--- a
+++ b/util/CollectionsEx.java
@@ -0,0 +1,95 @@
+package util;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Provides additional methods for collection operation
+ * @author zhengc
+ *
+ */
+public class CollectionsEx {
+	@SuppressWarnings("hiding")
+	/**
+	 * Returns sorted HashMap by descend Value
+	 * @param map the original HashMap 
+	 * @return sorted HashMap by descend Value
+	 */
+	public static <String, Double extends Comparable<? super Double>> Map<String, Double> sortByValueR( Map<String, Double> map ) {
+		/* sort map by value reversed*/
+		List<Map.Entry<String, Double>> list = new LinkedList<>(map.entrySet());
+		Collections.sort(list, Collections.reverseOrder(new Comparator<Map.Entry<String, Double>>() {
+			
+			public int compare( Map.Entry<String, Double> o1, Map.Entry<String, Double> o2 ) {
+				return ( o1.getValue() ).compareTo( o2.getValue() );
+			}
+		}) );
+		
+
+		Map<String, Double> result = new LinkedHashMap<>();
+		for (Map.Entry<String, Double> entry : list){
+			result.put( entry.getKey(), entry.getValue() );
+		}
+		return result;
+	}
+	
+	@SuppressWarnings("hiding")
+	/**
+	 * Returns sorted HashMap by Value
+	 * @param map the original HashMap 
+	 * @return sorted HashMap by Value
+	 */
+	public static <String, Double extends Comparable<? super Double>> Map<String, Double> sortByValue( Map<String, Double> map ) {
+		/* sort map by value*/
+		List<Map.Entry<String, Double>> list = new LinkedList<>(map.entrySet());
+		Collections.sort(list, new Comparator<Map.Entry<String, Double>>() {
+			
+			public int compare( Map.Entry<String, Double> o1, Map.Entry<String, Double> o2 ) {
+				return ( o1.getValue() ).compareTo( o2.getValue() );
+			}
+		} );
+		
+
+		Map<String, Double> result = new LinkedHashMap<>();
+		for (Map.Entry<String, Double> entry : list){
+			result.put( entry.getKey(), entry.getValue() );
+		}
+		return result;
+	}
+	
+//	public static Map<String, String> sortByKey(Map<String, String> map ) {
+//		/* sort map by key*/
+//		Map<String, String> sortedmap = new HashMap<String, String>();
+//		Set<String> keys = map.keySet();
+//		List<String> key_list = setToList(keys);
+//		Collections.sort(key_list);
+//		
+//		for(String key : key_list) {
+//			System.out.println(key);
+//			sortedmap.put(key, map.get(key));
+//		}
+//		return sortedmap;
+//	}
+	
+	/**
+	 * Creates a list from a set
+	 * @param set a set
+	 * @return a list
+	 */
+	public static List<String> setToList(Set<String> set) {
+		List<String> list = new ArrayList<String>();
+		for (String s: set) {
+			list.add(s);
+		}
+		Collections.sort(list);
+		return list;
+	}
+
+}