What happens when a duplicate key is put into a HashMap

Question

If I pass the same key multiple times to HashMap   s put method  what happens to the original value  And what if even the value repeats  I didn   t find any documentation on this   Case 1  Overwritten values for a key  Map mymap   new HashMap    mymap put  1   one    mymap put  1   not one    mymap put  1   surely not one    System out println mymap get  1       We get surely not one   Case 2  Duplicate value  Map mymap   new HashMap    mymap put  1   one    mymap put  1   not one    mymap put  1   surely not one       The following line was added  mymap put  1   one    System out println mymap get  1       We get one   But what happens to the other values  I was teaching basics to a student and I was asked this  Is the Map like a bucket where the last value is referenced  but in memory

User · Answer

By definition  the put command replaces the previous value associated with the given key in the map  conceptually like an array indexing operation for primitive types    The map simply drops its reference to the value   If nothing else holds a reference to the object  that object becomes eligible for garbage collection   Additionally  Java returns any previous value associated with the given key  or null if none present   so you can determine what was there and maintain a reference if necessary   More information here  HashMap Doc

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The prior value for the key is dropped and replaced with the new one   If you d like to keep all the values a key is given  you might consider implementing something like this   import org apache commons collections MultiHashMap  import java util Set  import java util Map  import java util Iterator  import java util List  public class MultiMapExample       public static void main String   args          MultiHashMap mp new MultiHashMap          mp put  a   10         mp put  a   11         mp put  a   12         mp put  b   13         mp put  c   14         mp put  e   15         List list   null         Set set   mp entrySet          Iterator i   set iterator          while i hasNext               Map Entry me    Map Entry i next             list  List mp get me getKey               for int j 0 j lt list size   j                         System out println me getKey      value    list get j

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BTW  if you want some semantics such as only put if this key is not exist  you can use concurrentHashMap with putIfAbsent   function  Check this out   https   docs oracle com javase 7 docs api java util concurrent ConcurrentHashMap html put K  20V   concurrentHashMap  is thread safe with high performance since it uses  lock striping  mechanism to improve the throughput

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You may find your answer in the javadoc of Map put K  V   which actually returns something     public V put K key               V value        Associates the specified value with the specified key in this map    optional operation   If the map   previously contained a mapping for   this key  the old value is replaced by   the specified value   A map m is said   to contain a mapping for a key k if   and only if m containsKey k  would   return true        Parameters            key - key with which the specified value is to be associated            value - value to be associated with the specified key        Returns            previous value associated with specified key  or null if there was no   mapping for key   A null return can also indicate that the map previously associated null with the specified key  if the implementation supports null values     So if you don t assign the returned value when calling mymap put  1    a string    it just becomes unreferenced and thus eligible for garbage collection

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Associates the specified value with the specified key in this map  If the map previously contained a mapping for the key  the old value is replaced

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Maps from JDK are not meant for storing data under duplicated keys    At best new value will override the previous ones  Worse scenario is exception  e g when you try to collect it as a stream     No duplicates   Stream of  one   collect Collectors toMap x - gt  x  x - gt  x       Ok  You will get   2      one one    Duplicated stream   Stream of  one    not one    surely not one   collect Collectors toMap x - gt  1  x - gt  x       Exception java lang IllegalStateException  Duplicate key 1  attempted merging values one and not one             at Collectors duplicateKeyException  Collectors java 133             at Collectors lambda uniqKeysMapAccumulator 1  Collectors java 180             at ReduceOps 3ReducingSink accept  ReduceOps java 169             at Spliterators ArraySpliterator forEachRemaining  Spliterators java 948             at AbstractPipeline copyInto  AbstractPipeline java 484             at AbstractPipeline wrapAndCopyInto  AbstractPipeline java 474             at ReduceOps ReduceOp evaluateSequential  ReduceOps java 913             at AbstractPipeline evaluate  AbstractPipeline java 234             at ReferencePipeline collect  ReferencePipeline java 578             at   4 1    To deal with duplicated keys - use other package  e g  https   google github io guava releases 19 0 api docs com google common collect Multimap html  There is a lot of other implementations dealing with duplicated keys  Those are needed for web  e g  duplicated cookie keys  Http headers can have same fields        Good luck

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HashMap lt Emp  Emp gt  empHashMap   new HashMap lt Emp  Emp gt               empHashMap put new Emp 1   new Emp 1             empHashMap put new Emp 1   new Emp 1             empHashMap put new Emp 1   new Emp              empHashMap put new Emp 1   new Emp              System out println empHashMap size              class Emp      public Emp                 public Emp int id           this id   id            public int id       Override     public boolean equals Object obj            return this id      Emp obj  id              Override     public int hashCode             return id            OUTPUT   is 1   Means hash map wont allow duplicates  if you have properly overridden equals and hashCode   methods    HashSet also uses HashMap internally  see the source doc  public class HashSet  public HashSet             map   new HashMap lt  gt

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Yes  this means all the 1 keys with value are overwriten with the last added value and here you add  surely not one  so it will display only  surely not one    Even if you are trying to display with a loop  it will also only display one key and value which have same key

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It replaces the existing value in the map for the respective key  And if no key exists with the same name then it creates a key with the value provided  eg   Map mymap   new HashMap    mymap put  1   one    mymap put  1   two      OUTPUT key    1   value    two   So  the previous value gets overwritten

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it s Key Value feature and you could not to have duplicate key for several values because when you want to get the actual value which one of values is belong to entered keyin your example when you want to get value of  1  which one is it   that s reasons to have unique key for every value but you could to have a trick by java standard lib    import java util ArrayList  import java util HashMap  import java util Map   public class DuplicateMap lt K  V gt         private Map lt K  ArrayList lt V gt  gt  m   new HashMap lt  gt          public void put K k  V v            if  m containsKey k                 m get k  add v             else               ArrayList lt V gt  arr   new ArrayList lt  gt                 arr add v               m put k  arr                         public ArrayList lt V gt  get K k            return m get k              public V get K k  int index            return m get k  size  -1  lt  index   null   m get k  get index             and you could to use it in this way       public static void main String   args        DuplicateMap lt String String gt  dm new DuplicateMap lt  gt         dm put  1    one        dm put  1    not one        dm put  1    surely not one        System out println dm get  1         System out println dm get  1  1        System out println dm get  1   5        and result of prints are     one  not one  surely not one  not one null

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I always used   HashMap lt String  ArrayList lt String gt  gt  hashy   new HashMap lt String  ArrayList lt String gt  gt       if I wanted to apply multiple things to one identifying key    public void MultiHash        HashMap lt String  ArrayList lt String gt  gt  hashy   new HashMap lt String  ArrayList lt String gt  gt         String key    Your key        ArrayList lt String gt  yourarraylist   hashy get key        for String valuessaved2key   yourarraylist           System out println valuessaved2key              you could always do something like this and create yourself a maze   public void LOOK AT ALL THESE HASHMAPS        HashMap lt String  HashMap lt String  HashMap lt String  HashMap lt String  String gt  gt  gt  gt  theultimatehashmap   new HashMap  lt String  HashMap lt String  HashMap lt String  HashMap lt String  String gt  gt  gt  gt         String ballsdeep into the hashmap   theultimatehashmap get  firststring   get  secondstring   get  thirdstring   get  forthstring

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To your question whether the map was like a bucket  no   It s like a list with name value pairs whereas name doesn t need to be a String  it can  though    To get an element  you pass your key to the get  -method which gives you the assigned object in return   And a Hashmap means that if you re trying to retrieve your object using the get-method  it won t compare the real object to the one you provided  because it would need to iterate through its list and compare   the key you provided with the current element   This would be inefficient  Instead  no matter what your object consists of  it calculates a so called hashcode from both objects and compares those  It s easier to compare two ints instead of two entire  possibly deeply complex  objects  You can imagine the hashcode like a summary having a predefined length  int   therefore it s not unique and has collisions  You find the rules for the hashcode in the documentation to which I ve inserted the link   If you want to know more about this  you might wanna take a look at articles on javapractices com and technofundo com  regards

[java] What happens when a duplicate key is put into a HashMap?

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