Map sort (sort by key, sort by value)

There are two main types, key sorting, sorting by value. Also, sorting by key is used primarily for treemap, while sorting by value is appropriate for the subclass of map.

first, the key sort

Sort by key is mainly used for treemap, can be implemented according to the size of the key value, when the object is inserted directly into the appropriate position, to maintain the order of the map.

Look at the constructor of TreeMap: TreeMap(Comparator<? super K> comparator)： constructs a new, empty tree map that is sorted by the given comparer.

The comparator here is the comparator for key. So the two parameters that are used for comparison when defining a comparer are objects of the data type of key.

The instance code is as follows:

 Public classMapsorttest { Public Static voidMain (string[] args) { Map <String,String> stu=new treemap<> (New Mycomparator ());//Pass in a key comparator object to construct the TreeMapStu.put ("Apple", "55"); Stu.put ("Boy", "32"); Stu.put ("Cat", "22"); Stu.put ("Dog", "12"); Stu.put ("Egg", "11"); //Map Traversal: Extract the key out of the set, then use the iterator to traverse the keyset, and use Map.get (key) to get the value corresponding to key. Set<String> keyset=Stu. KeySet　　(); Iterator it=Keyset.iterator ();  while(It.hasnext ()) {String Next=(String) it.next (); System.out.println (Next+","+Stu.get (next)); }}}//the comparator that defines the key, the comparison algorithm according to the first parameter O1, less than, equal to or greater than O2 respectively return a negative integer, 0 or a positive integer, to determine the position of the two deposit: return negative number is O1 before, positive number is O2 in front. class Mycomparator Implements Comparator<String>{ Public int Compare(String O1, String O2) {returnO1.compareto (O2); }}

second, sort by value

Sorting by value only uses treemap, sorting by value because of the uniformity of the type used by its methods, so it can be used for all subclasses of map .

The main points of knowledge used are;

1:map. EntrySet() takes each key-value pair out of the map and encapsulates it into a entry object and puts it into a set.

2: Generic map.entry<type1,type2> because Key-value is a Entry object, this indicates the data type of the two members in the Entry object.

3:collections.sort (list<t> List, comparator<? super t> C) The sorting method of the collection class, sorted by a custom comparer. The list here holds objects that are entry objects. Defines the comparer to compare the Value property in the Entry object .

The instance code is as follows:

 Public classMapsorttest { Public Static voidMain (string[] args) { Map <String,String> stu=new treemap<> ();//Storage with TreeMap//map<string,string> stu=new hashmap<> ();//Storage with HashMapStu.put ("Apple", "55"); Stu.put ("Boy", "32"); Stu.put ("Cat", "22"); Stu.put ("Dog", "12"); Stu.put ("Egg", "11"); //1: Convert map to EntrySet and convert to list to save entry object.  　　　　 list<map.entry<string,string>> entrys=new arraylist<>(Stu.entryset ());　　　　//2: Call Collections.sort (List,comparator) method to sort Entry-list 　　　　 Collections.sort (Entrys, Newmycomparator ());　　　　//3: Traverse the ordered entry-list, can get the result of output sequentially 　　　　  for(map.entry<string,string>Entry:entrys) {System.out.println (Entry.getkey ()+","+Entry.getvalue ()); }}}//the comparer for the custom entry object. Each entry object can be used by Getkey (), GetValue () to obtain a key or value for comparison. In other words: we can also sort by key by entry object implementation.  　　 class Mycomparator ImplementsComparator<map.entry>{ Public intCompare (Map.entry O1, Map.entry O2) {return(String) O1.getvalue ()). CompareTo (String) o2.getvalue ()); }}

Map sort (sort by key, sort by value)