How to clone ArrayList and also clone its contents

Question

How can I clone an ArrayList and also clone its items in Java   For example I have   ArrayList lt Dog gt  dogs   getDogs    ArrayList lt Dog gt  clonedList       something to do with dogs       And I would expect that objects in clonedList are not the same as in dogs list

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I have found a way  you can use json to serialize unserialize the list  The serialized list holds no reference to the original object when unserialized   Using gson   List lt CategoryModel gt  originalList   new ArrayList lt  gt        add some items later String listAsJson   gson toJson originalList   List lt CategoryModel gt  newList   new Gson   fromJson listAsJson  new TypeToken lt List lt CategoryModel gt  gt       getType       You can do that using jackson and any other json library too

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Some other alternatives for copying ArrayList as a Deep Copy Alernative 1 - Use of external package commons-lang3  method SerializationUtils clone    SerializationUtils clone    Let s say we have a class dog where the fields of the class are mutable and at least one field is an object of type String and mutable - not a primitive data type  otherwise shallow copy would be enough   Example of shallow copy  List lt Dog gt  dogs   getDogs       We assume it returns a list of Dogs List lt Dog gt  clonedDogs   new ArrayList lt  gt  dogs    Now back to deep copy of dogs  The Dog class does only have mutable fields  Dog class  public class Dog implements Serializable       private String name      private int age       public Dog                Class with only mutable fields          this name    quot NO NAME quot           this age   -1             public String getName             return name             public void setName String name            this name   name             public int getAge             return age             public void setAge int age            this age   age              Override     public String toString             return  quot Dog  quot                     quot name   quot    name                           quot   age  quot    age                                 Note that the  class Dog implements Serializable  This makes it possible to utilize method   quot SerializationUtils clone dog  quot  Read the comments in the main method to understand the outcome  It shows that we have successfully made a deep copy of ArrayList    See below  quot SerializationUtils clone dog  quot  in context  public static void main String   args        Dog dog1   new Dog        dog1 setName  quot Buddy quot        dog1 setAge 1        Dog dog2   new Dog        dog2 setName  quot Milo quot        dog2 setAge 2        List lt Dog gt  dogs   new ArrayList lt  gt  Arrays asList dog1 dog2            Output   List dogs   Dog name  Buddy   age 1   Dog name  Milo   age 2        System out println  quot List dogs   quot    dogs           Let s clone and make a deep copy of the dogs  ArrayList with external package commons-lang3      List lt Dog gt  clonedDogs   dogs stream   map dog - gt  SerializationUtils clone dog   collect Collectors toList            Output   Now list dogs are deep copied into list clonedDogs       System out println  quot Now list dogs are deep copied into list clonedDogs  quot            A change on dog1 or dog2 can not impact a deep copy         Let s make a change on dog1 and dog2  and test this        statement      dog1 setName  quot Bella quot        dog1 setAge 3       dog2 setName  quot Molly quot        dog2 setAge 4           The change is made on list dogs         Output   List dogs after change   Dog name  Bella   age 3   Dog name  Molly   age 4        System out println  quot List dogs after change   quot    dogs           There is no impact on list clonedDogs s inner objects after the deep copy         The deep copy of list clonedDogs was successful         If clonedDogs would be a shallow copy we would see the change on the field         quot private String name quot   the change made in list dogs  when setting the names        Bella and Molly         Output clonedDogs          After change in list dogs  no impact change in list clonedDogs  n           Dog name  Buddy   age 1   Dog name  Milo   age 2   n      System out println  quot After change in list dogs  no impact change in list clonedDogs   n quot    clonedDogs      Output  List dogs   Dog name  Buddy   age 1   Dog name  Milo   age 2   Now list dogs are deep copied into list clonedDogs  List dogs after change   Dog name  Bella   age 3   Dog name  Molly   age 4   After change in list dogs  no impact change in list clonedDogs   Dog name  Buddy   age 1   Dog name  Milo   age 2    Comment  Since there is no impact change on list clonedDogs after changing list dogs  then deep copy of ArrayList is successful  Alernative 2 - Use of no external packages  A new method  quot clone   quot  is introduced in the Dog class and  quot implements Serializable quot  is removed compare to alternative 1  clone    Dog class  public class Dog       private String name      private int age       public Dog                Class with only mutable fields          this name    quot NO NAME quot           this age   -1             public String getName             return name             public void setName String name            this name   name             public int getAge             return age             public void setAge int age            this age   age                        Returns a deep copy of the Dog         return new instance of   link Dog              public Dog clone             Dog newDog   new Dog            newDog setName this name           newDog setAge this age           return newDog              Override     public String toString             return  quot Dog  quot                     quot name   quot    name                           quot   age  quot    age                                 Read the comments in the main method below to understand the outcome  It shows that we have successfully made a deep copy of ArrayList    See below  quot clone   quot  method in context  public static void main String   args        Dog dog1   new Dog        dog1 setName  quot Buddy quot        dog1 setAge 1        Dog dog2   new Dog        dog2 setName  quot Milo quot        dog2 setAge 2        List lt Dog gt  dogs   new ArrayList lt  gt  Arrays asList dog1 dog2            Output   List dogs   Dog name  Buddy   age 1   Dog name  Milo   age 2        System out println  quot List dogs   quot    dogs           Let s clone and make a deep copy of the dogs  ArrayList      List lt Dog gt  clonedDogs   dogs stream   map dog - gt  dog clone    collect Collectors toList            Output   Now list dogs are deep copied into list clonedDogs       System out println  quot Now list dogs are deep copied into list clonedDogs  quot            A change on dog1 or dog2 can not impact a deep copy         Let s make a change on dog1 and dog2  and test this        statement      dog1 setName  quot Bella quot        dog1 setAge 3       dog2 setName  quot Molly quot        dog2 setAge 4           The change is made on list dogs         Output   List dogs after change   Dog name  Bella   age 3   Dog name  Molly   age 4        System out println  quot List dogs after change   quot    dogs           There is no impact on list clonedDogs s inner objects after the deep copy         The deep copy of list clonedDogs was successful         If clonedDogs would be a shallow copy we would see the change on the field         quot private String name quot   the change made in list dogs  when setting the names        Bella and Molly         Output clonedDogs          After change in list dogs  no impact change in list clonedDogs  n           Dog name  Buddy   age 1   Dog name  Milo   age 2   n      System out println  quot After change in list dogs  no impact change in list clonedDogs   n quot    clonedDogs      Output  List dogs   Dog name  Buddy   age 1   Dog name  Milo   age 2   Now list dogs are deep copied into list clonedDogs  List dogs after change   Dog name  Bella   age 3   Dog name  Molly   age 4   After change in list dogs  no impact change in list clonedDogs   Dog name  Buddy   age 1   Dog name  Milo   age 2    Comment  Since there is no impact change on list clonedDogs after changing list dogs  then deep copy of ArrayList is successful  Note1  Alternative 1 is much slower than Alternative 2  but easier to mainatain since you do not need to upadate any methods like clone    Note2  For alternative 1 the following maven dependency was used for method  quot SerializationUtils clone   quot    lt  -- https   mvnrepository com artifact org apache commons commons-lang3 -- gt   lt dependency gt       lt groupId gt org apache commons lt  groupId gt       lt artifactId gt commons-lang3 lt  artifactId gt       lt version gt 3 9 lt  version gt   lt  dependency gt   Find more releases of common-lang3 at  https   mvnrepository com artifact org apache commons commons-lang3

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The package import org apache commons lang SerializationUtils   There is a method SerializationUtils clone Object    Example  this myObjectCloned   SerializationUtils clone this object

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for you objects override clone   method  class You class        int a        Override     public You class clone             You class you class   new You class            you class a   this a          return you class            and call  clone   for Vector obj or ArraiList obj

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I have used this option always    ArrayList lt Dog gt  clonedList   new ArrayList lt Dog gt  name of arraylist that you need to Clone

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Basically there are three ways without iterating manually    1  Using constructor  ArrayList lt Dog gt  dogs   getDogs    ArrayList lt Dog gt  clonedList   new ArrayList lt Dog gt  dogs     2 Using addAll Collection lt   extends E gt  c   ArrayList lt Dog gt  dogs   getDogs    ArrayList lt Dog gt  clonedList   new ArrayList lt Dog gt     clonedList addAll dogs     3 Using addAll int index  Collection lt   extends E gt  c  method with int parameter  ArrayList lt Dog gt  dogs   getDogs    ArrayList lt Dog gt  clonedList   new ArrayList lt Dog gt     clonedList addAll 0  dogs     NB   The behavior of these operations will be undefined if the specified collection is modified while the operation is in progress

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The below worked for me    in Dog java  public Class Dog   private String a b   public Dog       no args constructor  public Dog Dog d      copy constructor    this a d a     this b d b         -------------------------   private List lt Dog gt  createCopy List lt Dog gt  dogs     List lt Dog gt  newDogsList  new ArrayList lt  gt      if  CollectionUtils isNotEmpty dogs      dogs stream   forEach dog- gt  newDogsList add  Dog  SerializationUtils clone dog         return newDogsList       Here the new list which got created from createCopy method is created through SerializationUtils clone    So any change made to new list will not affect original list

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I have just developed a lib that is able to clone an entity object and a java util List object  Just download the jar in https   drive google com open id 0B69Sui5ah93EUTloSktFUkctN0U and use the static method cloneListObject List list   This method not only clones the List but also all entity elements

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You will need to iterate on the items  and clone them one by one  putting the clones in your result array as you go   public static List lt Dog gt  cloneList List lt Dog gt  list        List lt Dog gt  clone   new ArrayList lt Dog gt  list size         for  Dog item   list  clone add item clone         return clone      For that to work  obviously  you will have to get your Dog class to implement the Cloneable interface and override the clone   method

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Java 8 provides a new way to call the copy constructor or clone method on the element dogs elegantly and compactly  Streams  lambdas and collectors   Copy constructor   List lt Dog gt  clonedDogs   dogs stream   map Dog  new  collect toList       The expression Dog  new is called a method reference  It creates a function object which calls a constructor on Dog which takes another dog as argument   Clone method 1    List lt Dog gt  clonedDogs   dogs stream   map d - gt  d clone    collect toList         Getting an ArrayList as the result  Or  if you have to get an ArrayList back  in case you want to modify it later    ArrayList lt Dog gt  clonedDogs   dogs stream   map Dog  new  collect toCollection ArrayList  new        Update the list in place  If you don t need to keep the original content of the dogs list you can instead use the replaceAll method and update the list in place   dogs replaceAll Dog  new       All examples assume import static java util stream Collectors        Collector for ArrayLists  The collector from the last example can be made into a util method  Since this is such a common thing to do I personally like it to be short and pretty  Like this   ArrayList lt Dog gt  clonedDogs   dogs stream   map d - gt  d clone    collect toArrayList      public static  lt T gt  Collector lt T     ArrayList lt T gt  gt  toArrayList         return Collectors toCollection ArrayList  new          1  Note on CloneNotSupportedException   For this solution to work the clone method of Dog must not declare that it throws CloneNotSupportedException  The reason is that the argument to map is not allowed to throw any checked exceptions   Like this          Note  Method is public and returns Dog  not Object      Override     public Dog clone      Note  No throws clause here            This should not be a big problem however  since that is the best practice anyway   Effectice Java for example gives this advice    Thanks to Gustavo for noting this     PS   If you find it prettier you can instead use the method reference syntax to do exactly the same thing   List lt Dog gt  clonedDogs   dogs stream   map Dog  clone  collect toList

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You will need to clone the ArrayList by hand  by iterating over it and copying each element to a new ArrayList   because clone   will not do it for you  Reason for this is that the objects contained in the ArrayList may not implement Clonable themselves   Edit      and that is exactly what Varkhan s code does

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List lt Dog gt  dogs  List lt Dog gt  copiedDogs   dogs stream   map dog - gt  SerializationUtils clone dog   Collectors toList       This will deep copy each dog

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I think the current green answer is bad   why you might ask    It can require to add a lot of code It requires you to list all Lists to be copied and do this   The way serialization is also bad imo  you might have to add Serializable all over the place   So what is the solution   Java Deep-Cloning library The cloning library is a small  open source  apache licence  java library which deep-clones objects  The objects don t have to implement the Cloneable interface  Effectivelly  this library can clone ANY java objects  It can be used i e  in cache implementations if you don t want the cached object to be modified or whenever you want to create a deep copy of objects    Cloner cloner new Cloner    XX clone   cloner deepClone someObjectOfTypeXX     Check it out at https   github com kostaskougios cloning

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I think I found a really easy way to make a deep copy ArrayList   Assuming you want to copy a String ArrayList arrayA   ArrayList lt String gt arrayB   new ArrayList lt String gt     arrayB addAll arrayA     Let me know if it doesn t work for you

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Simple way is ArrayList lt Dog gt  dogs   getDogs    ArrayList lt Dog gt  clonedList   new ArrayList lt Dog gt  dogs

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Here is a solution using a generic template type   public static  lt T gt  List lt T gt  copyList List lt T gt  source        List lt T gt  dest   new ArrayList lt T gt         for  T item   source    dest add item         return dest

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I  personally  would add a constructor to Dog   class Dog       public Dog                  Regular constructor      public Dog Dog dog               Copy all the fields of Dog            Then just iterate  as shown in Varkhan s answer    public static List lt Dog gt  cloneList List lt Dog gt  dogList        List lt Dog gt  clonedList   new ArrayList lt Dog gt  dogList size         for  Dog dog   dogList            clonedList add new Dog dog              return clonedList      I find the advantage of this is you don t need to screw around with the broken Cloneable stuff in Java   It also matches the way that you copy Java collections   Another option could be to write your own ICloneable interface and use that   That way you could write a generic method for cloning

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Easy way by using commons-lang-2 3 jar that library of java to clone list  link download commons-lang-2 3 jar  How to use  oldList          List lt YourObject gt  newList   new ArrayList lt YourObject gt     foreach YourObject obj   oldList      newList add  YourObject SerializationUtils clone obj        I hope this one can helpful    D

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All standard collections have copy constructors  Use them   List lt Double gt  original      some list List lt Double gt  copy   new ArrayList lt Double gt  original     This does a shallow copy   clone   was designed with several mistakes  see this question   so it s best to avoid it   From Effective Java 2nd Edition  Item 11  Override clone judiciously     Given all of the problems associated with Cloneable  it   s safe to say    that other interfaces should not extend it  and that classes   designed for inheritance  Item 17  should not implement it  Because of   its many shortcomings  some expert programmers simply choose never to   override the clone method and never to invoke it except  perhaps  to   copy arrays  If you design a class for inheritance  be aware that if   you choose not to provide a well-behaved protected clone method  it   will be impossible for subclasses to implement Cloneable    This book also describes the many advantages copy constructors have over Cloneable clone    They don t rely on a risk-prone extralinguistic object creation mechanism They don t demand unenforceable adherence to thinly documented conventions They don t conflict with the proper use of final fields They don t throw unnecessary checked exceptions They don t require casts    Consider another benefit of using copy constructors  Suppose you have a HashSet s  and you want to copy it as a TreeSet  The clone method can   t offer this functionality  but it   s easy with a conversion constructor  new TreeSet s

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A nasty way is to do it with reflection  Something like this worked for me   public static  lt T extends Cloneable gt  List lt T gt  deepCloneList List lt T gt  original        if  original    null    original size    lt  1            return new ArrayList lt  gt                try           int originalSize   original size            Method cloneMethod   original get 0  getClass   getDeclaredMethod  clone            List lt T gt  clonedList   new ArrayList lt  gt                 noinspection ForLoopReplaceableByForEach         for  int i   0  i  lt  originalSize  i                     noinspection unchecked             clonedList add  T  cloneMethod invoke original get i                       return clonedList        catch  NoSuchMethodException   InvocationTargetException   IllegalAccessException e            System err println  Couldn t clone list due to     e getMessage             return new ArrayList lt  gt

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The other posters are correct  you need to iterate the list and copy into a new list   However    If the objects in the list are immutable - you don t need to clone them  If your object has a complex object graph - they will need to be immutable as well    The other benefit of immutability is that they are threadsafe as well

[java] How to clone ArrayList and also clone its contents?

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