Accessing the last entry in a Map

Question

How to move a particular HashMap entry to Last position   For Example  I have HashMap values like this   HashMap lt String Integer gt  map   new HashMap lt String Integer gt      map   Not-Specified 1  test 2  testtest 3      Not-Specified  may come in any position  it may come first or in the middle of the map  But i want to move the  Not-Specified  to the last position   How can I do that  thanks in advance

User · Answer

When using numbers as the key  I suppose you could also try this           Map lt Long  String gt  map   new HashMap lt  gt             map put 4L   The First            map put 6L   The Second            map put 11L   The Last             long lastKey   0            you entered Map lt Long  String gt  entry         for  Map Entry lt Long  String gt  entry   map entrySet                  lastKey   entry getKey                      System out println lastKey      11

User · Answer

move does not make sense for a hashmap since its a dictionary with a hashcode for bucketing based on key and then a linked list for colliding hashcodes resolved via equals  Use a TreeMap for sorted maps and then pass in a custom comparator

User · Answer

HashMap doesn t have  the last position   as it is not sorted   You may use other Map which implements java util SortedMap  most popular one is TreeMap

User · Answer

In such scenario last used key is usually known so it can be used for accessing last value  inserted with the one    class PostIndexData       String  office name      Boolean  isGov      public PostIndexData String name  Boolean gov             office name   name           isGov   gov            ----------------------- class KgpData       String  postIndex      PostIndexData  postIndexData      public KgpData String postIndex  PostIndexData postIndexData             postIndex   postIndex           postIndexData   postIndexData            public class Office2ASMPro       private HashMap lt String PostIndexData gt   postIndexMap   new HashMap lt  gt         private HashMap lt String KgpData gt   kgpMap   new HashMap lt  gt         private void addOffice String kgp  String postIndex  String officeName  Boolean gov                if   postIndexMap get postIndex     null                     postIndexMap put postIndex  new PostIndexData officeName  gov                               kgpMap put  kgp  new KgpData postIndex   postIndexMap get postIndex

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Find missing all elements from array         int   array    3 5 7 8 2 1 32 5 7 9 30 5           TreeMap lt Integer  Integer gt  map   new TreeMap lt  gt             for int i 0 i lt array length i                  map put array i   1                     int maxSize   map lastKey            for int j 0 j lt maxSize j                  if null    map get j                   System out println  Missing  enter code here No   j

User · Answer

To answer your question in one sentence   Per default  Maps don t have a last entry  it s not part of their contract     And a side note  it s good practice to code against interfaces  not the implementation classes  see Effective Java by Joshua Bloch  Chapter 8  Item 52  Refer to objects by their interfaces    So your declaration should read   Map lt String Integer gt  map   new HashMap lt String Integer gt        All maps share a common contract  so the client need not know what kind of map it is  unless he specifies a sub interface with an extended contract      Possible Solutions  Sorted Maps   There is a sub interface SortedMap that extends the map interface with order-based lookup methods and it has a sub interface NavigableMap that extends it even further  The standard implementation of this interface  TreeMap  allows you to sort entries either by natural ordering  if they implement the Comparable interface  or by a supplied Comparator   You can access the last entry through the lastEntry method   NavigableMap lt String Integer gt  map   new TreeMap lt String  Integer gt        add some entries Entry lt String  Integer gt  lastEntry   map lastEntry      Linked maps   There is also the special case of LinkedHashMap  a HashMap implementation that stores the order in which keys are inserted  There is however no interface to back up this functionality  nor is there a  direct way to access the last key  You can only do it through tricks such as using a List in between   Map lt String String gt  map   new LinkedHashMap lt String  Integer gt        add some entries List lt Entry lt String Integer gt  gt  entryList       new ArrayList lt Map Entry lt String  Integer gt  gt  map entrySet     Entry lt String  Integer gt  lastEntry       entryList get entryList size  -1     Proper Solution   Since you don t control the insertion order  you should go with the NavigableMap interface  i e  you would write a comparator that positions the Not-Specified entry last   Here is an example   final NavigableMap lt String Integer gt  map            new TreeMap lt String  Integer gt  new Comparator lt String gt          public int compare final String o1  final String o2            int result          if  Not-Specified  equals o1                 result 1            else if  Not-Specified  equals o2                 result -1            else               result  o1 compareTo o2                     return result             map put  test   Integer valueOf 2    map put  Not-Specified   Integer valueOf 1    map put  testtest   Integer valueOf 3    final Entry lt String  Integer gt  lastEntry   map lastEntry    System out println  Last key    lastEntry getKey                 last value    lastEntry getValue       Output      Last key  Not-Specified  last value  1   Solution using HashMap   If you must rely on HashMaps  there is still a solution  using a  a modified version of the above comparator  b  a List initialized with the Map s entrySet and c  the Collections sort   helper method       final Map lt String  Integer gt  map   new HashMap lt String  Integer gt         map put  test   Integer valueOf 2        map put  Not-Specified   Integer valueOf 1        map put  testtest   Integer valueOf 3         final List lt Entry lt String  Integer gt  gt  entries           new ArrayList lt Entry lt String  Integer gt  gt  map entrySet         Collections sort entries  new Comparator lt Entry lt String  Integer gt  gt              public int compareKeys final String o1  final String o2               int result              if  Not-Specified  equals o1                    result   1                else if  Not-Specified  equals o2                    result   -1                else                  result   o1 compareTo o2                             return result                      Override         public int compare final Entry lt String  Integer gt  o1              final Entry lt String  Integer gt  o2               return this compareKeys o1 getKey    o2 getKey                             final Entry lt String  Integer gt  lastEntry           entries get entries size   - 1       System out println  Last key      lastEntry getKey        last value              lastEntry getValue          Output      Last key  Not-Specified  last value  1

User · Answer

A SortedMap is the logical best choice  however another option is to use a LinkedHashMap which maintains two order modes  most-recently-added goes last  and most-recently-accessed goes last  See the Javadocs for more details

[java] Accessing the last entry in a Map

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