Java ArrayList populating

Question

So I am creating an method that shuffles a group of numbers and the idea is that i create a population of these numbers. So I create a loop which shuffles the numbers and then adds it to an arraylist, however after a few debugging statements I found that it does shuffle the numbers but only adds the last shuffle to the arrayList. Can anyone help me figure out why?

solutionList is an arraylist further up the code if anyone was wondering

for(int k =0;k <100; k++){
        Collections.shuffle(solutionList);
        population2.add(new Object[]{solutionList}) ;
        System.out.println("In the loop  " + solutionList);

    }

    for(Object[] row : population2){
        System.out.println("Row = " + Arrays.toString(row));
    }

Jon Skeet · Accepted Answer · 2012-06-25 16:33:04Z

9

Each element of population2 is an array with a reference to the same ArrayList. If you want different lists, you need to create a new one for each iteration.

For example, to avoid populating the list with the right numbers each time, you could just shuffle solutionList and then add a reference to a copy:

for (int k = 0; k < 100; k++) {
    Collections.shuffle(solutionList);
    List<Integer> copy = new ArrayList<Integer>(solutionList);
    population2.add(new Object[]{ copy });
}

answered Jun 25, 2012 at 16:33

Jon Skeet

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Jochen · Accepted Answer · 2012-06-25 16:33:15Z

3

population2.add(new Object[]{solutionList}) ;

creates an Object array with a single element. That element happens to be a list.

answered Jun 25, 2012 at 16:33

Jochen

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JB Nizet · Accepted Answer · 2012-06-25 16:35:25Z

2

You're creating 100 arrays, each containing a reference to the same list. I think what you want is creating 100 arrays, each containing a copy of the elements of the list:

for(int k =0;k <100; k++){
    Collections.shuffle(solutionList);
    population2.add(solutionList.toArray()) ;
}

But I would advise avoiding arrays completely, and always using collections:

for(int k =0;k <100; k++){
    Collections.shuffle(solutionList);
    population2.add(new ArrayList<Something>(solutionList));
}

answered Jun 25, 2012 at 16:35

JB Nizet

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