How to create a HashMap with two keys Key-Pair Value

Question

I have a 2D array of Integers  I want them to be put into a HashMap  But I want to access the elements from the HashMap based on Array Index  Something like   For A 2  5   map get 2 5  which returns a value associated with that key  But how do I create a hashMap with a pair of keys  Or in general  multiple keys  Map lt   key1  key2    keyN   Value  in a way that I can access the element with using get key1 key2    keyN    EDIT    3 years after posting the question  I want to add a bit more to it  I came across another way for NxN matrix    Array indices  i and j can be represented as a single key the following way   int key   i   N   j    map put key  a i  j       queue add key      And the indices can be retrevied from the key in this way   int i   key   N  int j   key   N

User · Answer

You can t have an hash map with multiple keys  but you can have an object that takes multiple parameters as the key    Create an object called Index that takes an x and y value    public class Index        private int x      private int y       public Index int x  int y            this x   x          this y   y              Override     public int hashCode             return this x   this y              Override     public boolean equals Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          Index other    Index  obj          if  x    other x              return false          if  y    other y              return false          return true            Then have your HashMap lt Index  Value gt  to get your result

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Implemented in common-collections MultiKeyMap

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Two possibilities  Either use a combined key   class MyKey       int firstIndex      int secondIndex         important  override hashCode   and equals       Or a Map of Map   Map lt Integer  Map lt Integer  Integer gt  gt  myMap

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You can download it from the below link  https   github com VVS279 DoubleKeyHashMap blob master src com virtualMark doubleKeyHashMap DoubleKeyHashMap java  https   github com VVS279 DoubleKeyHashMap  You can use double key  value hashmap      DoubleKeyHashMap lt Integer  Integer  String gt  doubleKeyHashMap1   new     DoubleKeyHashMap lt Integer  Integer  String gt         DoubleKeyHashMap lt String  String  String gt  doubleKeyHashMap2   new     DoubleKeyHashMap lt String  String  String gt

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If they are two integers you can try a quick and dirty trick  Map lt String    gt  using the key as i     j   If the key i     j is the same as j     i try min i j      max i j

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There are several options   2 dimensions  Map of maps  Map lt Integer  Map lt Integer  V gt  gt  map                map get 2  get 5     Wrapper key object  public class Key        private final int x      private final int y       public Key int x  int y            this x   x          this y   y              Override     public boolean equals Object o            if  this    o  return true          if    o instanceof Key   return false          Key key    Key  o          return x    key x  amp  amp  y    key y              Override     public int hashCode             int result   x          result   31   result   y          return result             Implementing equals   and hashCode   is crucial here  Then you simply use   Map lt Key  V gt  map           and   map get new Key 2  5      Table from Guava  Table lt Integer  Integer  V gt  table   HashBasedTable create           table get 2  5     Table uses map of maps underneath   N dimensions  Notice that special Key class is the only approach that scales to n-dimensions  You might also consider   Map lt List lt Integer gt   V gt  map           but that s terrible from performance perspective  as well as readability and correctness  no easy way to enforce list size    Maybe take a look at Scala where you have tuples and case classes  replacing whole Key class with one-liner

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You could create your key object something like this   public class MapKey    public  Object key1  public Object key2   public Object getKey1         return key1     public void setKey1 Object key1        this key1   key1     public Object getKey2         return key2     public void setKey2 Object key2        this key2   key2     public boolean equals Object keyObject        if keyObject  null          return false       if  keyObject getClass     MapKey class          return false       MapKey key    MapKey keyObject       if key key1  null  amp  amp  this key1  null          return false       if key key2   null  amp  amp  this key2  null          return false       if this key1  null  amp  amp  key key1   null          return false       if this key2  null  amp  amp  key key2   null          return false       if this key1  null  amp  amp  key key1  null  amp  amp  this key2   null  amp  amp  key key2   null          return this key2 equals key key2        if this key2  null  amp  amp  key key2  null  amp  amp  this key1   null  amp  amp  key key1   null          return this key1 equals key key1        return  this key1 equals key key1   amp  amp  this key2 equals key2       public int hashCode        int key1HashCode key1 hashCode        int key2HashCode key2 hashCode        return key1HashCode  gt  gt  3   key2HashCode  lt  lt  5         The advantage of this is  It will always make sure you are covering all the scenario s of Equals as well   NOTE  Your key1 and key2 should be immutable  Only then will you be able to construct a stable key Object

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Create a value class that will represent your compound key  such as   class Index2D     int first  second        overrides equals and hashCode properly here     taking care to override equals   and hashCode   correctly   If that seems like a lot of work  you might consider some ready made generic containers  such as Pair provided by apache commons among others   There are also many similar questions here  with other ideas  such as using Guava s Table  although allows the keys to have different types  which might be overkill  in memory use and complexity  in your case since I understand your keys are both integers

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Use a Pair as keys for the HashMap  JDK has no Pair  but you can either use a 3rd party libraray such as http   commons apache org lang or write a Pair taype of your own

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we can create a class to pass more than one key or value and the object of this class can be used as a parameter in map   import java io BufferedReader   import java io FileReader  import java io IOException  import java util      public class key1       String b      String a      key1 String a String b                this a a          this b b             public class read2    private static final String FILENAME    E  studies JAVA ReadFile Project nn txt    public static void main String   args         BufferedReader br   null      FileReader fr   null      Map lt key1 String gt  map new HashMap lt key1 String gt         try            fr   new FileReader FILENAME           br   new BufferedReader fr            String sCurrentLine           br   new BufferedReader new FileReader FILENAME             while   sCurrentLine   br readLine       null                String   s1   sCurrentLine split                   key1 k1   new key1 s1 0  s1 2                map put k1 s1 2                      for Map Entry lt key1 String gt  m map entrySet                   key1 key   m getKey                String s3   m getValue                   System out println key a     key b       s3                                                      catch  IOException e             e printStackTrace           finally            try                if  br    null                  br close                 if  fr    null                  fr close               catch  IOException ex                 ex printStackTrace

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When you create your own key pair object  you should face a few thing   First  you should be aware of implementing hashCode   and equals    You will need to do this   Second  when implementing hashCode    make sure you understand how it works  The given user example  public int hashCode         return this x   this y      is actually one of the worst implementations you can do  The reason is simple  you have a lot of equal hashes  And the hashCode   should return int values that tend to be rare  unique at it s best  Use something like this   public int hashCode       return  X  lt  lt  16    Y      This is fast and returns unique hashes for keys between -2 16 and 2 16-1  -65536 to 65535   This fits in almost any case  Very rarely you are out of this bounds   Third  when implementing equals   also know what it is used for and be aware of how you create your keys  since they are objects  Often you do unnecessary if statements cause you will always have the same result   If you create keys like this  map put new Key x y  V   you will never compare the references of your keys  Cause everytime you want to acces the map  you will do something like map get new Key x y     Therefore your equals   does not need a statement like if  this    obj   It will never occure   Instead of if  getClass      obj getClass    in your equals   better use if    obj instanceof this    It will be valid even for subclasses   So the only thing you need to compare is actually X and Y  So the best equals   implementation in this case would be   public boolean equals  final Object O      if    O instanceof Key   return false    if    Key  O  X    X  return false    if    Key  O  Y    Y  return false    return true      So in the end your key class is like this   public class Key      public final int X    public final int Y     public Key final int X  final int Y        this X   X      this Y   Y         public boolean equals  final Object O        if    O instanceof Key   return false      if    Key  O  X    X  return false      if    Key  O  Y    Y  return false      return true         public int hashCode         return  X  lt  lt  16    Y           You can give your dimension indices X and Y a public access level  due to the fact they are final and do not contain sensitive information  I m not a 100  sure whether private access level works correctly in any case when casting the Object to a Key   If you wonder about the finals  I declare anything as final which value is set on instancing and never changes - and therefore is an object constant

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You can also use guava Table implementation for this   Table represents a special map where two keys can be specified in combined fashion to refer to a single value  It is similar to creating a map of maps     create a table   Table lt String  String  String gt  employeeTable   HashBasedTable create         initialize the table with employee details   employeeTable put  IBM    101   Mahesh      employeeTable put  IBM    102   Ramesh      employeeTable put  IBM    103   Suresh       employeeTable put  Microsoft    111   Sohan      employeeTable put  Microsoft    112   Mohan      employeeTable put  Microsoft    113   Rohan       employeeTable put  TCS    121   Ram      employeeTable put  TCS    122   Shyam      employeeTable put  TCS    123   Sunil         get Map corresponding to IBM   Map lt String String gt  ibmEmployees    employeeTable row  IBM

[java] How to create a HashMap with two keys (Key-Pair, Value)?

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