Adding multiple "randomly generated" objects to ArrayList results in adding the same object multiple times

Question

I have a class Ttp which has a ArrayList<City>loaded from file. In constructor of Ttp I randomly shuffle a list read from file and assign it to the object.

public class Ttp {
    private ArrayList<City> cities;

    public Ttp() {
        cities = Utils.shuffleArray(Loader.getCities());
    }
}

This way I get 10 objects with nicely shuffled arrays:

public static void main(String args[]) {
    Loader.readFile("easy_0.ttp");
    for(int i=0; i<10; i++){
        System.out.println(new Ttp());
    }
}

But in this scenario, when I try to create ArrayList<Ttp> I get a collection full of the same objects (instances of Ttp with the same arrays of cities)

public static void main(String args[]) {
    Loader.readFile("easy_0.ttp");
    ArrayList<Ttp> arrayList = new ArrayList<>();
    for(int i=0; i<10; i++){
        arrayList.add(new Ttp());
    }
    arrayList.forEach(System.out::println);
}

Shuffle function:

public static <T> ArrayList<T> shuffleArray(ArrayList<T> arrayList) {
    if (arrayList != null && arrayList.size() > 0) {
        int numberOfRolls = Random.getGenerator().nextInt((arrayList.size() - arrayList.size() / 3) + 1) + arrayList.size() / 3;
        int indexA;
        int indexB;
        T objectA;
        for (int i = 0; i < numberOfRolls; i++) {
            indexA = Random.getGenerator().nextInt(arrayList.size());
            indexB = Random.getGenerator().nextInt(arrayList.size());
            objectA = arrayList.get(indexA);
            arrayList.set(indexA, arrayList.get(indexB));
            arrayList.set(indexB, objectA);
        }
    }
    return arrayList;
}

To pick random indexes in shuffle function I am using java.util.Random:

public class Random {
    private static final java.util.Random generator = new java.util.Random();

    public static java.util.Random getGenerator() {
        return generator;
    }
}

John Kugelman · Accepted Answer · 2019-03-15 12:24:19Z

1

If Loader.getCities() returns the same list every time that means shuffleArray() is shuffling the same list over and over and every Ttp.cities has a reference to the same unitary list.

The fix is to make a copy somewhere. It could be in getCities(), it could be in shuffleArray(), or it could be in the Ttp constructor:

cities = Utils.shuffleArray(new ArrayList<>(Loader.getCities()));

answered Mar 15, 2019 at 12:24

John Kugelman

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