CODEWITHSUNDEEP
javahashmapdeep-copy
Smolda
Smolda

Reputation: 902

Java HashMap - deep copy

I am just trying to find out the best solution how to make a deep copy of HashMap. There are no objects in this map which implement Cloneable. I would like to find better solution than serialization and deserialization.

Upvotes: 19

Views: 47227

Answers (3)

Sam YC
Sam YC

Reputation: 11617

This is not easy, we are using some sort of workaround:

1) convert the map to json string. (for example, using Google Gson)

2) convert the json string back to map.

Do note that there is performance issue, but this is kind of easiest way.

Upvotes: 3

stealthjong
stealthjong

Reputation: 11093

Take a look at Deep Cloning, on Google Code you can find a library. You can read it on https://github.com/kostaskougios/cloning.

How it works is easy. This can clone any object, and the object doesnt have to implement any interfaces, like serializable.

Cloner cloner = new Cloner();
MyClass clone = cloner.deepClone(o);
// clone is a deep-clone of o

Be aware though: this may clone thousands of objects (if the cloned object has that many references). Also, copying Files or Streams may crash the JVM.

You can, however, ignore certain instances of classes, like streams et cetera. It's worth checking this library and its source out.

Upvotes: 13

hyperman
hyperman

Reputation: 1344

I don't think it can be implemented in a generic way.

  • If you have the chance to simply implement clone, I'd go that way.
  • A bit more complex is creating a type map, where you look up some kind of clone implmentation class based on the class of each object
  • When the objects might form a Directed Acyclic Graph, I'd in general keep a Map from the original to the clone of every object I've ever seen, and check if I've already made it
  • When you have a general graph, the problem gets really nasty. You might have strange constraints of the object creation order, it might even be impossible when you have final fields.

For now, I'd propose to re-write your question in a less general way

Upvotes: 3

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