How do I do a Java-deep copy of an object?

Question

I have a class (Literal). I need to be able to keep an intact Literal instance in memory throughout my application, and have a copy of it which I can alter. I have used two ways to do this:

class Literal implements Cloneable and override Object.clone() method.

Factory constructor:

public Literal(Literal lit){ this = lit; }

In both cases copying doesn't work. Every change I make to the copy, changes the original. Does anybody have any idea what I'm doing wrong?

What you doing is only shallow copying.check this post stackoverflow.com/questions/64036/… — Umesh Awasthi
– Umesh Awasthi, Commented Dec 21, 2011 at 10:32
Are you sure that you work with Java? I am asking this because the second attempt with a Factory constructor (usually, this is called copy constructor) will result in a compile error "cannot assign a value to final variable this" — nd.
– nd., Commented Dec 21, 2011 at 10:44

akappa · Accepted Answer · 2011-12-21 10:34:13Z

5

since Literal is not a integral type, variables of type Literal holds a reference to the actual value, so this = lit just copies the reference, so your behavior.

You must copy all the member fields recursively to do a "real" copy.

This article has been linked in the accepted answer to the question linked by Umesh in his comment, and I think it clears the conceptual problems you're hitting with your problem.

answered Dec 21, 2011 at 10:34

akappa

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4 Comments

nkatz Over a year ago

Actually I have and it still doesn't work.My actual constructor is public Literal(Literal lit){ this.functor = lit.getFunctor(); this.arguments = lit.getArguments(); this.isCompound = lit.isCompound(); this.isVarOrConst = lit.isVarOrConst(); this.varOrconstName = lit.getVarOrconstName(); this.arity = lit.getArity(); this.subsumedBy = lit.getSubsumedBy(); this.asString = lit.getLiteralAsString(); }

akappa Over a year ago

you must do that recursively, since the same effect happens in the case your member fields are non-integral types.

akappa Over a year ago

Check out the answers of this question: stackoverflow.com/questions/1175620/…

nkatz Over a year ago

Οκ Ι did that, calling the constructor recursively from within itself. It works now. Thanks a lot!!

ewan.chalmers · Accepted Answer · 2011-12-21 10:51:59Z

If you are going to use copy-constructor, then you will need to make a deep copy of every mutable member variable.

Say your Literal has member variables like this:

private String[] args; private String s;

Then your copy-constructor would need to do something like this

public Literal(Literal l) { // have to deep copy the array, otherwise both instances are sharing the reference this.args = Arrays.copyOf(l.args, l.args.length); // safe to just copy the reference, String is immutable this.s = l.s; }

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