How do I copy an object in Java

Question

Consider the code below   DummyBean dum   new DummyBean    dum setDummy  foo    System out println dum getDummy        prints  foo   DummyBean dumtwo   dum  System out println dumtwo getDummy        prints  foo   dum setDummy  bar    System out println dumtwo getDummy        prints  bar  but it should print  foo    So  I want to copy the dum to dumtwo and change dum without affecting the dumtwo  But the code above is not doing that  When I change something in dum  the same change is happening in dumtwo also   I guess  when I say dumtwo   dum  Java copies the reference only  So  is there any way to create a fresh copy of dum and assign it to dumtwo

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Use a deep cloning utility    SomeObjectType copy   new Cloner   deepClone someObject     This will deep copy any java object  check it out at https   github com kostaskougios cloning

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Yes  You need to Deep Copy your object

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Pass the object that you want to copy and get the object you want   private Object copyObject Object objSource            try               ByteArrayOutputStream bos   new ByteArrayOutputStream                ObjectOutputStream oos   new ObjectOutputStream bos               oos writeObject objSource               oos flush                oos close                bos close                byte   byteData   bos toByteArray                ByteArrayInputStream bais   new ByteArrayInputStream byteData               try                   objDest   new ObjectInputStream bais  readObject                  catch  ClassNotFoundException e                    e printStackTrace                            catch  IOException e                e printStackTrace                      return objDest           Now parse the objDest to desired object   Happy Coding

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class DB     private String dummy     public DB DB one        this dummy   one dummy

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In 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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Alternative to egaga s constructor method of copy  You probably already have a POJO  so just add another method copy   which returns a copy of the initialized object   class DummyBean       private String dummyStr      private int dummyInt       public DummyBean String dummyStr  int dummyInt            this dummyStr   dummyStr          this dummyInt   dummyInt             public DummyBean copy             return new DummyBean dummyStr  dummyInt                    Getters  amp  Setters     If you already have a DummyBean and want a copy   DummyBean bean1   new DummyBean  peet   2   DummyBean bean2   bean1 copy        lt -- Create copy of bean1   System out println  bean1      bean1 getDummyStr           bean1 getDummyInt     System out println  bean2      bean2 getDummyStr           bean2 getDummyInt        Change bean1 bean1 setDummyStr  koos    bean1 setDummyInt 88    System out println  bean1      bean1 getDummyStr           bean1 getDummyInt     System out println  bean2      bean2 getDummyStr           bean2 getDummyInt          Output   bean1  peet 2 bean2  peet 2  bean1  koos 88 bean2  peet 2    But both works well  it is ultimately up to you

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Add Cloneable and below code to your class  public Object clone   throws CloneNotSupportedException           return super clone            Use this  clonedObject    YourClass  yourClassObject clone

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Create a copy constructor   class DummyBean     private String dummy     public DummyBean DummyBean another        this dummy   another dummy     you can access           Every object has also a clone method which can be used to copy the object  but don t use it  It s way too easy to create a class and do improper clone method  If you are going to do that  read at least what Joshua Bloch has to say about it in Effective Java

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To do that you have to clone the object in some way  Although Java has a cloning mechanism  don t use it if you don t have to  Create a copy method that does the copy work for you  and then do   dumtwo   dum copy      Here is some more advice on different techniques for accomplishing a copy

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You can deep copy automatically with XStream  from http   x-stream github io       XStream is a simple library to serialize objects to XML and back   again    Add it to your project  if using maven    lt dependency gt       lt groupId gt com thoughtworks xstream lt  groupId gt       lt artifactId gt xstream lt  artifactId gt       lt version gt 1 3 1 lt  version gt                   lt  dependency gt    Then  DummyBean dum   new DummyBean    dum setDummy  foo    DummyBean dumCopy    DummyBean  XSTREAM fromXML XSTREAM toXML dum      With this you have a copy without the need to implement any cloning interface

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Other than explicitly copying  another approach is to make the object immutable  no set or other mutator methods   In this way the question never arises  Immutability becomes more difficult with larger objects  but that other side of that is that it pushes you in the direction of splitting into coherent small objects and composites

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Why is there no answer for using Reflection API   private static Object cloneObject Object obj           try              Object clone   obj getClass   newInstance                for  Field field   obj getClass   getDeclaredFields                      field setAccessible true                   field set clone  field get obj                              return clone           catch Exception e               return null                    It s really simple   EDIT  Include child object via recursion  private static Object cloneObject Object obj           try              Object clone   obj getClass   newInstance                for  Field field   obj getClass   getDeclaredFields                      field setAccessible true                   if field get obj     null    Modifier isFinal field getModifiers                          continue                                    if field getType   isPrimitive      field getType   equals String class                             field getType   getSuperclass   equals Number class                             field getType   equals Boolean class                        field set clone  field get obj                     else                      Object childObj   field get obj                       if childObj    obj                           field set clone  clone                        else                          field set clone  cloneObject field get obj                                                                       return clone           catch Exception e               return null

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Basic  Object Copying in Java   Let us Assume an object- obj1  that contains two objects  containedObj1 and containedObj2     shallow copying  shallow copying creates a new instance of the same class and copies all the fields to the new instance and returns it  Object class provides a clone method and provides support for the shallow copying     Deep copying  A deep copy occurs when an object is copied along with the objects to which it refers  Below image shows obj1 after a deep copy has been performed on it  Not only has obj1 been copied  but the objects contained within it have been copied as well  We can use Java Object Serialization to make a deep copy  Unfortunately  this approach has some problems too detailed examples      Possible Problems  clone is tricky to implement correctly  It s better to use Defensive copying  copy constructors as  egaga reply  or static factory methods    If you have an object  that you know has a public clone   method  but you don   t know the type of the object at compile time  then you have problem  Java has an interface called Cloneable  In practice  we should implement this interface if we want to make an object Cloneable  Object clone is protected  so we must override it with a public method in order for it to be accessible  Another problem arises when we try deep copying of a complex object  Assume that the clone   method of all member object variables also does deep copy  this is too risky of an assumption  You must control the code in all classes    For example org apache commons lang SerializationUtils will have method for Deep clone using serialization Source   If we need to clone Bean then there are couple of utility methods in org apache commons beanutils  Source     cloneBean will Clone a bean based on the available property getters and setters  even if the bean class itself does not implement Cloneable  copyProperties will Copy property values from the origin bean to the destination bean for all cases where the property names are the same

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Yes  you are just making a reference to the object  You can clone the object if it implements Cloneable   Check out this wiki article about copying objects   Refer here  Object copying

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public class MyClass implements Cloneable    private boolean myField  false     and other fields or objects  public MyClass        Override public MyClass clone   throws CloneNotSupportedException      try             MyClass clonedMyClass    MyClass super clone              if you have custom object  then you need create a new one in here        return clonedMyClass        catch  CloneNotSupportedException e           e printStackTrace           return new MyClass                  and in your code   MyClass myClass   new MyClass       do some work with this object MyClass clonedMyClass   myClass clone

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Use gson for duplicating an object  public static  lt T gt T copyObject Object object       Gson gson   new Gson        JsonObject jsonObject   gson toJsonTree object  getAsJsonObject        return gson fromJson jsonObject  Type  object getClass        Assume I have an object person So Person copyPerson   copyObject person    Note  The performance is much slower

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This works too  Assuming model  class UserAccount     public int id     public String name      First add compile  com google code gson gson 2 8 1  to your app gradle  amp  sync  Then  Gson gson   new Gson    updateUser   gson fromJson gson toJson mUser  UserAccount class     You can exclude using a field by using transient keyword after access modifier   Note  This is bad practice  Also don t recommend to use Cloneable or JavaSerialization It s slow and broken  Write copy constructor for best performance ref   Something like  class UserAccount          public int id          public String name            empty constructor         public UserAccount               parameterize constructor         public UserAccount int id  String name                this id   id              this name   name                       copy constructor         public UserAccount UserAccount in               this in id in name                     Test stats of 90000 iteration  Line UserAccount clone   gson fromJson gson toJson aO   UserAccount class   takes 808ms  Line UserAccount clone   new UserAccount aO   takes less than 1ms  Conclusion  Use gson if your boss is crazy and you prefer speed  Use second copy constructor if you prefer quality   You can also use copy constructor code generator plugin in Android Studio

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You can try to implement Cloneable and use the clone   method  however  if you use the clone method you should - by standard - ALWAYS override Object s public Object clone   method

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Here s a decent explanation of clone   if you end up needing it     Here  clone  Java method

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Just follow as below   public class Deletable implements Cloneable       private String str      public Deletable              public void setStr String str           this str   str            public void display            System out println  The String is   str             protected Object clone   throws CloneNotSupportedException           return super clone              and wherever you want to get another object  simple perform cloning  e g    Deletable del   new Deletable    Deletable delTemp    Deletable   del clone       this line will return you an independent                                     object  the changes made to this object will                                     not be reflected to other object

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Deep Cloning is your answer  which requires implementing the Cloneable interface and overriding the clone   method   public class DummyBean implements Cloneable       private String dummy      public void setDummy String dummy          this dummy   dummy           public String getDummy           return dummy            Override    public Object clone   throws CloneNotSupportedException         DummyBean cloned    DummyBean super clone          cloned setDummy cloned getDummy              the above is applicable in case of primitive member types like String           however  in case of non primitive types          cloned setNonPrimitiveType cloned getNonPrimitiveType   clone           return cloned           You will call it like this DummyBean dumtwo   dum clone

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If you can add an annotation to the source file  an annotation processor or code generator like this one can be used   import net zerobuilder BeanBuilder   BeanBuilder public class DummyBean         bean stuff     A class DummyBeanBuilders will be generates  which has a static method dummyBeanUpdater to create shallow copies  the same way as you would do it manually   DummyBean bean   new DummyBean       Call some setters        Now make a copy DummyBean copy   DummyBeanBuilders dummyBeanUpdater bean  done

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I use Google s JSON library to serialize it then create a new instance of the serialized object  It does deep copy with a few restrictions    there can t be any recursive references it won t copy arrays of disparate types arrays and lists should be typed or it won t find the class to instantiate you may need to encapsulate strings in a class you declare yourself   I also use this class to save user preferences  windows and whatnot to be reloaded at runtime  It is very easy to use and effective   import com google gson     public class SerialUtils                                                                                           public static String serializeObject Object o        Gson gson   new Gson        String serializedObject   gson toJson o       return serializedObject                                                                                           public static Object unserializeObject String s  Object o       Gson gson   new Gson        Object object   gson fromJson s  o getClass         return object                                                                                                 public static Object cloneObject Object o       String s   serializeObject o       Object object   unserializeObject s o       return object

[java] How do I copy an object in Java?

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