Getting an element from a Set

Question

Why doesn t Set provide an operation to get an element that equals another element   Set lt Foo gt  set            Foo foo   new Foo 1  2  3   Foo bar   set get foo        get the Foo element from the Set that equals foo   I can ask whether the Set contains an element equal to bar  so why can t I get that element      To clarify  the equals method is overridden  but it only checks one of the fields  not all  So two Foo objects that are considered equal can actually have different values  that s why I can t just use foo

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Following can be an approach      SharedPreferences se get   getSharedPreferences  points  MODE PRIVATE      Set lt String gt  main   se get getStringSet  mydata  null      for int jk   0   jk  lt  main size   jk               Log i  data  String valueOf main toArray   jk

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Default Set in Java is  unfortunately  not designed to provide a  get  operation  as jschreiner accurately explained   The solutions of using an iterator to find the element of interest  suggested by dacwe  or to remove the element and re-add it with its values updated  suggested by KyleM   could work  but can be very inefficient   Overriding the implementation of equals so that non-equal objects are  equal   as stated correctly by David Ogren  can easily cause maintenance problems   And using a Map as an explicit replacement  as suggested by many   imho  makes the code less elegant   If the goal is to get access to the original instance of the element contained in the set  hope I understood correctly your use case   here is another possible solution     I personally had your same need while developing a client-server videogame with Java  In my case  each client had copies of the components stored in the server and the problem was whenever a client needed to modify an object of the server   Passing an object through the internet meant that the client had different instances of that object anyway  In order to match this  copied  instance with the original one  I decided to use Java UUIDs   So I created an abstract class UniqueItem  which automatically gives a random unique id to each instance of its subclasses   This UUID is shared between the client and the server instance  so this way it could be easy to match them by simply using a Map   However directly using a Map in a similar usecase was still inelegant  Someone might argue that using an Map might be more complicated to mantain and handle   For these reasons I implemented a library called MagicSet  that makes the usage of an Map  transparent  to the developer   https   github com ricpacca magicset    Like the original Java HashSet  a MagicHashSet  which is one of the implementations of MagicSet provided in the library  uses a backing HashMap  but instead of having elements as keys and a dummy value as values  it uses the UUID of the element as key and the element itself as value  This does not cause overhead in the memory use compared to a normal HashSet    Moreover  a MagicSet can be used exactly as a Set  but with some more methods providing additional functionalities  like getFromId    popFromId    removeFromId    etc   The only requirement to use it is that any element that you want to store in a MagicSet needs to extend the abstract class UniqueItem     Here is a code example  imagining to retrieve the original instance of a city from a MagicSet  given another instance of that city with the same UUID  or even just its UUID    class City extends UniqueItem           Somewhere in this class      public void doSomething                Whatever          public class GameMap       private MagicSet lt City gt  cities       public GameMap Collection lt City gt  cities            cities   new MagicHashSet lt  gt  cities                        cityId is the UUID of the city you want to retrieve         If you have a copied instance of that city  you can simply         call copiedCity getId   and pass the return value to this method              public void doSomethingInCity UUID cityId            City city   cities getFromId cityId           city doSomething                  Other methods can be called on a MagicSet too

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it looks like the proper object to use is the Interner from guava       Provides equivalent behavior to String intern   for other immutable   types  Common implementations are available from the Interners   class    It also has a few very interesting levers  like concurrencyLevel  or the type of references used  it might be worth noting that it doesn t offer a SoftInterner which I could see as more useful than a WeakInterner

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Because any particular implementation of Set may or may not be random access   You can always get an iterator and step through the Set  using the iterators  next   method to return the result you want once you find the equal element   This works regardless of the implementation   If the implementation is NOT random access  picture a linked-list backed Set   a get E element  method in the interface would be deceptive  since it would have to iterate the collection to find the element to return  and a get E element  would seem to imply this would be necessary  that the Set could jump directly to the element to get   contains   may or may not have to do the same thing  of course  depending on the implementation  but the name doesn t seem to lend itself to the same sort of misunderstandings

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If you look at the first few lines of the implementation of java util HashSet you will see   public class HashSet lt E gt               private transient HashMap lt E Object gt  map    So HashSet uses HashMap interally anyway  which means that if you just use a HashMap directly and use the same value as the key and the value you will get the effect you want and save yourself some memory

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I know  this has been asked and answered long ago  however if anyone is interested  here is my solution - custom set class backed by HashMap   http   pastebin com Qv6S91n9  You can easily implement all other Set methods

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If you have an equal object  why do you need the one from the set  If it is  equal  only by a key  an Map would be a better choice   Anyway  the following will do it   Foo getEqual Foo sample  Set lt Foo gt  all      for  Foo one   all        if  one equals sample           return one               return null      With Java 8 this can become a one liner   return all stream   filter sample  equals  findAny   orElse null

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Convert set to list  and then use get method of list  Set lt Foo gt  set        List lt Foo gt  list   new ArrayList lt Foo gt  set   Foo obj   list get 0

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If your set is in fact a NavigableSet lt Foo gt   such as a TreeSet   and Foo implements Comparable lt Foo gt   you can use  Foo bar   set floor foo      or  ceiling if  foo equals bar            use bar         Thanks to  eliran-malka   s comment for the hint

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What about using the Arrays class     import java util Arrays  import java util List  import java util HashSet  import java util Arrays   public class MyClass       public static void main String args              Set mySet   new HashSet            mySet add  one            mySet add  two            List list   Arrays asList mySet toArray             Object o0   list get 0           Object o1   list get 1           System out println  items     o0     o1             output  items one two

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Been there done that   If you are using Guava a quick way to convert it to a map is   Map lt Integer Foo gt  map   Maps uniqueIndex fooSet  Foo  getKey

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Try using an array   ObjectClass   arrayName   SetOfObjects toArray new ObjectClass setOfObjects size

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Yes  use HashMap     but in a specialised way  the trap I foresee in trying to use a HashMap as a pseudo-Set is the possible confusion between  actual  elements of the Map Set  and  candidate  elements  i e  elements used to test whether an equal element is already present   This is far from foolproof  but nudges you away from the trap   class SelfMappingHashMap lt V gt  extends HashMap lt V  V gt        Override     public String toString               otherwise you get lots of      object1 object1  object2 object2     stuff         return keySet   toString                Override     public V get  Object key            throw new UnsupportedOperationException   use tryToGetRealFromCandidate                  Override     public V put  V key  V value              thorny issue here  if you were indavertently to  put            a  candidate instance  with the element already in the  Map Set              these will obviously be considered equivalent         assert key equals  value           return super put  key  value               public V tryToGetRealFromCandidate  V key            return super get key             Then do this   SelfMappingHashMap lt SomeClass gt  selfMap   new SelfMappingHashMap lt SomeClass gt         SomeClass candidate   new SomeClass    if  selfMap contains  candidate          SomeClass realThing   selfMap tryToGetRealFromCandidate  candidate                realThing useInSomeWay          But    you now want the candidate to self-destruct in some way unless the programmer actually immediately puts it in the Map Set    you d want contains to  taint  the candidate so that any use of it unless it joins the Map makes it  anathema    Perhaps you could make SomeClass implement a new Taintable interface   A more satisfactory solution is a GettableSet  as below  However  for this to work you have either to be in charge of the design of SomeClass in order to make all constructors non-visible  or    able and willing to design and use a wrapper class for it    public interface NoVisibleConstructor          again  this is a  nudge  technique  in the sense that there is no known method of         making an interface enforce  no visible constructor  in its implementing classes         - of course when Java finally implements full multiple inheritance some reflection         technique might be used        NoVisibleConstructor addOrGetExisting  GettableSet lt   extends NoVisibleConstructor gt  gettableSet        public interface GettableSet lt V extends NoVisibleConstructor gt  extends Set lt V gt        V getGenuineFromImpostor  V impostor       see below for naming     Implementation    public class GettableHashSet lt V extends NoVisibleConstructor gt  implements GettableSet lt V gt        private Map lt V  V gt  map   new HashMap lt V  V gt           Override     public V getGenuineFromImpostor V impostor             return map get  impostor                Override     public int size             return map size                Override     public boolean contains Object o            return map containsKey  o                Override     public boolean add V e            assert e    null          V result   map put  e   e            return result    null              Override     public boolean remove Object o            V result   map remove  o            return result    null              Override     public boolean addAll Collection lt   extends V gt  c               for example          throw new UnsupportedOperationException                Override     public void clear             map clear                  implement the other methods from Set         Your NoVisibleConstructor classes then look like this   class SomeClass implements NoVisibleConstructor        private SomeClass  Object param1  Object param2                              static SomeClass getOrCreate  GettableSet lt SomeClass gt  gettableSet  Object param1  Object param2             SomeClass candidate   new SomeClass  param1  param2            if  gettableSet contains candidate                    obviously this then means that the candidate  fails   or is revealed                to be an  impostor  if you will    Return the existing element              return gettableSet getGenuineFromImpostor candidate                     gettableSet add  candidate            return candidate              Override     public NoVisibleConstructor addOrGetExisting  GettableSet lt   extends NoVisibleConstructor gt  gettableSet              more elegant implementation-hiding  see below           PS one technical issue with such a NoVisibleConstructor class  it may be objected that such a class is inherently final  which may be undesirable  Actually you could always add a dummy parameterless protected constructor   protected SomeClass        throw new UnsupportedOperationException            which would at least let a subclass compile  You d then have to think about whether you need to include another getOrCreate   factory method in the subclass   Final step is an abstract base class  NB  element  for a list   member  for a set  like this for your set members  when possible - again  scope for using a wrapper class where the class is not under your control  or already has a base class  etc    for maximum implementation-hiding   public abstract class AbstractSetMember implements NoVisibleConstructor        Override     public NoVisibleConstructor             addOrGetExisting GettableSet lt   extends NoVisibleConstructor gt  gettableSet            AbstractSetMember member   this           SuppressWarnings  unchecked      unavoidable          GettableSet lt AbstractSetMembers gt  set    GettableSet lt AbstractSetMember gt   gettableSet          if  gettableSet contains  member                  member   set getGenuineFromImpostor  member                cleanUpAfterFindingGenuine  set              else               addNewToSet  set                      return member             abstract public void addNewToSet GettableSet lt   extends AbstractSetMember gt  gettableSet        abstract public void cleanUpAfterFindingGenuine GettableSet lt   extends AbstractSetMember gt  gettableSet            usage is fairly obvious  inside your SomeClass s static factory method    SomeClass setMember   new SomeClass  param1  param2   addOrGetExisting  set

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There would be no point of getting the element if it is equal  A Map is better suited for this usecase     If you still want to find the element you have no other option but to use the iterator   public static void main String   args         Set lt Foo gt  set   new HashSet lt Foo gt         set add new Foo  Hello          for  Iterator lt Foo gt  it   set iterator    it hasNext                Foo f   it next            if  f equals new Foo  Hello                 System out println  foo found             static class Foo       String string      Foo String string            this string   string             Override     public int hashCode              return string hashCode                Override     public boolean equals Object obj            return string equals   Foo  obj  string

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If you want nth Element from HashSet  you can go with below solution   here i have added object of ModelClass in HashSet   ModelClass m1   null  int nth scanner nextInt    for int index 0 index lt hashset1 size   index         m1    ModelClass  itr next        if nth    index            System out println m1           break

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you can use Iterator class  import java util Iterator  import java util HashSet   public class MyClass    public static void main String    args     HashSet lt String gt  animals   new HashSet lt String gt     animals add  fox    animals add  cat    animals add  dog    animals add  rabbit     Iterator lt String gt  it   animals iterator    while it hasNext        String value   it next      System out println value

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Here s what you can do if you have a NavigableSet  e g  a TreeSet   public static  lt E gt  E get NavigableSet lt E gt  set  E key        return set tailSet key  true  floor key      The things are slightly trickier for HashSet and its descendants like LinkedHashSet  import java util    import java lang reflect Field  import java lang reflect Method   public class Test       private static final Field mapField      private static final Method hashMethod      private static final Method getNodeMethod      private static final Field keyField      static           try               mapField   HashSet class getDeclaredField  quot map quot                mapField setAccessible true               hashMethod   HashMap class getDeclaredMethod  quot hash quot   Object class               hashMethod setAccessible true               getNodeMethod   HashMap class getDeclaredMethod  quot getNode quot                       Integer TYPE  Object class               getNodeMethod setAccessible true               keyField   Class forName  quot java util HashMap Node quot   getDeclaredField  quot key quot                keyField setAccessible true             catch  ReflectiveOperationException e                throw new RuntimeException e                        public static  lt E gt  E get HashSet lt E gt  set  E key            try               Object map   mapField get set               Object hash   hashMethod invoke null  key               Object node   getNodeMethod invoke map  hash  key               if  node    null                  return null               SuppressWarnings  quot unchecked quot               E result    E keyField get node               return result            catch  ReflectiveOperationException e                throw new RuntimeException e                        public static  lt E gt  E get NavigableSet lt E gt  set  E key            return set tailSet key  true  floor key              public static void main String   args            HashSet lt Integer gt  s   new HashSet lt  gt             HashSet lt Integer gt  s   new LinkedHashSet lt  gt             TreeSet lt Integer gt  s   new TreeSet lt  gt             for  int i   0  i  lt  100 000  i                s add i           Integer key   java awt event KeyEvent VK FIND          Integer hidden   get s  key           System out println key           System out println hidden           System out println key equals hidden            System out println key    hidden

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Quick helper method that might address this situation    lt T gt  T onlyItem Collection lt T gt  items        if  items size      1          throw new IllegalArgumentException  Collection must have single item  instead it has     items size          return items iterator   next

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With Java 8 you can do   Foo foo   set stream   filter item- gt item equals theItemYouAreLookingFor   findFirst   get      But be careful   get   throws a NoSuchElementException  or you can manipulate a Optional item

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The contract of the hash code makes clear that        If two objects are equal according to the Object method  then calling the hashCode method on each of the two objects must produce the same integer result     So your assumption        To clarify  the equals method is overridden  but it only checks one of   the fields  not all  So two Foo objects that are considered equal can   actually have different values  that s why I can t just use foo      is wrong and you are breaking the contract  If we look at the  contains  method of Set interface  we have that       boolean contains Object o      Returns true if this set contains the specified element  More   formally  returns true if and only if this set  contains an element    e  such that o  null   e  null   o equals e    To accomplish what you want  you can use a Map where you define the key and store your element with the key that defines how objects are different or equal to each other

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Why   It seems that Set plays a useful role in providing a means of comparison  It is designed not to store duplicate elements   Because of this intention design  if one were to get   a reference to the stored object  then mutate it  it is possible that the design intentions of Set could be thwarted and could cause unexpected behavior   From the JavaDocs     Great care must be exercised if mutable objects are used as set elements  The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set    How   Now that Streams have been introduced one can do the following  mySet stream    filter object - gt  object property equals myProperty    findFirst   get

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You better use the Java HashMap object for that purpose http   download oracle com javase 1 5 0 docs api java util HashMap html

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Object objectToGet       Map lt Object  Object gt  map   new HashMap lt Object  Object gt  set size     for  Object o   set        map put o  o     Object objectFromSet   map get objectToGet     If you only do one get this will not be very performing because you will loop over all your elements but when performing multiple retrieves on a big set you will notice the difference

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To answer the precise question  Why doesn t Set provide an operation to get an element that equals another element    the answer would be  because the designers of the collection framework were not very forward looking  They didn t anticipate your very legitimate use case  naively tried to  model the mathematical set abstraction   from the javadoc  and simply forgot to add the useful get   method   Now to the implied question  how do you get the element then   I think the best solution is to use a Map lt E E gt  instead of a Set lt E gt   to map the elements to themselves  In that way  you can efficiently retrieve an element from the  set   because the get   method of the Map will find the element using an efficient hash table or tree algorithm  If you wanted  you could write your own implementation of Set that offers the additional get   method  encapsulating the Map   The following answers are in my opinion bad or wrong    You don t need to get the element  because you already have an equal object   the assertion is wrong  as you already showed in the question  Two objects that are equal still can have different state that is not relevant to the object equality  The goal is to get access to this state of the element contained in the Set  not the state of the object used as a  query     You have no other option but to use the iterator   that is a linear search over a collection which is totally inefficient for large sets  ironically  internally the Set is organized as hash map or tree that could be queried efficiently   Don t do it  I have seen severe performance problems in real-life systems by using that approach  In my opinion what is terrible about the missing get   method is not so much that it is a bit cumbersome to work around it  but that most programmers will use the linear search approach without thinking of the implications

[java] Getting an element from a Set

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