Question

I ran across this problem, which has been driving me nuts. In a nutshell, I instantiate two objects of the same class. When I run a method in one object, the other object is affected too as if I called a method on a 2nd object explicitly. I was wondering if someone could please give me a hand on this.

Suppose, I have class Portfolio…

public class Portfolio implements Cloneable {

public ArrayList<Instrument> portfolio;
private String name;
private String description;

public Portfolio() {
    portfolio = new ArrayList<Instrument>();
}

public Portfolio(Portfolio copyFrom) {
    //attempt to copy the object by value
    this.portfolio = (ArrayList<Instrument>) copyFrom.portfolio.clone();
    this.name = copyFrom.name;
    this.description = copyFrom.description;
}

public void applyStress(Stress stress) {
    this.portfolio.set(0,this.portfolio.get(0)+1;
}

1st constructor is used to instantiate an object, etc. 2nd constructor is used to copy an object by value.

A method applyStress is used to run through sum calculations. in our case I simplified the method, so that it does nothing but adds +1 to whatever is in the object.

So I would instantiate an object as

Portfolio p = new Portfolio();

then I would assign to a portfolio field, some instruments;

p.portfolio = someInstrumentList;

then I would copy by value the portfolio p into pCopy:

Portfolio pCopy = new Portfolio(p);

So at this time I am have two objects that are the same. Further one is a copy-by-value object. Changing values of fields in pCopy does not affect same fields in p.

Now, when I run a method applyStress on p, then the values of the instrument list in pCopy will also change.

In other words, if p.portfolio.get(0) == 1, then after p.applyStress, I would expect to see that p.portfolio.get(0) is 2 and pCopy.portfolio.get(0) is 1

But what I see instead is p.portfolio.get(0) is 2 and pCopy.portfolio.get(0) is also 2

I do not understand why this happens. It is not the static modifier issue, as there is no static modifiers. Anyone has any ideas?

Answer 1

The clone method applied to you your ArrayList reference does a shallow copy, not a deep copy. This implies that whatever you had in the original collection is shared by the cloned one.

This means that you need to clone every instrument as well, or provide a copy constructor for every one of them.

this.portfolio = new ArrayList<Instrument>();
for(Instrument toBeCopiedInstrument : copyFrom.portfolio){
   this.portfolio.add(new Instrument(toBeCopiedInstrument ));
}