Using Pairs or 2-tuples in Java

Question

My Hashtable in Java would benefit from a value having a tuple structure  What data structure can I use in Java to do that   Hashtable lt Long  Tuple lt Set lt Long gt  Set lt Long gt  gt  gt  table

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Though the article is pretty old now  and though I understand that I m not really very helpful  I think the work done here  http   www pds ewi tudelft nl pubs papers cpe2005 pdf  would have been nice in mainstream Java   You can do things like   int a  char b  float c   a b c     3  a  2 33     or   int int char  x    1 2  a      or  public  int boolean  Find int i      int idx   FindInArray A i     return  idx idx gt  0       idx  found    Find 7     Here tuples are    Defined as primitive types - no templates generics Stack-allocated if declared locally Assigned using pattern-matching   This approach increases   Performance Readability Expressiveness

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Android Tuple Utils  This object provides a sensible implementation of equals    returning true if equals   is true on each of the contained objects

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I don t think there is a general purpose tuple class in Java but a custom one might be as easy as the following   public class Tuple lt X  Y gt       public final X x     public final Y y     public Tuple X x  Y y         this x   x       this y   y             Of course  there are some important implications of how to design this class further regarding equality  immutability  etc   especially if you plan to use instances as keys for hashing

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You can use Google Guava Table

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I will start from a general point of view about tuples in Java and finish with an implication for your concrete problem   1  The way tuples are used in non-generic languages is avoided in Java because they are not type-safe  e g  in Python  tuple    4  7 9   python     If you still want to use something like a general purpose tuple  which is not recommended   you should use Object   or List lt Object gt  and cast the elements after a check with instanceof to assure type-safety   Usually  tuples in a certain setting are always used the same way with containing the same structure  In Java  you have to define this structure explicitly in a class to provide well-defined  type-safe values and methods  This seems annoying and unnecessairy at first but prevents errors already at compile-time   2  If you need a tuple containing the same  super- classes Foo  use Foo    List lt Foo gt   or List lt   extends Foo gt   or the lists s immutable counterparts   Since a tuple is not of a defined length  this solution is equivalent   3  In your case  you seem to need a Pair  i e  a tuple of well-defined length 2   This renders maerics s answer or one of the supplementary answers the most efficient since you can reuse the code in the future

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Apache Commons provided some common java utilities including a Pair  It implements Map Entry  Comparable and Serializable

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Here s this exact same question elsewhere  that includes a more robust equals  hash that maerics alludes to   http   groups google com group comp lang java help browse thread thread f8b63fc645c1b487 1d94be050cfc249b  That discussion goes on to mirror the maerics vs ColinD approaches of  should I re-use a class Tuple with an unspecific name  or make a new class with specific names each time I encounter this situation    Years ago I was in the latter camp  I ve evolved into supporting the former

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Create a class that describes the concept you re actually modeling and use that  It can just store two Set lt Long gt  and provide accessors for them  but it should be named to indicate what exactly each of those sets is and why they re grouped together

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If you are looking for a built-in Java two-element tuple  try AbstractMap SimpleEntry

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javatuples is a dedicated project for tuples in Java   Unit lt A gt   1 element  Pair lt A B gt   2 elements  Triplet lt A B C gt   3 elements

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With lombok it s easy to declare a Pair class    Data staticConstructor    of   public class Pair lt A  B gt        private final A left      private final B right      This will generates getters  static constructor named  of   equals    hashcode   and toString     see  Data documentation for more information

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To supplement  maerics s answer  here is the Comparable tuple   import java util            A tuple of two classes that implement Comparable     public class ComparableTuple lt X extends Comparable lt   super X gt   Y extends Comparable lt   super Y gt  gt         extends Tuple lt X  Y gt         implements Comparable lt ComparableTuple lt X  Y gt  gt      public ComparableTuple X x  Y y        super x  y                   Implements lexicographic order         public int compareTo ComparableTuple lt X  Y gt  other        int d   this x compareTo other x       if  d    0        return this y compareTo other y       return d

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As an extension to  maerics nice answer  I ve added a few useful methods   public class Tuple lt X  Y gt         public final X x       public final Y y       public Tuple X x  Y y             this x   x           this y   y               Override     public String toString             return       x         y                    Override     public boolean equals Object other            if  other    this                return true                     if    other instanceof Tuple                return false                     Tuple lt X Y gt  other     Tuple lt X Y gt   other              this may cause NPE if nulls are valid values for x or y  The logic may be improved to handle nulls properly  if needed          return other  x equals this x   amp  amp  other  y equals this y               Override     public int hashCode             final int prime   31          int result   1          result   prime   result     x    null    0   x hashCode             result   prime   result     y    null    0   y hashCode             return result

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Another 2 cents   Starting with Java 7  there is now a class for this in standard Lib   javafx util Pair   And Yes  It is standard Java  now that JavaFx is included in the JDK

[java] Using Pairs or 2-tuples in Java

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