Java LinkedHashMap get first or last entry

Question

I have used LinkedHashMap because it is important the order in which keys entered in the map   But now I want to get the value of key in the first place  the first entered entry  or the last   Should there be a method like first   and last   or something like that   Do I need to have an iterator to just get the first key entry  That is why I used LinkedHashMap    Thanks

User · Answer

Though linkedHashMap doesn t provide any method to get first  last or any specific object  But its pretty trivial to get   Map lt Integer String gt  orderMap   new LinkedHashMap lt Integer String gt       Set lt Integer gt  al     orderMap keySet     now using iterator on al object   you can get any object

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The semantics of LinkedHashMap are still those of a Map  rather than that of a LinkedList  It retains insertion order  yes  but that s an implementation detail  rather than an aspect of its interface   The quickest way to get the  first  entry is still entrySet   iterator   next    Getting the  last  entry is possible  but will entail iterating over the whole entry set by calling  next   until you reach the last   while  iterator hasNext      lastElement   iterator next      edit  However  if you re willing to go beyond the JavaSE API  Apache Commons Collections has its own LinkedMap implementation  which has methods like firstKey and lastKey  which do what you re looking for  The interface is considerably richer

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Suggestion   map remove map keySet   iterator   next

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One more way to get first and last entry of a LinkedHashMap is to use toArray   method of Set interface  But I think iterating over the entries in the entry set and getting the first and last entry is a better approach  The usage of array methods leads to warning of the form  quot     needs unchecked conversion to conform to     quot   which cannot be fixed  but can be only be suppressed by using the annotation  SuppressWarnings  quot unchecked quot     Here is a small example to demonstrate the usage of toArray   method      public static void main final String   args            final Map lt Integer String gt  orderMap   new LinkedHashMap lt Integer String gt             orderMap put 6   quot Six quot            orderMap put 7   quot Seven quot            orderMap put 3   quot Three quot            orderMap put 100   quot Hundered quot            orderMap put 10   quot Ten quot             final Set lt Entry lt Integer  String gt  gt  mapValues   orderMap entrySet            final int maplength   mapValues size            final Entry lt Integer String gt    test   new Entry maplength           mapValues toArray test            System out print  quot First Key  quot  test 0  getKey             System out println  quot  First Value  quot  test 0  getValue              System out print  quot Last Key  quot  test maplength-1  getKey             System out println  quot  Last Value  quot  test maplength-1  getValue                   the output geneated is       First Key 6 First Value Six     Last Key 10 Last Value Ten

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I know that I came too late but I would like to offer some alternatives  not something extraordinary but some cases that none mentioned here  In case that someone doesn t care so much for efficiency but he wants something with more simplicity perhaps find the last entry value with one line of code   all this will get quite simplified with the arrival of   Java 8   I provide some useful scenarios    For the sake of the completeness  I compare these alternatives with the solution of arrays that already mentioned in this post by others users  I sum up all the cases and i think they would be useful when performance does matter or no  especially for new developers  always depends on the matter of each problem  Possible Alternatives  Usage of Array Method  I took it from the previous answer to to make the follow comparisons  This solution belongs  feresr     public static String FindLasstEntryWithArrayMethod             return String valueOf linkedmap entrySet   toArray   linkedmap size   - 1            Usage of ArrayList Method  Similar to the first solution with a little bit different performance  public static String FindLasstEntryWithArrayListMethod             List lt Entry lt Integer  String gt  gt  entryList   new ArrayList lt Map Entry lt Integer  String gt  gt  linkedmap entrySet             return entryList get entryList size   - 1  getValue            Reduce Method  This method will reduce the set of elements until getting the last element of stream  In addition  it will  return only deterministic results   public static String FindLasstEntryWithReduceMethod             return linkedmap entrySet   stream   reduce  first  second  - gt  second  orElse null  getValue            SkipFunction Method  This method will get the last element of the stream  by simply skipping all the elements before it  public static String FindLasstEntryWithSkipFunctionMethod             final long count   linkedmap entrySet   stream   count            return linkedmap entrySet   stream   skip count - 1  findFirst   get   getValue            Iterable Alternative     Iterables getLast from Google Guava  It has some optimization for   Lists and SortedSets too     public static String FindLasstEntryWithGuavaIterable             return Iterables getLast linkedmap entrySet    getValue            Here is the full source code   import com google common collect Iterables  import java math BigDecimal  import java math RoundingMode  import java util ArrayList  import java util LinkedHashMap  import java util List  import java util Map  import java util Map Entry   public class PerformanceTest        private static long startTime      private static long endTime      private static LinkedHashMap lt Integer  String gt  linkedmap       public static void main String   args            linkedmap   new LinkedHashMap lt Integer  String gt              linkedmap put 12   Chaitanya            linkedmap put 2   Rahul            linkedmap put 7   Singh            linkedmap put 49   Ajeet            linkedmap put 76   Anuj               call a useless action  so that the caching occurs before the jobs starts          linkedmap entrySet   forEach x - gt                  startTime   System nanoTime            FindLasstEntryWithArrayListMethod            endTime   System nanoTime            System out println  FindLasstEntryWithArrayListMethod        took     new BigDecimal  endTime - startTime    1000000 000  setScale 3  RoundingMode CEILING      milliseconds               startTime   System nanoTime            FindLasstEntryWithArrayMethod            endTime   System nanoTime            System out println  FindLasstEntryWithArrayMethod        took     new BigDecimal  endTime - startTime    1000000 000  setScale 3  RoundingMode CEILING      milliseconds             startTime   System nanoTime            FindLasstEntryWithReduceMethod            endTime   System nanoTime             System out println  FindLasstEntryWithReduceMethod        took     new BigDecimal  endTime - startTime    1000000 000  setScale 3  RoundingMode CEILING      milliseconds             startTime   System nanoTime            FindLasstEntryWithSkipFunctionMethod            endTime   System nanoTime             System out println  FindLasstEntryWithSkipFunctionMethod        took     new BigDecimal  endTime - startTime    1000000 000  setScale 3  RoundingMode CEILING      milliseconds             startTime   System currentTimeMillis            FindLasstEntryWithGuavaIterable            endTime   System currentTimeMillis            System out println  FindLasstEntryWithGuavaIterable        took      endTime - startTime      milliseconds                 public static String FindLasstEntryWithReduceMethod             return linkedmap entrySet   stream   reduce  first  second  - gt  second  orElse null  getValue               public static String FindLasstEntryWithSkipFunctionMethod             final long count   linkedmap entrySet   stream   count            return linkedmap entrySet   stream   skip count - 1  findFirst   get   getValue               public static String FindLasstEntryWithGuavaIterable             return Iterables getLast linkedmap entrySet    getValue               public static String FindLasstEntryWithArrayListMethod             List lt Entry lt Integer  String gt  gt  entryList   new ArrayList lt Map Entry lt Integer  String gt  gt  linkedmap entrySet             return entryList get entryList size   - 1  getValue               public static String FindLasstEntryWithArrayMethod             return String valueOf linkedmap entrySet   toArray   linkedmap size   - 1              Here is the output with performance of each method   FindLasstEntryWithArrayListMethod   took 0 162 milliseconds FindLasstEntryWithArrayMethod   took 0 025 milliseconds FindLasstEntryWithReduceMethod   took 2 776 milliseconds FindLasstEntryWithSkipFunctionMethod   took 3 396 milliseconds FindLasstEntryWithGuavaIterable   took 11 milliseconds

User · Answer

Perhaps something like this    LinkedHashMap lt Integer  String gt  myMap   public String getFirstKey       String out   null    for  int key   myMap keySet          out   myMap get key       break        return out     public String getLastKey       String out   null    for  int key   myMap keySet          out   myMap get key         return out

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public static List lt Fragment gt  pullToBackStack         List lt Fragment gt  fragments   new ArrayList lt  gt         List lt Map Entry lt String  Fragment gt  gt  entryList   new ArrayList lt  gt  backMap entrySet         int size   entryList size        if  size  gt  0            for  int i   size - 1  i  gt   0  i--      last Fragments             fragments add entryList get i  getValue                 backMap remove entryList get i  getKey                       return fragments            return null

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It s a bit dirty  but you can override the removeEldestEntry method of LinkedHashMap  which it might suit you to do as a private anonymous member   private Splat eldest   null  private LinkedHashMap lt Integer  Splat gt  pastFutures   new LinkedHashMap lt Integer  Splat gt            Override     protected boolean removeEldestEntry Map Entry lt Integer  Splat gt  eldest             eldest   eldest getValue            return false             So you will always be able to get the first entry at your eldest member  It will be updated every time you perform a put   It should also be easy to override put and set youngest           Override     public Splat put Integer key  Splat value             youngest   value          return super put key  value           It all breaks down when you start removing entries though  haven t figured out a way to kludge that   It s very annoying that you can t otherwise get access to head or tail in a sensible way

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LinkedHashMap current implementation  Java 8  keeps track of its tail  If performance is a concern and or the map is large in size  you could access that field via reflection   Because the implementation may change it is probably a good idea to have a fallback strategy too  You may want to log something if an exception is thrown so you know that the implementation has changed   It could look like   public static  lt K  V gt  Entry lt K  V gt  getFirst Map lt K  V gt  map      if  map isEmpty    return null    return map entrySet   iterator   next       public static  lt K  V gt  Entry lt K  V gt  getLast Map lt K  V gt  map      try       if  map instanceof LinkedHashMap  return getLastViaReflection map       catch  Exception ignore        return getLastByIterating map      private static  lt K  V gt  Entry lt K  V gt  getLastByIterating Map lt K  V gt  map      Entry lt K  V gt  last   null    for  Entry lt K  V gt  e   map entrySet    last   e    return last     private static  lt K  V gt  Entry lt K  V gt  getLastViaReflection Map lt K  V gt  map  throws NoSuchFieldException  IllegalAccessException     Field tail   map getClass   getDeclaredField  tail      tail setAccessible true     return  Entry lt K  V gt   tail get map

User · Answer

For first element use entrySet   iterator   next   and stop iterating after 1 iteration  For last one the easiest way is to preserve the key in a variable whenever you do a map put

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I would recommend using ConcurrentSkipListMap which has firstKey   and lastKey   methods

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Yea I came across the same problem  but luckily I only need the first element    - This is what I did for it   private String getDefaultPlayerType         String defaultPlayerType           for LinkedHashMap Entry lt String Integer gt  entry   getLeagueByName currentLeague  getStatisticsOrder   entrySet                  defaultPlayerType   entry getKey            break            return defaultPlayerType      If you need the last element as well - I d look into how to reverse the order of your map - store it in a temp variable  access the first element in the reversed map therefore it would be your last element   kill the temp variable    Here s some good answers on how to reverse order a hashmap    How to iterate hashmap in reverse order in Java  If you use help from the above link  please give them up-votes    Hope this can help someone

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right  you have to manually enumerate keyset till the end of the linkedlist  then retrieve the entry by key and return this entry

User · Answer

Can you try doing something like  to get the last entry    linkedHashMap entrySet   toArray   linkedHashMap size   -1

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import java util Arrays          import java util LinkedHashMap          import java util List          import java util Map           public class Scratch              public static void main String   args                      Plain java version                Map lt String  List lt Integer gt  gt  linked   new LinkedHashMap lt  gt                   linked put  quot a quot   Arrays asList 1  2  3                  linked put  quot aa quot   Arrays asList 1  2  3  4                  linked put  quot b quot   Arrays asList 1  2  3  4  5                  linked put  quot bb quot   Arrays asList 1  2  3  4  5  6                   System out println  quot linked    quot    linked                  String firstKey   getFirstKey linked                 System out println  quot firstKey    quot    firstKey                 List lt Integer gt  firstEntry   linked get firstKey                 System out println  quot firstEntry    quot    firstEntry                  String lastKey   getLastKey linked                 System out println  quot lastKey    quot    lastKey                 List lt Integer gt  lastEntry   linked get lastKey                 System out println  quot lastEntry    quot    lastEntry                               private static String getLastKey Map lt String  List lt Integer gt  gt  linked                  int index   0                for  String key   linked keySet                   index                 if  index    linked size                      return key                                               return null                           private static String getFirstKey Map lt String  List lt Integer gt  gt  linked                  for  String key   linked keySet                   return key                                return null

[java] Java LinkedHashMap get first or last entry

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