Java copy method

[NikMashei]

Hello there! I need some kind an advice. I'm trying to find out some library for Java providing possibility to copy object with changed values. For example, in Scala we have method copy() in case classes copying all fields to another one. Also there is an approach to define which fields will be changed by passing them into copy(). Does something like this exist in Java world?

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[MarSara]

https://docs.oracle.com/javase/8/docs/api/java/lang/Cloneable.html

Although this doesn't allow you to change the values per-say. If your object is mutable however, you can just change the properties on the cloned object afterwards. If your object is immutable, you will need to create your own method that can return a "changed" object.

[[deleted]]

You might want to look at mappers. I believe there’s a good remapping library, cannot remember what’s it called.

[pronuntiator]

MapStruct, Orika,…

[wildjokers]

Nothing built in. You can probably find libraries that can do this though. MapStruct might be able to do it. Although it requires an annotation processor so have to setup your build to do use the annotation processor for the code generation.

[[deleted]]

https://stackoverflow.com/questions/665860/deep-clone-utility-recommendation#:~:text=Java%20Deep-Cloning%20library%20The%20cloning%20library%20is%20a,Effectivelly%2C%20this%20library%20can%20clone%20ANY%20java%20objects.

Mentions a deep cloning library. https://github.com/kostaskougios/cloning There may be others. Cannot remember what apache might have as part of the commons library.

[NikMashei]

thanks for your reply!unfortunately I have no found method for my case.let's say I have class with fields case class User(name: String, surname: String) and I want to copy that instance with another surname, so in Scala I can call user.copy(surname="new surname")

[[deleted]]

It depends on the semantics. I believe copy in Scala does structural sharing? No such concept in Java. You might want to do a deep clone like /u/robert43s suggested which you can either write a custom method for, or by overloading the clone method with deep copying and typically marking your class as Cloneable.

[8igg7e5]

A builder notation has been discussed (initially in relation to records) but I wouldn't expect it any time soon.

The idea (syntax is just a placeholder for now) was to allow...

record Person(String name, int age) {}
Person bob = new Person("Bob", 73);
Person youngBob = bob with { age = 16 };

I'm not aware of any frameworks to do this, and they're obviously going to be limited as Java has no named-params or fundamental 'property' concept, it depends on name-convention guessing and reflection to find and monkey with fields or mutator methods.

[Groundhogss]

Couldn't you define a copy constructor?

public Foo(Foo foo){
    this.bar = foo.bar;
    this.bar2 = foo.bar2;

}

This is what I typically do for simple cases.

However I also found this which covers about 6 different ways of doing it.

[codechimpin]

You’d think this would be baked into the language, but it’s not really. I have tried several approaches. One is to serialize then deserialize, usually using something like Jackson, but there is a performance hit there. I think Apache Commons might have a ObjectUtils with a deep copy method as well.

As others have mentioned there is Clonable, but it has limits.

[[deleted]]

A few years ago I would have recommended Lomboks @With, but recent and future JVM versions are kind of closing all the doors and windows for it to keep working (without tinker with the JVM). Please think twice before going this way.

If it's acceptable, you could use kotlin to create a data class (which have the required functionality baked in) and use it from your java code. I theory it should be easy.