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

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.

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

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_
}

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.

I believe .clone() is what you're looking for, so long as the class supports it.

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;       
}

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.

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"

Here are the few techniques I've heard of:

1. 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.

2. 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 ).

3. 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.

4. Use the state pattern.

5. 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