How to copy an object by value not by reference

Question

I want to make a copy of an object  then after some logic  re-assign the original object the value of the copy   example   User userCopy     make a copy  foreach          user Age   1    user ID   -1     UserDao Update user      user   userCopy        I don t want a copy by reference  it has to be a copy by value   The above is just a sample  not how I really want to use it but I need to learn how to copy by value

User · Answer

Here are the few techniques I ve heard of    Use clone   if the class implements Cloneable  This API is a bit flawed in java and I never quite understood why clone is not defined in the interface  but in Object  Still  it might work  Create a clone manually  If there is a constructor that accepts all parameters  it might be simple  e g new User  user ID  user Age         You might even want a constructor that takes a User  new User  anotherUser    Implement something to copy from to a user  Instead of using a constructor  the class may have a method copy  User    You can then first snapshot the object backupUser copy  user   and then restore it user copy  backupUser    You might have a variant with methods named backup restore snapshot  Use the state pattern  Use serialization  If your object is a graph  it might be easier to serialize deserialize it to get a clone    That all depends on the use case  Go for the simplest   EDIT  I also recommend to have a look at these questions    Clone   vs  Copy constructor How to properly override clone method

User · Answer

I believe  clone   is what you re looking for  so long as the class supports it

User · Answer

You may use  clone   which works well if your object has immutable objects and or primitives  but it may be a  little problematic when you don t have these   such as collections   for which you may need to perform a deep clone   User userCopy    User  user clone     make a copy  for            user age   1      user id   -1       UserDao update user      user   userCopy       It seems like you just want to preserve the attributes  age and id which are of type int so  why don t you give it a try and see if it works    For more complex scenarios you could create a  copy  method   publc class User        public static User copy  User other              User newUser   new User             newUser age   other age           newUser id   other id                 etc            return newUser            It should take you about 10 minutes    And then you can use that instead        User userCopy   User copy  user      make a copy         etc     To read more about clone read this chapter in Joshua Bloch  Effective Java  Override clone judiciously

User · Answer

Can t you just make a copy constructor  By the way Java always passes references by value  so you keep pointing to the same object

User · Answer

You need to do a deep copy from user to usercopy  and then after your login you can reassign your userCopy reference to user   User userCopy   new User    userCopy Age   user Age userCopy ID   user ID  foreach            user Age   1     user ID   -1      UserDao Update user          user   userCopy

User · Answer

what language is this  If you re using a language that passes everything by reference like Java  except for native types   typically you can call  clone   method  The  clone   method is typically implemented by copying cloning all relevant instance fields into the new object

User · Answer

I know this is a little bit too late but it might just help someone   In my case I already had a method to make the Object from a json Object and make json from the object  with this you can simply create a new instance of the object and use it to restore  For instance in a function parsing a final object    x000D   x000D  public void update final Object object    x000D    final Object original   Object makeFromJSON object toJSON      x000D       the original is not affected by changes made to object  x000D    x000D   x000D   x000D

[java] How to copy an object by value, not by reference

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