How to get the unique ID of an object which overrides hashCode

Question

When a class in Java doesn t override hashCode     printing an instance of this class gives a nice unique number   The Javadoc of Object says about hashCode        As much as is reasonably practical  the hashCode method defined by class Object does return distinct integers for distinct objects    But when the class overrides hashCode    how can I get its unique number

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I came up with this solution which works in my case where I have objects created on multiple threads and are serializable   public abstract class ObjBase implements Serializable     private static final long serialVersionUID   1L      private static final AtomicLong atomicRefId   new AtomicLong            transient field is not serialized     private transient long refId          default constructor will be called on base class even during deserialization     public ObjBase            refId   atomicRefId incrementAndGet              public long getRefId             return refId

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Maybe this quick  dirty solution will work   public class A       static int UNIQUE ID   0      int uid     UNIQUE ID       public int hashCode             return uid            This also gives the number of instance of a class being initialized

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looking for that last hex  org joda DateTime 57110da6   If you re looking into the hashcode Java types when you do a  toString   on an object the underlying code is this   Integer toHexString hashCode

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The javadoc for Object specifies that      This is typically implemented by converting the internal address of the object into an integer  but this implementation technique is not required by the JavaTM programming language    If a class overrides hashCode  it means that it wants to generate a specific id  which will  one can hope  have the right behaviour   You can use System identityHashCode to get that id for any class

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There is a difference between hashCode   and identityHashCode   returns  It is possible that for two unequal  tested with     objects o1  o2 hashCode   can be the same  See the example below how this is true   class SeeDifferences       public static void main String   args                String s1    stackoverflow           String s2   new String  stackoverflow            String s3    stackoverflow           System out println s1 hashCode             System out println s2 hashCode             System out println s3 hashCode             System out println System identityHashCode s1            System out println System identityHashCode s2            System out println System identityHashCode s3            if  s1    s2                        System out println  s1 and s2 equal                       else                       System out println  s1 and s2 not equal                      if  s1    s3                        System out println  s1 and s3 equal                      else                       System out println  s1 and s3 not equal

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If it s a class that you can modify  you could declare a class variable static java util concurrent atomic AtomicInteger nextInstanceId   You ll have to give it an initial value in the obvious way   Then declare an instance variable int instanceId   nextInstanceId getAndIncrement

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System identityHashCode yourObject  will give the  original  hash code of yourObject as an integer  Uniqueness isn t necessarily guaranteed  The Sun JVM implementation will give you a value which is related to the original memory address for this object  but that s an implementation detail and you shouldn t rely on it   EDIT  Answer modified following Tom s comment below re  memory addresses and moving objects

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Just to augment the other answers from a different angle   If you want to reuse hashcode s  from  above  and derive new ones using your class  immutatable state  then a call to super will work   While this may may not cascade all the way up to Object  i e  some ancestor may not call super   it will allow you to derive hashcodes by reuse    Override public int hashCode         int ancestorHash   super hashCode           now derive new hash from ancestorHash plus immutable instance vars  id fields

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I had the same issue and was not satisfied with any of the answers so far since none of them guaranteed unique IDs   I too wanted to print object IDs for debugging purposed  I knew there must be some way to do it  because in the Eclipse debugger  it specifies unique IDs for each object   I came up with a solution based on the fact that the      operator for objects only returns true if the two objects are actually the same instance   import java util HashMap  import java util Map           Utility for assigning a unique ID to objects and fetching objects given     a specified ID     public class ObjectIDBank           Singleton instance       private static ObjectIDBank instance          Counting value to ensure unique incrementing IDs       private long nextId   1           Map from ObjectEntry to the objects corresponding ID       private Map lt ObjectEntry  Long gt  ids   new HashMap lt ObjectEntry  Long gt              Map from assigned IDs to their corresponding objects        private Map lt Long  Object gt  objects   new HashMap lt Long  Object gt             Private constructor to ensure it is only instantiated by the singleton pattern       private ObjectIDBank             Fetches the singleton instance of ObjectIDBank        public static ObjectIDBank instance             if instance    null              instance   new ObjectIDBank             return instance                 Fetches a unique ID for the specified object  If this method is called multiple        times with the same object  it is guaranteed to return the same value  It is also guaranteed        to never return the same value for different object instances  until we run out of IDs that can        be represented by a long of course          param obj The object instance for which we want to fetch an ID         return Non zero unique ID or 0 if obj    null             public long getId Object obj             if obj    null              return 0           ObjectEntry objEntry   new ObjectEntry obj            if  ids containsKey objEntry                 ids put objEntry  nextId               objects put nextId    obj                      return ids get objEntry                         Fetches the object that has been assigned the specified ID  or null if no object is        assigned the given id         param id Id of the object         return The corresponding object or null             public Object getObject long id            return objects get id                          Wrapper around an Object used as the key for the ids map  The wrapper is needed to        ensure that the equals method only returns true if the two objects are the same instance        and to ensure that the hash code is always the same for the same instance              private class ObjectEntry           private Object obj               Instantiates an ObjectEntry wrapper around the specified object           public ObjectEntry Object obj                this obj   obj                          Returns true if and only if the objects contained in this wrapper and the other            wrapper are the exact same object  same instance  not just equivalent             Override         public boolean equals Object other                return obj      ObjectEntry other  obj                                     Returns the contained object s identityHashCode  Note that identityHashCode values            are not guaranteed to be unique from object to object  but the hash code is guaranteed to            not change over time for a given instance of an Object                       Override         public int hashCode                 return System identityHashCode obj                       I believe that this should ensure unique IDs throughout the lifetime of the program  Note  however  that you probably don t want to use this in a production application because it maintains references to all of the objects for which you generate IDs  This means that any objects for which you create an ID will never be garbage collected   Since I m using this for debug purposes  I m not too concerned with the memory being freed   You could modify this to allow clearing Objects or removing individual objects if freeing memory is a concern

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hashCode   method is not for providing a unique identifier for an object  It rather digests the object s state  i e  values of member fields  to a single integer  This value is mostly used by some hash based data structures like maps and sets to effectively store and retrieve objects   If you need an identifier for your objects  I recommend you to add your own method instead of overriding hashCode  For this purpose  you can create a base interface  or an abstract class  like below   public interface IdentifiedObject lt I gt        I getId        Example usage   public class User implements IdentifiedObject lt Integer gt        private Integer studentId       public User Integer studentId            this studentId   studentId              Override     public Integer getId             return studentId

[java] How to get the unique ID of an object which overrides hashCode()?

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