java中key值可以重复的map:IdentityHashMap

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(new String("xx"), "first");
        map.put(new String("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 = new String("xx");
        map.put(fsString, "first");
        map.put(new String("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 = new String("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 = new String("xx");
        String secString = new String("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

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。

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