Java - How to create new Entry key value

Question

I d like to create new item that similarly to Util Map Entry that will contain the structure key  value   The problem is that I can t instantiate a Map Entry because it s an interface    Does anyone know how to create a new generic key value object for Map Entry

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If you are using Clojure  you have another option    defn map-entry    k v     clojure lang MapEntry create k v

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Why Map Entry  I guess something like a key-value pair is fit for the case   Use java util AbstractMap SimpleImmutableEntry or java util AbstractMap SimpleEntry

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I defined a generic Pair class that I use all the time  It s great  As a bonus  by defining a static factory method  Pair create  I only have to write the type arguments half as often   public class Pair lt A  B gt         private A component1      private B component2       public Pair                 super               public Pair A component1  B component2                this component1   component1              this component2   component2             public A fst                 return component1             public void setComponent1 A component1                this component1   component1             public B snd                 return component2             public void setComponent2 B component2                this component2   component2              Override     public String toString                 return   lt     component1         component2     gt                Override     public int hashCode                 final int prime   31              int result   1              result   prime   result                                 component1    null    0   component1 hashCode                 result   prime   result                                 component2    null    0   component2 hashCode                 return result              Override     public boolean equals Object obj                if  this    obj                      return true              if  obj    null                      return false              if  getClass      obj getClass                        return false              final Pair lt      gt  other    Pair lt      gt   obj              if  component1    null                        if  other component1    null                              return false                else if   component1 equals other component1                       return false              if  component2    null                        if  other component2    null                              return false                else if   component2 equals other component2                       return false              return true             public static  lt A  B gt  Pair lt A  B gt  create A component1  B component2                return new Pair lt A  B gt  component1  component2

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org apache commons lang3 tuple Pair implements java util Map Entry and can also be used standalone   Also as others mentioned Guava s com google common collect Maps immutableEntry K  V  does the trick   I prefer Pair for its fluent Pair of L  R  syntax

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You can just implement the Map Entry lt K  V gt  interface yourself   import java util Map   final class MyEntry lt K  V gt  implements Map Entry lt K  V gt        private final K key      private V value       public MyEntry K key  V value            this key   key          this value   value              Override     public K getKey             return key              Override     public V getValue             return value              Override     public V setValue V value            V old   this value          this value   value          return old            And then use it   Map Entry lt String  Object gt  entry   new MyEntry lt String  Object gt   Hello   123   System out println entry getKey     System out println entry getValue

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You could actually go with  Map Entry lt String  String gt  en  Maps immutableEntry key  value

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If you look at the documentation of Map Entry you will find that it is a static interface  an interface which is defined inside the Map interface an can be accessed through Map Entry  and it has two implementations  All Known Implementing Classes  AbstractMap SimpleEntry  AbstractMap SimpleImmutableEntry  The class AbstractMap SimpleEntry provides 2 constructors   Constructors and Description AbstractMap SimpleEntry K key  V value  Creates an entry representing a mapping from the specified key to the specified value  AbstractMap SimpleEntry Map Entry lt   extends K   extends V gt  entry  Creates an entry representing the same mapping as the specified entry   An example use case  import java util Map  import java util AbstractMap SimpleEntry   public class MyClass       public static void main String args            Map Entry e   new SimpleEntry lt String  String gt   quot Hello quot   quot World quot           System out println e getKey    quot   quot  e getValue

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There s public static class AbstractMap SimpleEntry lt K V gt   Don t let the Abstract part of the name mislead you  it is in fact NOT an abstract class  but its top-level AbstractMap is    The fact that it s a static nested class means that you DON T need an enclosing AbstractMap instance to instantiate it  so something like this compiles fine   Map Entry lt String Integer gt  entry       new AbstractMap SimpleEntry lt String  Integer gt   exmpleString   42     As noted in another answer  Guava also has a convenient static factory method Maps immutableEntry that you can use     You said      I can t use Map Entry itself because apparently it s a read-only object that I can t instantiate new instanceof   That s not entirely accurate  The reason why you can t instantiate it directly  i e  with new  is because it s an interface Map Entry     Caveat and tip  As noted in the documentation  AbstractMap SimpleEntry is  since 1 6  so if you re stuck to 5 0  then it s not available to you   To look for another known class that implements Map Entry  you can in fact go directly to the javadoc  From the Java 6 version     Interface Map Entry      All Known Implementing Classes          AbstractMap SimpleEntry  AbstractMap SimpleImmutableEntry      Unfortunately the 1 5 version does not list any known implementing class that you can use  so you may have be stuck with implementing your own

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Starting from Java 9  there is a new utility method allowing to create an immutable entry which is Map entry Object  Object     Here is a simple example   Entry lt String  String gt  entry   Map entry  foo    bar      As it is immutable  calling setValue will throw an UnsupportedOperationException  The other limitations are the fact that it is not serializable and null as key or value is forbidden  if it is not acceptable for you  you will need to use AbstractMap SimpleImmutableEntry or AbstractMap SimpleEntry instead   NB  If your need is to create directly a Map with 0 to up to 10  key  value  pairs  you can instead use the methods of type Map of K key1  V value1

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Example of AbstractMap SimpleEntry   import java util Map   import java util AbstractMap  import java util AbstractMap SimpleEntry    Instantiate   ArrayList lt Map Entry lt Integer  Integer gt  gt  arr        new ArrayList lt Map Entry lt Integer  Integer gt  gt       Add rows   arr add new AbstractMap SimpleEntry 2  3    arr add new AbstractMap SimpleEntry 20  30    arr add new AbstractMap SimpleEntry 2  4      Fetch rows   System out println arr get 0  getKey     System out println arr get 0  getValue     System out println arr get 1  getKey     System out println arr get 1  getValue     System out println arr get 2  getKey     System out println arr get 2  getValue       Should print   2 3 20 30 2 4   It s good for defining edges of graph structures   Like the ones between neurons in your head

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Try Maps immutableEntry from Guava  This has the advantage of being compatible with Java 5  unlike AbstractMap SimpleEntry which requires Java 6

[java] Java - How to create new Entry (key, value)

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