HashMap中如何按键排序(sort by key)或按值排序(sort by value)呢?

我们只需定义比较器
    即可实现 
	   按键排序(sort by key)
       按值排序(sort by value)
	 的操作

1、按键排序

public class MapSortDemo {
    public static void main(String[] args) {
        Map<String, String> map = new TreeMap<String, String>();
 
        map.put("KFC", "kfc");
        map.put("WNBA", "wnba");
        map.put("NBA", "nba");
        map.put("CBA", "cba");
 
        Map<String, String> resultMap = sortMapByKey(map);  //按Key进行排序
 
        for (Map.Entry<String, String> entry : resultMap.entrySet()) {
            System.out.println(entry.getKey() + " " + entry.getValue());
        }
    }
 
    /**
     * 使用 Map按key进行排序
     * @param map
     * @return
     */
    public static Map<String, String> sortMapByKey(Map<String, String> map) {
        if (map == null || map.isEmpty()) {
            return null;
        }
        Map<String, String> sortMap = new TreeMap<String, String>(
                new MapKeyComparator());
        sortMap.putAll(map);
        return sortMap;
    }
}

比较器类:
class MapKeyComparator implements Comparator<String>{
 
    @Override
    public int compare(String str1, String str2) {
 
        return str1.compareTo(str2);
    }
}

2、按值排序

public class MapSortDemo {
 
    public static void main(String[] args) {
 
        Map<String, String> map = new TreeMap<String, String>();
 
        map.put("KFC", "kfc");
        map.put("WNBA", "wnba");
        map.put("NBA", "nba");
        map.put("CBA", "cba");
 
        Map<String, String> resultMap = sortMapByKey(map);  //按Key进行排序
//      Map<String, String> resultMap = sortMapByValue(map); //按Value进行排序
 
        for (Map.Entry<String, String> entry : resultMap.entrySet()) {
            System.out.println(entry.getKey() + " " + entry.getValue());
        }
    }
 
    /**
     * 使用 Map按value进行排序
     * @param map
     * @return
     */
    public static Map<String, String> sortMapByValue(Map<String, String> oriMap) {
        if (oriMap == null || oriMap.isEmpty()) {
            return null;
        }
        Map<String, String> sortedMap = new LinkedHashMap<String, String>();
        list<Map.Entry<String, String>> entryList =
		    new ArrayList<Map.Entry<String, String>>(
                oriMap.entrySet());
        Collections.sort(entryList, new MapValueComparator());
 
        Iterator<Map.Entry<String, String>> iter = entryList.iterator();
        Map.Entry<String, String> tmpEntry = null;
        while (iter.hasNext()) {
            tmpEntry = iter.next();
            sortedMap.put(tmpEntry.getKey(), tmpEntry.getValue());
        }
        return sortedMap;
    }
}

比较器类

class MapValueComparator implements Comparator<Map.Entry<String, String>> {
 
    @Override
    public int compare(Entry<String, String> me1, Entry<String, String> me2) {
 
        return me1.getValue().compareTo(me2.getValue());
    }
}

//或有一个可以优化的写法
//可以将比较器类
//用匿名内部类的方式来写,而不用专门创建一个类
//例
// 按值排序中的Collections.sort()方法可以修改如下：

Collections.sort(entryList, new Comparator<Map.Entry<String, String>>(){
		@Override
		public int compare(Entry<String, String> o1, Entry<String, String> o2) {
			return o1.getValue().compareTo(o2.getValue());
		};
	});