TreeMap: 能够把它保存的记录根据键(key)排序,默认是按升序排序，也可以指定排序的比较器，当用Iterator 遍历TreeMap时，得到的记录是排过序的。TreeMap不允许key的值为null。非同步的。
TreeMap默认按key进行升序排序，如果想改变默认的顺序，可以使用比较器:
Map<String,String> map = new TreeMap<String,String>(new Comparator<String>(){ public int compare(String obj1,String obj2){ //降序排序 return obj2.compareTo(obj1);
}
});
map.put("month", "The month"); map.put("bread", "The bread"); map.put("attack", "The attack"); Set<String> keySet = map.keySet(); Iterator<String> iter = keySet.iterator(); while(iter.hasNext()){ String key = iter.next(); System.out.println(key+":"+map.get(key)); }
如果要对TreeMap按照value的值进行排序，或者对HashMap,Hashtable,LinkedHashMap进行排序，则可以使用Map.Entry<K,V>接口结合List实现：
eg.1 对TreeMap按照value值升序：
List<Map.Entry<String,String>> mappingList = null; Map<String,String> map = new TreeMap<String,String>(); map.put("aaaa", "month"); map.put("bbbb", "bread"); map.put("ccccc", "attack"); //通过ArrayList构造函数把map.entrySet()转换成list mappingList = new ArrayList<Map.Entry<String,String>>(map.entrySet()); //通过比较器实现比较排序 Collections.sort(mappingList, new Comparator<Map.Entry<String,String>>(){ public int compare(Map.Entry<String,String> mapping1,Map.Entry<String,String> mapping2){ return mapping1.getValue().compareTo(mapping2.getValue()); } }); for(Map.Entry<String,String> mapping:mappingList){ System.out.println(mapping.getKey()+":"+mapping.getValue()); }
eg.2 对HashMap(或Hashtable,LinkedHashMap)按照key的值升序:
List<Map.Entry<String,String>> mappingList = null; Map<String,String> map = new HashMap<String,String>(); map.put("month", "month"); map.put("bread", "bread"); map.put("attack", "attack"); //通过ArrayList构造函数把map.entrySet()转换成list mappingList = new ArrayList<Map.Entry<String,String>>(map.entrySet()); //通过比较器实现比较排序 Collections.sort(mappingList, new Comparator<Map.Entry<String,String>>(){ public int compare(Map.Entry<String,String> mapping1,Map.Entry<String,String> mapping2){ return mapping1.getKey().compareTo(mapping2.getKey()); } }); for(Map.Entry<String,String> mapping:mappingList){ System.out.println(mapping.getKey()+":"+mapping.getValue()); }
原文：http://www.cnblogs.com/xiaoyunxia/p/5965510.html