Java Ordered Map

Question

In Java  Is there an object that acts like a Map for storing and accessing key value pairs  but can return an ordered list of keys and an ordered list of values  such that the key and value lists are in the same order   So as explanation-by-code  I m looking for something that behaves like my fictitious OrderedMap   OrderedMap lt Integer  String gt  om   new OrderedMap lt  gt     om put 0   Zero    om put 7   Seven     String o   om get 7      o is  Seven  List lt Integer gt  keys   om getKeys    List lt String gt  values   om getValues     for int i   0  i  lt  keys size    i          Integer key   keys get i       String value   values get i       Assert om get key     value

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Is there an object that acts like a Map for storing and accessing key/value pairs, but can return an ordered list of keys and an ordered list of values, such that the key and value lists are in the same order?

You're looking for java.util.LinkedHashMap. You'll get a list of Map.Entry<K,V> pairs, which always get iterated in the same order. That order is the same as the order by which you put the items in. Alternatively, use the java.util.SortedMap, where the keys must either have a natural ordering or have it specified by a Comparator.

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You can leverage NavigableMap interface that may be accessed and traversed in either ascending or descending key order  This interface is intended to supersede the SortedMap interface  The Navigable map is usually sorted according to the natural ordering of its keys  or by a Comparator provided at map creation time   There are three most useful implementations of it  TreeMap  ImmutableSortedMap  and ConcurrentSkipListMap    TreeMap example   TreeMap lt String  Integer gt  users   new TreeMap lt String  Integer gt     users put  Bob   1   users put  Alice   2   users put  John   3    for  String key  users keySet        System out println key      ID      users get key              Output   Alice  ID   2  Bob  ID   1  John  ID   3

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I have used Simple Hash map  linked list and Collections to sort a Map by values   import java util    import java util Map    public class Solution        public static void main String   args               create a simple hash map and insert some key-value pairs into it         Map lt String  Integer gt  map   new HashMap lt String  Integer gt             map put  Python   3           map put  C   0           map put  JavaScript   4           map put  C     1           map put  Golang   5           map put  Java   2              Create a linked list from the above map entries         List lt Entry lt String  Integer gt  gt  list   new LinkedList lt Entry lt String  Integer gt  gt  map entrySet                sort the linked list using Collections sort           Collections sort list  new Comparator lt Entry lt String  Integer gt  gt              Override          public int compare Entry lt String  Integer gt  m1  Entry lt String  Integer gt  m2            return m1 getValue   compareTo m2 getValue                               for Entry lt String  Integer gt  value  list             System out println value                   The output is   C 0 C   1 Java 2 Python 3 JavaScript 4 Golang 5

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Modern Java version of Steffi Keran s answer public class Solution       public static void main String   args               create a simple hash map and insert some key-value pairs into it         Map lt String  Integer gt  map   new HashMap lt  gt             map put  quot Python quot   3           map put  quot C quot   0           map put  quot JavaScript quot   4           map put  quot C   quot   1           map put  quot Golang quot   5           map put  quot Java quot   2              Create a linked list from the above map entries         List lt Map Entry lt String  Integer gt  gt  list   new LinkedList lt  gt  map entrySet                sort the linked list using Collections sort           list sort Comparator comparing Map Entry  getValue            list forEach System out  println

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I think the SortedMap interface enforces what you ask for and TreeMap implements that    http   java sun com j2se 1 5 0 docs api java util SortedMap html http   java sun com j2se 1 5 0 docs api java util TreeMap html

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I think the closest collection you ll get from the framework is the SortedMap

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Since Java 6 there is also non-blocking thread-safe alternative to TreeMap  See ConcurrentSkipListMap

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The SortedMap interface  with the implementation TreeMap  should be your friend   The interface has the methods    keySet   which returns a set of the keys in ascending order values   which returns a collection of all values in the ascending order of the corresponding keys   So this interface fulfills exactly your requirements  However  the keys must have a meaningful order  Otherwise you can used the LinkedHashMap where the order is determined by the insertion order

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tl dr  To keep Map lt  Integer   String  gt  in an order sorted by key  use either of the two classes implementing the SortedMap NavigableMap interfaces    TreeMap ConcurrentSkipListMap   If manipulating the map within a single thread  use the first  TreeMap  If manipulating across threads  use the second  ConcurrentSkipListMap    For details  see the table below and the following discussion   Details  Here is a graphic table I made showing the features of the ten Map implementations bundled with Java 11   The NavigableMap interface is what SortedMap should have been in the first place  The SortedMap logically should be removed but cannot be as some 3rd-party map implementations may be using interface   As you can see in this table  only two classes implement the SortedMap NavigableMap interfaces     TreeMap ConcurrentSkipListMap   Both of these keep keys in sorted order  either by their natural order  using compareTo method of the Comparable https   docs oracle com en java javase 11 docs api java base java lang Comparable html  interface  or by a Comparator implementation you pass  The difference between these two classes is that the second one  ConcurrentSkipListMap  is thread-safe  highly concurrent   See also the Iteration order column in the table below     The LinkedHashMap class returns its entries by the order in which they were originally inserted   EnumMap returns entries in the order by which the enum class of the key is defined  For example  a map of which employee is covering which day of the week  Map lt  DayOfWeek   Person  gt   uses the DayOfWeek enum class built into Java  That enum is defined with Monday first and Sunday last  So entries in an iterator will appear in that order    The other six implementations make no promise about the order in which they report their entries

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LinkedHashMap maintains the order of the keys   java util LinkedHashMap appears to work just like a normal HashMap otherwise

[java] Java Ordered Map

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