Java map sorted by value

Map is a set interface of key-value pairs, and its implementation classes mainly include: Hashmap,treemap,hashtable and Linkedhashmap.

TREEMAP: Based on the navigablemap implementation of the red-black tree (red-black trees), the map is sorted according to the natural order of its keys or sorted based on the Comparator provided when the mapping was created, depending on the construction method used.
The value of HashMap is not sequential, it is according to the key hashcode to achieve, for this unordered hashmap how do we want to implement the sort? Sorts the value of reference TreeMap.


Map.entry returns the collections view.

Sort by key

TreeMap is ascending by default, and if we need to change the sorting method, we need to use the comparator: Comparator. Comparator a comparer interface that can sort a collection object or array, implementing the public compare (T O1,to2) method of the interface to achieve sorting.

Note: The following code has been passed in the Jdk1.6 test

TreeMap by key in ascending order by default

Public static void keyupsort ()  {        //  By default, TreeMap is sorted by key in ascending order         map<string, integer> map  = new TreeMap<String, Integer> ();         Map.put ("ACB1",  5);         map.put ("Bac1",  3);         map.put ("BCA1",  20);         Map.put ("Cab1",  80);         map.put ("Cba1",  1);         map.put ("ABC1",  10);         Map.put ("ABC2",  12);        //  by default, TreeMap sort keys in ascending order          system.out.println ("------------normal situation, TreeMap sorted by key in ascending order--------------------");         for  (map.entry<string, integer> entry :  Map.entryset ())  {             System.out.println (Entry.getkey ()  +  ":"  + entry.getvalue ());         }    }

Modify the Sort method for TreeMap, sort by key descending

Public static void keydownsort ()  {        //  TreeMap, sort by key         //  descending sort comparer          Comparator<String> keyComparator = new Comparator<String> ()  {             @Override              public int compare (String o1, String &NBSP;O2)  {                 // TODO Auto-generated method stub                 return o2.compareto (O1);             }        };         Map<String, Integer> map = new TreeMap<String, Integer> ( Keycomparator);         map.put ("ACB1",  5);         map.put ("Bac1",  3);         map.put ( "BCA1",  20);         map.put ("Cab1",  80);         map.put ("Cba1",  1);         map.put ( "ABC1",  10);         map.put ("ABC2",  12);         system.out.println ("------------treemap sorted by key in descending order--------------------");         for  (map.entry<string, integer> entry :  map.entryset ())  {             System.out.println (Entry.getkey ()  +&nBSP; ":"  + entry.getvalue ());        }     }

Sort by value



modifies the sort method of TreeMap, sorted by value ascending

Note: Normally, map cannot be sorted using the Collections.sort () method, but you can convert the map to list before sorting.

Public static void valueupsort ()  {        //  By default, TreeMap is sorted by key in ascending order         map<string, integer> map  = new TreeMap<String, Integer> ();         Map.put ("ACB1",  5);         map.put ("Bac1",  3);         map.put ("BCA1",  20);         Map.put ("Cab1",  80);         map.put ("Cba1",  1);         map.put ("ABC1",  10);         Map.put ("ABC2",  12);        //  Ascending Comparator          comparator<map.entry<string, integer>> valuecomparator =  new comparaTor<map.entry<string,integer>> ()  {              @Override             public  int compare (entry<string, integer> o1,                     entry<string, integer &GT;&NBSP;O2)  {                 // TODO Auto-generated method stub                 return o1.getvalue ()-o2.getvalue ();             }        };         // map Convert to list sort          list<map.entry<string,&Nbsp;integer>> list = new arraylist<map.entry<string,integer>> ( Map.entryset ());        //  sort          collections.sort (List,valuecomparator);        //  By default, TreeMap sorts keys in ascending order         system.out.println ("------------ Map is sorted in ascending order of Value--------------------");        for  (map.entry< String, integer> entry : list)  {             system.out.println (Entry.getkey ()  +  ":"  + entry.getvalue ());         }    }

Test results

------------Normal, TreeMap is sorted by key in ascending order--------------------abc1:10abc2:12acb1:5bac1:3bca1:20cab1:80cba1:1------------ TreeMap sort by key in descending order--------------------cba1:1cab1:80bca1:20bac1:3acb1:5abc2:12abc1:10------------ The map is sorted in ascending order of value--------------------cba1:1bac1:3acb1:5abc1:10abc2:12bca1:20cab1:80

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Java map sorted by value