Creating a deep copy method, Java

Question

I want to make a deep copy method. I seeked help here the other day with this issue, but that was for a copy constructor. Now I need a regular method. I have the code created (nonworking), but I'm just not understanding it completely.

public GhostList deepCopy(){
        int length=this.getLength();
        GhostList jadeed=new GhostList();
        Ghost[] data = new Ghost[length];
        for (int i=0;i<this.getLength();i++){
            data[i] = new Ghost();
            data[i].setX(this.ghosts[i].getX());
            data[i].setY(this.ghosts[i].getY());
            data[i].setColor(this.ghosts[i].getColor());
            data[i].setDirection(this.ghosts[i].getDirection());
        }

        return jadeed;
    }

Now when I create a new GhostList named jadeed, and then under that I create a new data array of ghosts, does it know that data belongs to the jadeed GhostList? I dont see how the two can be associated, even though they should be.

Also, I'm not getting the lengths to match up for the copy and this.object. What is my problem?

Answer 1

You created a new GhostList and a new Ghost array.
You fill in the Ghost array and return the GhostList but the returned GhostList has nothing to do with the Ghost array.
You should add all the new ghosts to the GhostList

Answer 2

First, you mentioned a copy constructor. If you already have that working, then all you need to do in your deepCopy method is:

return new GhostList(this);

Let's forget that for now and get back to the code you posted. You are creating an array named data but you never used it anywhere. Aren't you supposed to assign this array to jadeed? Something like:

jadeed.ghosts = data;

And finally, instead of calling the method deepCopy, it would be better to call it clone and implement the Cloneable interface. Doing this allows everyone to know how to get a copy of your object using a standard interface.

Answer 3

Your GhostList class will have as its data member a reference to the array of Ghost. You've not shown us the class definition, so lets say that member is named foo. Now all you need to do is make the foo reference of the newly created jadeed object refer to the array of Ghost which you've created and populated. You can do it as:

jadeed.foo = data;

before you return jadeed.

Answer 4

If GhostList and everything it's composed of is Serializable, you can serialize the GhostList instance into a byte array and re-read it. It's a few lines of code, unless you use `Jakarta Commons Lang - one line of code:

http://commons.apache.org/lang/api-2.5/org/apache/commons/lang/SerializationUtils.html#clone%28java.io.Serializable%29