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

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.

Answer 1

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