Copying a HashMap in Java

Question

I am trying to keep a temporary container of a class that contains member   HashMap lt Integer myObject gt  myobjectHashMap  A class called myobjectsList Then I do myobjectsListA   new myobjectsList    myobjectsListB   new myobjectsList     then  Add some hashmap items to A  like 2  then  myobjectListB   myobjectListA    B has 2  then  Add hashmap items to A  like 4 more  then  return A to the items stored in B myobjectListA   myobjectListb   But when I do this  B grows with A while I am adding hashmap items to A  A now has 6 items in it because B had 6  I want A to still have the original 2 at the end after last assignment  In C   I would use copy with objects  what is the java equivalent  Added  OK I left something out explaining this  MyObjectsList does not contain the HashMap  it is derived from a class MyBaseOjbectsList which has the HashMap member and MyObjectsList extends MyBaseOjbectsList  Does this make a difference

User · Answer

The difference is that in C   your object is on the stack  whereas in Java  your object is in the heap  If A and B are Objects  any time in Java you do   B   A   A and B point to the same object  so anything you do to A you do to B and vice versa   Use new HashMap   if you want two different objects   And you can use Map putAll      to copy data between two Maps

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If we want to copy an object in Java  there are two possibilities that we need to consider  a shallow copy and a deep copy    The shallow copy is the approach when we only copy field values  Therefore  the copy might be dependent on the original object  In the deep copy approach  we make sure that all the objects in the tree are deeply copied  so the copy is not dependent on any earlier existing object that might ever change   This question is the perfect definition for the application of the deep copy approach    First  if you have a simple HashMap lt Integer  List lt T gt  gt  map then we just create a workaround like this  Creating a new instance of the List lt T gt    public static  lt T gt  HashMap lt Integer  List lt T gt  gt  deepCopyWorkAround HashMap lt Integer  List lt T gt  gt  original        HashMap lt Integer  List lt T gt  gt  copy   new HashMap lt  gt         for  Map Entry lt Integer  List lt T gt  gt  entry   original entrySet              copy put entry getKey    new ArrayList lt  gt  entry getValue                return copy      This one uses Stream collect   method to create the clone map  but uses the same idea as the previous method   public static  lt T gt  Map lt Integer  List lt T gt  gt  deepCopyStreamWorkAround Map lt Integer  List lt T gt  gt  original        return original              entrySet                stream                collect Collectors toMap Map Entry  getKey  valueMapper - gt  new ArrayList lt  gt  valueMapper getValue              But  if the instances inside T are also mutable objects we have a big problem  In this case a real  deep copy is an alternative that solves this problem  Its advantage is that at least each mutable object in the object graph is recursively copied  Since the copy is not dependent on any mutable object that was created earlier  it won   t get modified by accident like we saw with the shallow copy   To solve that this deep copy implementations will do the work   public class DeepClone       public static void main String   args                Map lt Long  Item gt  itemMap   Stream of                  entry 0L  new Item 2558584                    entry 1L  new Item 254243232                    entry 2L  new Item 986786                    entry 3L  new Item 672542                    entry 4L  new Item 4846                    entry 5L  new Item 76867467                    entry 6L  new Item 986786                    entry 7L  new Item 7969768                    entry 8L  new Item 68868486                    entry 9L  new Item 923                    entry 10L  new Item 986786                    entry 11L  new Item 549768                    entry 12L  new Item 796168                    entry 13L  new Item 868421                    entry 14L  new Item 923                    entry 15L  new Item 986786                    entry 16L  new Item 549768                    entry 17L  new Item 4846                    entry 18L  new Item 4846                    entry 19L  new Item 76867467                    entry 20L  new Item 986786                    entry 21L  new Item 7969768                    entry 22L  new Item 923                    entry 23L  new Item 4846                    entry 24L  new Item 986786                    entry 25L  new Item 549768             collect entriesToMap               Map lt Long  Item gt  clone   DeepClone deepClone itemMap           clone remove 1L           clone remove 2L            System out println itemMap           System out println clone              private DeepClone           public static  lt T gt  T deepClone final T input                if  input    null  return null           if  input instanceof Map lt      gt                 return  T  deepCloneMap  Map lt      gt   input             else if  input instanceof Collection lt   gt                 return  T  deepCloneCollection  Collection lt   gt   input             else if  input instanceof Object                  return  T  deepCloneObjectArray  Object    input             else if  input getClass   isArray                  return  T  clonePrimitiveArray  Object  input                      return input             private static Object clonePrimitiveArray final Object input                final int length   Array getLength input           final Object output   Array newInstance input getClass   getComponentType    length           System arraycopy input  0  output  0  length           return output             private static  lt E gt  E   deepCloneObjectArray final E   input                final E   clone    E    Array newInstance input getClass   getComponentType    input length           for  int i   0  i  lt  input length  i                  clone i    deepClone input i                       return clone             private static  lt E gt  Collection lt E gt  deepCloneCollection final Collection lt E gt  input                Collection lt E gt  clone          if  input instanceof LinkedList lt   gt                 clone   new LinkedList lt  gt               else if  input instanceof SortedSet lt   gt                 clone   new TreeSet lt  gt               else if  input instanceof Set                clone   new HashSet lt  gt               else               clone   new ArrayList lt  gt                        for  E item   input                clone add deepClone item                       return clone             private static  lt K  V gt  Map lt K  V gt  deepCloneMap final Map lt K  V gt  map                Map lt K  V gt  clone          if  map instanceof LinkedHashMap lt      gt                 clone   new LinkedHashMap lt  gt               else if  map instanceof TreeMap lt      gt                 clone   new TreeMap lt  gt               else               clone   new HashMap lt  gt                        for  Map Entry lt K  V gt  entry   map entrySet                  clone put deepClone entry getKey     deepClone entry getValue                         return clone

User · Answer

In Java  when you write   Object objectA   new Object    Object objectB   objectA    objectA and objectB are the same and point to the same reference  Changing one will change the other  So if you change the state of objectA  not its reference  objectB will reflect that change too   However  if you write   objectA   new Object     Then objectB is still pointing to the first object you created  original objectA  while objectA is now pointing to a new Object

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Java supports shallow not deep  copy concept You can archive it using   constructor clone   putAll

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Since Java 10 it is possible to use   Map copyOf   for creating a shallow copy  which is also immutable   Here is its Javadoc   For a deep copy  as mentioned in this answer you  need some kind of value mapper to make a safe copy of values  You don t need to copy keys though  since they must be immutable

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There is a small  HUGE  understatement here  If you want to copy a HashMap with nested structures  HashMap putAll   will copy by reference  because it doesn t know how to exactly copy your object  For example   import java util    class Playground       public static void main String    args            Map lt Integer  Map lt Integer List lt Float gt  gt  gt  dataA   new HashMap lt  gt             Map lt Integer  Map lt Integer List lt Float gt  gt  gt  dataB   new HashMap lt  gt              dataA put 1  new HashMap lt  gt              dataB putAll dataA            assert dataB get 1  size      0            dataA get 1  put 2  new ArrayList lt  gt               if  dataB get 1  size      1       true             System out println                   Sorry object reference was copied - not the values                        So basically you will need to copy the fields yourself like here  List  lt Float gt  aX   new ArrayList lt  gt  accelerometerReadingsX   List  lt Float gt  aY   new ArrayList lt  gt  accelerometerReadingsY    List  lt Float gt  gX   new ArrayList lt  gt  gyroscopeReadingsX   List  lt Float gt  gY   new ArrayList lt  gt  gyroscopeReadingsY    Map lt Integer  Map lt Integer  Float gt  gt  readings   new HashMap lt  gt      Map lt Integer List lt Float gt  gt  accelerometerReadings   new HashMap lt  gt     accelerometerReadings put X axis  aX   accelerometerReadings put Y axis  aY   readings put Sensor TYPE ACCELEROMETER  accelerometerReadings    Map lt Integer List lt Float gt  gt  gyroscopeReadings   new HashMap lt  gt     gyroscopeReadings put X axis  gX   gyroscopeReadings put Y axis  gY   readings put Sensor TYPE GYROSCOPE  gyroscopeReadings

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What you assign one object to another  all you re doing is copying the reference to the object  not the contents of it  What you need to do is take your object B and manually copy the contents of object A into it    If you do this often  you might consider implementing a clone   method on the class that will create a new object of the same type  and copy all of it s contents into the new object

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If you want a copy of the HashMap you need to construct a new one with   myobjectListB   new HashMap lt Integer myObject gt  myobjectListA     This will create a  shallow  copy of the map

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Since the OP has mentioned he does not have access to the base class inside of which exists a HashMap - I am afraid there are very few options available   One  painfully slow and resource intensive  way of performing a deep copy of an object in Java is to abuse the  Serializable  interface which many classes either intentionally - or unintentionally extend - and then utilise this to serialise your class to ByteStream  Upon de-serialisation you will have a deep copy of the object in question   A guide for this can be found here  https   www avajava com tutorials lessons how-do-i-perform-a-deep-clone-using-serializable html

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Since this question is still unanswered and I had a similar problem  I will try to answer this  The problem  as others already mentioned  is that you just copy references to the same object and thus a modify on the copy will also modify the origin object  So what you have to to is to copy the object  your map value  itself  The far easiest way to do so is to make all your objects implementing the serializeable interface  Then serialize and deserialize your map to get a real copy  You can do this by yourself or use the apache commons SerializationUtils clone   which you can find here   https   commons apache org proper commons-lang javadocs api-2 6 org apache commons lang SerializationUtils html But be aware this is the simplest approach but it is an expensive task to serialize and deserialize a lot of objects

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You can also use  clone     Method to copy all elements from one hashmap to another hashmap   Program for copy all elements from one hashmap to another  import java util HashMap   public class CloneHashMap            public static void main String a                  HashMap hashMap   new HashMap                HashMap hashMap1   new HashMap                hashMap put 1   One            hashMap put 2   Two            hashMap put 3   Three            System out println  Original HashMap       hashMap           hashMap1    HashMap  hashMap clone            System out println  Copied HashMap       hashMap1                     source    http   www tutorialdata com examples java collection-framework hashmap copy-all-elements-from-one-hashmap-to-another

[java] Copying a HashMap in Java

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