Class instance change another instance

Question

The code is :

package classes;

public class Test {
    private static double mutationRate = 0.5;

    public static void main(String[] args) {
        Population pop = new Population();
        pop.initialise();
        Population po = new Population();
        po.getIndividusList().add(pop.getFittest());
        po.getIndividusList().add(mutate(pop.getIndividusList().get(1)));
    }

    private static Chromosom mutate(Chromosom l) { // changer les couples d'interventions des parcs)
        // loop through genes
        Chromosom ch = new Chromosom();
        for (int i = 0; i < l.size(); i++)
            ch.put(i, l.get(i));

        for (int i = 0; i < ch.size(); i++) {
            double alea = Math.random() * 13;
            int moisIntervention1 = (int) alea;

            Intervention interv1 = new Intervention(1, moisIntervention1);
            ch.get(i).modInterventions(ch.get(i).intervention2(interv1));
        }
        return ch;
    }
}

The problem is that I did not change the instance pop but when I change the other instance po, pop changes too.

As you populate po with the contents of pop, when you change po, pop still points at the same changed objects. — Smutje
– Smutje, Commented Aug 11, 2014 at 12:07
You need to actually copy the contents of pop into po... not point like the above comment — nbz
– nbz, Commented Aug 11, 2014 at 12:07
What does class Population look like? Are its member variables static (which means they are shared between all instances of Population)? — Jesper
– Jesper, Commented Aug 11, 2014 at 12:09
The problem is likely in your implementation of the class Population — Philipp
– Philipp, Commented Aug 11, 2014 at 12:09

Sanjay Rabari · Accepted Answer · 2014-08-11 12:37:27Z

1

java pass by value.

when you call this mutate(pop.getIndividusList().get(1))

you are sending pop's instance, so it will get change.

Supose pop.getIndividusList().get(1) return String varibale do this way

String var=pop.getIndividusList().get(1);

then call mutate(var)

edited Aug 11, 2014 at 12:37

answered Aug 11, 2014 at 12:20

Sanjay Rabari

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4 Comments

Sarah Over a year ago

so what can i do ? there is any solution to not sending pop's instance ? thanks

Sanjay Rabari Over a year ago

what pop.getIndividusList().get(1) return? store it in some variable and then mutate(variableHavingValue)

Sarah Over a year ago

what pop.getIndividusList().get(1) return an instance of another class. i do what you told me to do and i still have the same probleme. i did: Chromosom var= what pop.getIndividusList().get(1) then i call mutate(var)

Sarah Over a year ago

pop.getIndividusList().get(1) implement hash table

Diego Martinoia · Accepted Answer · 2014-08-11 14:23:50Z

0

I'm unsure about whether I understood the problem, but I think that you mean that when you alter the items in Population po, the items in Population pop mirror those changes.

That is, indeed, the expected behavior of your code: to populate po, you are adding items from pop - (pop.getFittest, pop.getList.get(1) ).

But the individuals are, I believe, instances of objects, so add/remove and similar operations work with references to the objects, and not with copies of them. Therefore, as you have 2 references to the same obj, any change is mirrored.

IF you want to create a copy, you should add to po a new object with the same state, either by creating a constructor that takes another instance as parameter, implementing a copy method, or something similar.

It should be something like this:

    Population po = new Population();
    Individual fittest = pop.getFittest();
    Individual poCopy = new Individual();
    //ADD CODE HERE TO COPY ALL THE FIELDS FROM fittest TO poCopy
    //....

    po.getIndividusList().add(poCopy);

edited Aug 11, 2014 at 14:23

answered Aug 11, 2014 at 12:10

Diego Martinoia

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1 Comment

Sarah Over a year ago

thank you for your answer i try this methode but i still have the same problem

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