在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