Java Retrieving an element from a HashSet

Question

Hoping someone can explain why I cannot retrieve an element from a HashSet   Consider my HashSet containing a list of MyHashObjects with their hashCode   and equals   methods overridden correctly   What I was hoping to do was to construct a MyHashObject myself  and set the relevant hash code properties to certain values  I can query the HashSet to see if there  equivalent  objects in the set using the contains   method  So even though contains   returns true for the 2 objects  they may not be    true   How come then there is no get   method similar to how the contains   works   Interested to know the thinking behind this API decision

User · Answer

If you know the order of elements in your Set  you can retrieve them by converting the Set to an Array   Something like this   Set mySet   MyStorageObject getMyStringSet    Object   myArr   mySet toArray    String value1   myArr 0  toString    String value2   myArr 1  toString

User · Answer

The idea that you need to get the reference to the object that is contained inside a Set object is common  It can be archived by 2 ways    Use HashSet as you wanted  then    public Object getObjectReference HashSet lt Xobject gt  set  Xobject obj        if  set contains obj             for  Xobject o   set                if  obj equals o                   return o                      return null       For this approach to work  you need to override both hashCode   and equals Object o  methods In the worst scenario we have O n    Second approach is to use TreeSet  public Object getObjectReference TreeSet lt Xobject gt  set  Xobject obj        if  set contains obj             return set floor obj             return null       This approach gives O log n    more efficient  You don t need to override hashCode for this approach but you have to implement Comparable interface    define function compareTo Object o

User · Answer

If I know for sure in my application that the object is not used in search in any of the list or hash data structure and not used equals method elsewhere except the one used indirectly in hash data structure while adding  Is it advisable to update the existing object in set in equals method  Refer the below code  If I add the this bean to HashSet  I can do group aggregation on the matching object on key  id   By this way I am able to achieve aggregation functions such as sum  max  min      as well  If not advisable  please feel free to share me your thoughts    public class MyBean        String id             name      double amountSpent        Override     public int hashCode             return id hashCode                Override     public boolean equals Object obj            if obj  null  amp  amp  obj instanceof MyBean                 MyBean tmpObj    MyBean  obj              if tmpObj id  null  amp  amp  tmpObj id equals this id                     tmpObj amountSpent    this amountSpent                  return true                                  return false

User · Answer

First of all convert your set to Array  Then  get item by index of array   Set uniqueItem   new HashSet     uniqueItem add  quot 0 quot    uniqueItem add  quot 1 quot    uniqueItem add  quot 0 quot     Object   arrayItem   uniqueItem toArray     for int i   0  i  lt  uniqueItem size   i         System out println  quot Item  quot  i  quot   quot  arrayItem i  toString

User · Answer

It sounds like you re essentially trying to use the hash code as a key in a map  which is what HashSets do behind the scenes    You could just do it explicitly  by declaring HashMap lt Integer  MyHashObject gt    There is no get for HashSets because typically the object you would supply to the get method as a parameter is the same object you would get back

User · Answer

You can HashMap lt MyHashObject MyHashObject gt  instead of HashSet lt MyHashObject gt    Calling ContainsKey   on your  reconstructed  MyHashObject will first hashCode  -check the collection  and if a duplicate hashcode is hit  finally equals  -check your  reconstructed  against the original  at which you can retrieve the original using get    This is O 1  but the downside is you will likely have to override both equals   and hashCode   methods

User · Answer

If you could use List as a data structure to store your data  instead of using Map to store the result in the value of the Map  you can use following snippet and store the result in the same object   Here is a Node class   private class Node       public int row  col  distance       public Node int row  int col  int distance            this row   row          this col   col          this distance   distance             public boolean equals Object o            return  o instanceof Node  amp  amp                  row      Node  o  row  amp  amp                  col      Node  o  col             If you store your result in distance variable and the items in the list are checked based on their coordinates  you can use the following to change the distance to a new one with the help of lastIndexOf method as long as you only need to store one element for each data       List lt Node gt  nodeList      nodeList   new ArrayList lt  gt  Arrays asList new Node 1  2  1   new Node 3  4  5         Node tempNode   new Node 1  2  10       if nodeList contains tempNode           nodeList get nodeList lastIndexOf tempNode   distance    tempNode distance    It is basically reimplementing Set whose items can be accessed and changed

User · Answer

One of the easiest ways is to convert to Array   for int i   0  i  lt  set size    i          System out println set toArray   i

User · Answer

If you know what element you want to retrieve  then you already have the element   The only question for a Set to answer  given an element  is whether it contains   it or not   If you want to iterator over the elements  just use a Set iterator     It sounds like what you re trying to do is designate a canonical element for an equivalence class of elements   You can use a Map lt MyObject MyObject gt  to do this   See this SO question or this one for a discussion   If you are really determined to find an element that  equals   your original element with the constraint that you MUST use the HashSet  I think you re stuck with iterating over it and checking equals   yourself   The API doesn t let you grab something by its hash code   So you could do   MyObject findIfPresent MyObject source  HashSet lt MyObject gt  set       if  set contains source           for  MyObject obj   set            if  obj equals source              return obj                   return null      Brute force and O n  ugly  but if that s what you need to do

[java] Java: Retrieving an element from a HashSet

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