ArrayList of Arraylist a remove function affects all Arraylists

Question

I created an ArrayList of 80 Arraylists. Each ArrayList inside, has the values 1-9.

However, when I delete a value from one of the Arraylists its deleted in all of them.

ArrayList<List> available = new ArrayList<List>();
ArrayList<Integer> possibleValues = new ArrayList<Integer>();



    for(int j = 1; j<=9; j++){
        possibleValues.add(j);
    }

    for (int i = 0; i<=80; i++){
        available.add(possibleValues);  
    }

     int b = (int) available.get(0).get(2);
     available.get(0).remove(0);
     String s = available.get(80).toString();
     System.out.println(" " + s);

}

Any help is appreciated.

rgettman · Accepted Answer · 2013-10-25 16:13:09Z

6

The problem here is that you've added the same ArrayList possibleValues to available 81 times. So, the available list contains 81 references to the same possibleValues list.

Any changes made to the list are visible through any of those 81 references.

If you don't want the changes visible to all references, just one, then you need to make 81 copies of the list to add:

for (int i = 0; i<=80; i++){
    available.add(new ArrayList<Integer>(possibleValues));  
}

answered Oct 25, 2013 at 16:13

rgettman

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Comments

Suresh Atta · Accepted Answer · 2013-10-25 16:16:11Z

1

That's because you're adding the same ArrayList to every index of the outer ArrayList . Any change to this ArrayList will automatically result in a change in the other ArrayList .

Nest your loops to always create a new ArrayList or construct a new ArrayList from the already existing one.

edited Oct 25, 2013 at 16:16

Suresh Atta

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answered Oct 25, 2013 at 16:12

Jeroen Vannevel

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Comments

Prabhakaran Ramaswamy · Accepted Answer · 2013-10-25 16:19:02Z

1

Because you are adding the values to same ArrayList. You need to create new Arraylist for 0 to 80 iterations.

for(int j = 1; j<=9; j++){
        possibleValues.add(j);// one ArrayList with values.
}


for (int i = 0; i<=80; i++){
        available.add(possibleValues); //Here adding same Arraylist 80 times.  
}

Try this one

  ArrayList<List> available = new ArrayList<List>();
  ArrayList<Integer> possibleValues = new ArrayList<Integer>();

  for (int i = 0; i<=80; i++){
    possibleValues = new ArrayList<Integer>();
    for(int j = 1; j<=9; j++){           
        possibleValues.add(j);
    } 
    available.add(possibleValues);  
  }

edited Oct 25, 2013 at 16:19

answered Oct 25, 2013 at 16:13

Prabhakaran Ramaswamy

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1 Comment

Suresh Atta Over a year ago

Please explain why this code is working instead simply providing, +1 though :)

Poornima · Accepted Answer · 2013-10-25 16:20:17Z

0

You are adding the same ArrayList 81 times. ArrayList is a reference type, so any change to it will affect all items. It should be:

available.add(new ArrayList(possibleValues));

answered Oct 25, 2013 at 16:20

Poornima

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