How do you make a deep copy of an object?

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.

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.

You can do a serialization-based deep clone using org.apache.commons.lang3.SerializationUtils.clone(T) in Apache Commons Lang, but be careful—the performance is abysmal.

In general, it is best practice to write your own clone methods for each class of an object in the object graph needing cloning.

XStream is really useful in such instances. Here is a simple code to do cloning

private static final XStream XSTREAM = new XStream();
...

Object newObject = XSTREAM.fromXML(XSTREAM.toXML(obj));

Use XStream(http://x-stream.github.io/). You can even control which properties you can ignore through annotations or explicitly specifying the property name to XStream class. Moreover you do not need to implement clonable interface.

Deep copying can only be done with each class's consent. If you have control over the class hierarchy then you can implement the clonable interface and implement the Clone method. Otherwise doing a deep copy is impossible to do safely because the object may also be sharing non-data resources (e.g. database connections). In general however deep copying is considered bad practice in the Java environment and should be avoided via the appropriate design practices.

Using Jackson to serialize and deserialize the object. This implementation does not require the object to implement the Serializable class.

  <T> T clone(T object, Class<T> clazzType) throws IOException {

    final ObjectMapper objMapper = new ObjectMapper();
    String jsonStr= objMapper.writeValueAsString(object);

    return objMapper.readValue(jsonStr, clazzType);

  }

Here is a generic deep cloning method using object serialization and deserialization with byte array streams (to avoid writing to a file).

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

@SuppressWarnings("unchecked")
public static <T extends Serializable> T deepClone(T t) {
    try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);) {
        oos.writeObject(t);
        byte[] bytes = baos.toByteArray();
        try (ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes))) {
            return (T) ois.readObject();
        }
    } catch (IOException | ClassNotFoundException e) {
        throw new RuntimeException(e);
    }
}

Here is an easy example on how to deep clone any object: Implement serializable first

public class CSVTable implements Serializable{
    Table<Integer, Integer, String> table; 
    public CSVTable() {
        this.table = HashBasedTable.create();
    }
    
    public CSVTable deepClone() {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            oos.writeObject(this);

            ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
            ObjectInputStream ois = new ObjectInputStream(bais);
            return (CSVTable) ois.readObject();
        } catch (IOException e) {
            return null;
        } catch (ClassNotFoundException e) {
            return null;
        }
    }

}

And then

CSVTable table = new CSVTable();
CSVTable tempTable = table.deepClone();

is how you get the clone.

A very quick and simple one-liner solution could be to use Jackson.

Have a look at the example snippet :

ObjectMapper objectMapper = new ObjectMapper();

MyClass deepCopyObject = objectMapper
  .readValue(objectMapper.writeValueAsString(originalObject), MyClass.class);

In the above example : "MyClass" refers to the class of the object you want to be copied .

• Explanation : We are simply trying to serialize the original object to string and then de-serialize the string back to object and thus getting a deep copy.
• Learn More about ObjectMapper here : https://fasterxml.github.io/jackson-databind/javadoc/2.7/com/fasterxml/jackson/databind/ObjectMapper.html