。

 

参考:http://blog.csdn.net/stoneok07/article/details/7262676