When learning the usage of map collection in java, I found serveral beneficial methods that was encountered in the daily life. Now  I made a summary: 

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;

public class MapUtil
{

    private static final Map<String, String> contents = new HashMap<String, String>();
    @SuppressWarnings("unchecked")
    public static void initMap()
    {
        Map testMap = new HashMap<String, String>();
        testMap.put("Albert", "Shao");
        contents.putAll(testMap);
    }

    /**
     * Four methods to list map.
     * output:
     * Albert:Shao
       Albert:Shao
       Shao
       Albert:Shao
       
     * @time Jul 18, 2014 11:39:46 AM
     * @return void
     */
    public static void listMap()
    {
        Map<String, String> testMap = new HashMap<String, String>();
        testMap.put("Albert", "Shao");
        for (Map.Entry<String, String> entry : testMap.entrySet())
        {
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }

        for (String key : testMap.keySet())
        {
            System.out.println(key + ":" + testMap.get(key));
        }

        for (String value : testMap.values())
        {
            System.out.println(value);
        }

        Iterator<Map.Entry<String, String>> keyIt = testMap.entrySet()
                .iterator();
        while (keyIt.hasNext())
        {
            Map.Entry<String, String> entry = keyIt.next();
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }
    }

    /**
     * Use the treeMap order by key asc.
     * Watch out: if key is repeated, the latter element will replace the former.
     * output: {Apple=five, Banana=three, Grape=one, Pair=four} 
     * 
     * @time Jul 18, 2014 11:37:51 AM
     * @return void
     */
    public static void sort()
    {
        SortedMap<String, String> sortMap = new TreeMap<String, String>();
        sortMap.put("Pair", "four");
        sortMap.put("Apple", "two");
        sortMap.put("Grape", "one");
        sortMap.put("Banana", "three");
        sortMap.put("Apple", "five");
        System.out.println(sortMap);
    }

    /**
     * Sort the Map by map.value, then set the result to map.
     * output : [Apple=1, Pair=2, Banana=3, Grape=4]
     * 
     * @time Jul 18, 2014 11:36:28 AM
     * @return void
     */
    public static void sortByValue()
    {
        Map<String, Integer> testMap = new HashMap<String, Integer>();
        testMap.put("Pair", 2);
        testMap.put("Apple", 1);
        testMap.put("Grape", 4);
        testMap.put("Banana", 3);

        List<Map.Entry<String, Integer>> entryList = new ArrayList<Map.Entry<String, Integer>>(
                testMap.entrySet());
        Collections.sort(entryList,
                new Comparator<Map.Entry<String, Integer>>()
                {
                    public int compare(Map.Entry<String, Integer> c1,
                            Map.Entry<String, Integer> c2)
                    {
                        return (c1.getValue() - c2.getValue());
                    }
                });

        System.out.println(entryList);
    }
     
    /**
     *Sort map by value when value is object. 
     * use compareTo method to replace simple '-'
     * output:[Apple=AB, Grape=AF, Pair=BB, Banana=XY]
     * 
     * @time Jul 18, 2014 11:48:35 AM
     * @return void
     */
    public static void sortByObject()
    {
        Map<String, String> testMap = new HashMap<String, String>();
        testMap.put("Pair", "BB");
        testMap.put("Apple", "AB");
        testMap.put("Grape", "AF");
        testMap.put("Banana", "XY");

        List<Map.Entry<String, String>> entryList = new ArrayList<Map.Entry<String, String>>(
                testMap.entrySet());
        Collections.sort(entryList,
                new Comparator<Map.Entry<String, String>>()
                {
                    public int compare(Map.Entry<String, String> c1,
                            Map.Entry<String, String> c2)
                    {
                        return (c1.getValue().compareTo(c2.getValue()));
                    }
                });

        System.out.println(entryList);
    }
    
    public static void main(String[] args)
    {
        MapUtil.listMap();
        MapUtil.sort();
        MapUtil.sortByValue();
        MapUtil.sortByObject();
    }
}

http://blog.csdn.net/sxb0841901116/article/details/20635267