Java Class that implements Map and keeps insertion order

Question

I m looking for a class in java that has key-value association  but without using hashes   Here is what I m currently doing    Add values to a Hashtable  Get an iterator for the Hashtable entrySet    Iterate through all values and    Get a Map Entry for the iterator  Create an object of type Module  a custom class  based on the value  Add the class to a JPanel   Show the panel    The problem with this is that I do not have control over the order that I get the values back  so I cannot display the values in the a given order  without hard-coding the order    I would use an ArrayList or Vector for this  but later in the code I need to grab the Module object for a given Key  which I can t do with an ArrayList or Vector   Does anyone know of a free open-source Java class that will do this  or a way to get values out of a Hashtable based on when they were added   Thanks

User · Answer

You can maintain a Map  for fast lookup  and List  for order  but a LinkedHashMap may be the simplest  You can also try a SortedMap e g  TreeMap  which an have any order you specify

User · Answer

Whenever i need to maintain the natural order of things that are known ahead of time  i use a EnumMap  the keys will be enums and you can insert in any order you want but when you iterate it will iterate in the enum order  the natural order      Also when using EnumMap there should be no collisions which can be more efficient     I really find that using enumMap makes for clean readable code   Here is an example

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I suggest a LinkedHashMap or a TreeMap  A LinkedHashMap keeps the keys in the order they were inserted  while a TreeMap is kept sorted via a Comparator or the natural Comparable ordering of the elements   Since it doesn t have to keep the elements sorted  LinkedHashMap should be faster for most cases  TreeMap has O log n  performance for containsKey  get  put  and remove  according to the Javadocs  while LinkedHashMap is O 1  for each   If your API that only expects a predictable sort order  as opposed to a specific sort order  consider using the interfaces these two classes implement  NavigableMap or SortedMap  This will allow you not to leak specific implementations into your API and switch to either of those specific classes or a completely different implementation at will afterwards

User · Answer

If an immutable map fits your needs then there is a library by google called guava  see also guava questions    Guava provides an ImmutableMap with reliable user-specified iteration order  This ImmutableMap has O 1  performance for containsKey  get  Obviously put and remove are not supported    ImmutableMap objects are constructed by using either the elegant static convenience methods of   and copyOf   or a Builder object

User · Answer

You could try my Linked Tree Map implementation

User · Answer

I don t know if it is opensource  but after a little googling  I found this implementation of Map using ArrayList  It seems to be pre-1 5 Java  so you might want to genericize it  which should be easy  Note that this implementation has O N  access  but this shouldn t be a problem if you don t add hundreds of widgets to your JPanel  which you shouldn t anyway

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LinkedHashMap will return the elements in the order they were inserted into the map when you iterate over the keySet    entrySet   or values   of the map    Map lt String  String gt  map   new LinkedHashMap lt String  String gt      map put  id    1    map put  name    rohan    map put  age    26     for  Map Entry lt String  String gt  entry   map entrySet          System out println entry getKey             entry getValue         This will print the elements in the order they were put into the map    id   1 name   rohan  age   26

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Either You can use LinkedHashMap lt K  V gt  or you can implement you own CustomMap which maintains insertion order   You can use the Following CustomHashMap with the following features    Insertion order is maintained  by using LinkedHashMap internally  Keys with null or empty strings are not allowed  Once key with value is created  we are not overriding its value    HashMap vs LinkedHashMap vs CustomHashMap  interface CustomMap lt K  V gt  extends Map lt K  V gt        public boolean insertionRule K key  V value       SuppressWarnings    rawtypes    unchecked     public class CustomHashMap lt K  V gt  implements CustomMap lt K  V gt        private Map lt K  V gt  entryMap         SET  Adds the specified element to this set if it is not already present      private Set lt K gt  entrySet       public CustomHashMap             super            entryMap   new LinkedHashMap lt K  V gt             entrySet   new HashSet                Override     public boolean insertionRule K key  V value               KEY as null and EMPTY String is not allowed          if  key    null     key instanceof String  amp  amp    String  key  trim   equals                       return false                        If key already available then  we are not overriding its value          if  entrySet contains key        Then override its value  but we are not allowing             return false            else      Add the entry             entrySet add key               entryMap put key  value               return true                      public V put K key  V value            V oldValue   entryMap get key           insertionRule key  value           return oldValue            public void putAll Map lt   extends K    extends V gt  t            for  Iterator i   t keySet   iterator    i hasNext                   K key    K  i next                insertionRule key  t get key                         public void clear             entryMap clear            entrySet clear              public boolean containsKey Object key            return entryMap containsKey key             public boolean containsValue Object value            return entryMap containsValue value             public Set entrySet             return entryMap entrySet              public boolean equals Object o            return entryMap equals o             public V get Object key            return entryMap get key             public int hashCode             return entryMap hashCode              public boolean isEmpty             return entryMap isEmpty              public Set keySet             return entrySet            public V remove Object key            entrySet remove key           return entryMap remove key             public int size             return entryMap size              public Collection values             return entryMap values              Usage of CustomHashMap   public static void main String   args        System out println     LinkedHashMap           Map lt Object  String gt  map2   new LinkedHashMap lt Object  String gt         addData map2        System out println     CustomHashMap           Map lt Object  String gt  map   new CustomHashMap lt Object  String gt         addData map     public static void addData Map lt Object  String gt  map        map put null   1        map put  name    Yash        map put  1    1 - Str        map put  1    2 - Str       Overriding value     map put      1       Empty String     map put       1       Empty String     map put 1   Int        map put null   2       Null      for  Map Entry lt Object  String gt  entry   map entrySet              System out println entry getKey             entry getValue               O P      LinkedHashMap         CustomHashMap    null   2              name   Yash name   Yash           1   1 - Str 1   2 - Str           1   Int    1                      1                 1   Int                   If you know the KEY s are fixed then you can use EnumMap  Get the values form Properties XML files  EX   enum ORACLE       IP  URL  USER NAME  PASSWORD  DB Name     EnumMap lt ORACLE  String gt  props   new EnumMap lt ORACLE  String gt  ORACLE class   props put ORACLE IP   127 0 0 1    props put ORACLE URL          props put ORACLE USER NAME   Scott    props put ORACLE PASSWORD   Tiget    props put ORACLE DB Name   MyDB

User · Answer

You can use LinkedHashMap to main insertion order in Map   The important points about Java LinkedHashMap class are    It contains onlyunique elements  A LinkedHashMap contains values based on the key 3 It may have one null key and multiple null values  4 It is same as HashMap instead maintains insertion order  public class LinkedHashMap lt K V gt  extends HashMap lt K V gt  implements Map lt K V gt      But if you want sort values in map using User-defined object or any primitive data type key then you should use TreeMap For more information  refer this link

[java] Java Class that implements Map and keeps insertion order?

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