Using Arraylist addAll() to create a new list with new objects with different reference

Question

I want to merge two arraylists to one new arraylist, and iterate through the list. Before I make actions on my new arraylist, I want to multiple some value times -1. The problem however is, that when using addAll() I also multiple my old arraylist.

public Polynoom subtract (Polynoom that) {
    Polynoom subtract = new Polynoom(); 
    subtract.termen.addAll(that.termen);
    for (int i = 0; i < subtract.termen.size(); i++) {
        subtract.termen.get(i).coef = subtract.termen.get(i).coef * -1; 
    }
    subtract.termen.addAll(this.termen);
    Collections.sort(subtract.termen);
    subtract.removeDoubles(); 
    return subtract; 

}

My Polynoom class looks like this:

   class Polynoom implements PolynoomInterface {


        ArrayList<Paar> termen = new ArrayList<Paar>(); 

        Polynoom() {
            termen = new ArrayList<Paar>();
        }
}

The problem is (i think): that when using addAll() it creates REFERENCES instead of new copies, therefore changing the elements will change both. What is nice way to overcome this problem?

I can of course create a new pair instead of a Polynoom an add this pair to a new Polynoom but I don't think that is really nice code. Does someone has a better idea?

Why not first iterate over `that.termen` and do the calculation then do `addAll`? — A4L, Oct 12 '13 at 19:02
As a recommendation, it would be better if you define your `Polynoom` class to have an immutable state. — Luiggi Mendoza, Oct 12 '13 at 19:34

JungJoo · Answer 1 · 2013-10-12T19:34:30.367

1

try this

subtract.termen = new ArrayList<Paar>(that.termen);

before the loop and remove the subract.termen.addall();

or you can just create new object

ArrayList <Paar> temp = new ArrayList<Paar>(that.termen);
subtract.termen.addAll(temp);

edited Oct 12 '13 at 19:34

answered Oct 12 '13 at 19:05

JungJoo

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libik · Answer 2 · 2013-10-12T19:31:13.163

First you can use much nicer methods for iterating :

    for (Paar par : substract.termen){
        par.coef *= -1;
    }

If you want nice approach, your method should look like this :

public Polynoom subtract(Polynoom that) {
    Polynoom subtract = this.clone(); //Creates same Polynom, you have to create this method
    //Polynoom subtract = new Polynoom(this) => the same as line above using constructor
    for (Paar par : that.termen){
        substract.sub(par));
    }
    return subtract;
}

So you have to add function public Paar sub(Paar that) which substract only one Paar from your polynom. It should be something like this :

public void sub(Paar that){
    Paar paar = new Paar(that.coef);
    if (this.existsPaarWithSameCoef()){
        paar.multipliedBy -= this.findsPaarWithSameCoef().multipliedBy;
    } else {
        paar.multipliedBy *= -1;
    }
    return paar;
}

As noted [here](http://stackoverflow.com/q/2156120/1065197), it is not a good idea to use the `clone` method. — Luiggi Mendoza, Oct 12 '13 at 19:38

Using Arraylist addAll() to create a new list with new objects with different reference

2 Answers2