How do you make a deep copy of an object?

A few people have mentioned using or overriding Object.clone(). Don't do it. Object.clone() has some major problems, and its use is discouraged in most cases. Please see Item 11, from "Effective Java" by Joshua Bloch for a complete answer. I believe you can safely use Object.clone() on primitive type arrays, but apart from that you need to be judicious about properly using and overriding clone.

The schemes that rely on serialization (XML or otherwise) are kludgy.

There is no easy answer here. If you want to deep copy an object you will have to traverse the object graph and copy each child object explicitly via the object's copy constructor or a static factory method that in turn deep copies the child object. Immutables (e.g. Strings) do not need to be copied. As an aside, you should favor immutability for this reason.

A safe way is to serialize the object, then deserialize. This ensures everything is a brand new reference.

Here's an article about how to do this efficiently.

Caveats: It's possible for classes to override serialization such that new instances are not created, e.g. for singletons. Also this of course doesn't work if your classes aren't Serializable.

You can make a deep copy with serialization without creating files.

Your object you wish to deep copy will need to implement serializable. If the class isn't final or can't be modified, extend the class and implement serializable.

Convert your class to a stream of bytes:

ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos);
oos.writeObject(object);
oos.flush();
oos.close();
bos.close();
byte[] byteData = bos.toByteArray();

Restore your class from a stream of bytes:

ByteArrayInputStream bais = new ByteArrayInputStream(byteData);
Object object = new ObjectInputStream(bais).readObject();