Java TreeMap Comparator

Question

I need a comparator for a TreeMap  Should I write this anonymously in the constructor for my TreeMap  How else could I write my comparator  Currently  Java does not like my code  can I do this anonymously     SortedMap lt String  Double gt  myMap        new TreeMap lt String  Double gt  new Comparator lt Entry lt String  Double gt  gt                  public int compare Entry lt String  Double gt  o1  Entry lt String  Double gt  o2                        return o1 getValue   compareTo o2 getValue                           Can I do the above anonymously  How else could I do this  I want to sort myMap by the Value not the Key

User · Answer

you can swipe the key and the value  For example          String   k     Elena    Thomas    Hamilton    Suzie    Phil            int   v    341  273  278  329  445           TreeMap lt Integer String gt a new TreeMap            for  int i   0  i  lt  k length  i                a put v i  k i                        System out println a firstEntry   getValue     t  a firstEntry   getKey             a remove a firstEntry   getKey             System out println a firstEntry   getValue     t  a firstEntry   getKey

User · Answer

You can not sort TreeMap on values      A Red-Black tree based NavigableMap implementation  The map is sorted according to the natural ordering of its keys  or by a Comparator provided at map creation time  depending on which constructor is used   You will need to provide comparator for Comparator lt   super K gt  so your comparator should compare on keys    To provide sort on values you will need SortedSet  Use   SortedSet lt Map Entry lt String  Double gt  gt  sortedset   new TreeSet lt Map Entry lt String  Double gt  gt               new Comparator lt Map Entry lt String  Double gt  gt                       Override                 public int compare Map Entry lt String  Double gt  e1                          Map Entry lt String  Double gt  e2                        return e1 getValue   compareTo e2 getValue                                          sortedset addAll myMap entrySet       To give you an example      SortedMap lt String  Double gt  myMap   new TreeMap lt String  Double gt         myMap put  a   10 0       myMap put  b   9 0       myMap put  c   11 0       myMap put  d   2 0       sortedset addAll myMap entrySet         System out println sortedset     Output      d 2 0  b 9 0  a 10 0  c 11 0

User · Answer

The comparator should be only for the key  not for the whole entry  It sorts the entries based on the keys   You should change it to something as follows  SortedMap lt String  Double gt  myMap        new TreeMap lt String  Double gt  new Comparator lt String gt                  public int compare String o1  String o2                        return o1 compareTo o2                    Update  You can do something as follows  create a list of entries in the map and sort the list base on value  but note this not going to sort the map itself  -  List lt Map Entry lt String  Double gt  gt  entryList   new ArrayList lt Map Entry lt String  Double gt  gt  myMap entrySet         Collections sort entryList  new Comparator lt Map Entry lt String  Double gt  gt               Override         public int compare Entry lt String  Double gt  o1  Entry lt String  Double gt  o2                return o1 getValue   compareTo o2 getValue

[java] Java TreeMap Comparator

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