Creating List of Lists

Question

I want to create a list of lists in Java and this is the code I am using for that.

public ArrayList<ArrayList<Object>> getRows(){
    ArrayList<ArrayList<Object>> listOfRows = new ArrayList<ArrayList<Object>>();
    ArrayList<Object> temporalRow = new ArrayList<Object>();
    IntNode rowSentinel = newTable.sentinel;
    for (int y = 0; y < this.numberOfRows; y++) {
        for (int x = 0; x < this.numberOfColumns; x++) {
            rowSentinel =rowSentinel.next;
            Object rowElement = rowSentinel.arrayCol.get(y);
            temporalRow.add(rowElement);
        }
        rowSentinel = newTable.sentinel;
        listOfRows.add(temporalRow);
        System.out.print(temporalRow);
        System.out.println(Arrays.toString(listOfRows.toArray()));
        temporalRow.clear();
    }
    return listOfRows;
}

However, when I see the final list of lists the console gives me the following output:

[oddy1, minino1, libertad1][[oddy1, minino1, libertad1]]

[oddy2, minino2, libertad2][[oddy2, minino2, libertad2], [oddy2, minino2, libertad2]] 

[oddy3, minino3, libertad3][[oddy3, minino3, libertad3], [oddy3, minino3, libertad3], [oddy3, minino3, libertad3]] 

[[], [], []]

I also tried defining listOfRows as an array of Objects, Object[], with size the number of rows, and adding elements by doing Object[0],Object[1], etc. but I ended up getting the same result.

I wanted to get a final list of the form

[[oddy1, minino1, libertad1],[oddy2, minino2, libertad2],[oddy3, minino3, libertad3]]

I really don't see what I'm doing wrong with the code. Could anyone help me with this?

possible duplicate with: [http://stackoverflow.com/questions/1474954/working-with-a-list-of-lists-in-java](http://stackoverflow.com/questions/1474954/working-with-a-list-of-lists-in-java) — Mouaici_Med, Mar 08 '17 at 17:51
Try using a debugger , it will help out in finding the logical error. — Amit Kumar, Mar 08 '17 at 19:44
I'm trying to do that, but I don't see it. I think for some reason whenever I use add on listOfRows, it is adding whatever temporalRow is refering to at that point in the program. I really don't know what's going on. — user284639, Mar 08 '17 at 19:48
step 1 is learn java. step 2 is pay attention. If you store a reference to a list then call list.clear() on that reference, the stored reference (which points to the same object) is also cleared. — DwB, Mar 08 '17 at 20:08
Thanks. I am trying to learn Java, that's why I'm asking and trying to find a solution to my problem. I don't see the need to be rude with your steps. — user284639, Mar 08 '17 at 20:11

Jakob · Accepted Answer · 2017-03-08T20:11:26.447

0

Replace temporalRow.clear() with temporalRow = new ArrayList<Object>(), that should fix it. Right now, your listOfRows contains three times the reference to the same object, and the elements you added previously always get deleted by temporalRow.clear().

edited Mar 08 '17 at 20:11

answered Mar 08 '17 at 20:04

Jakob

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opensam · Answer 2 · 2017-03-08T20:23:52.433

Check Out the code below

ArrayList<Test> temporalRow = new ArrayList<Test>();

    Test t = new Test();
    t.i = 5;
    temporalRow.add(t);
    System.out.println(temporalRow.get(0).i);
    t.i = 6;
    System.out.println(temporalRow.get(0).i);

    class Test {
public int i;
}

The output of above code is 5 and 6 because temporalRow list contains the reference to object t and if that object is modified somewhere else, the changes are, obviously, reflected in temporalRow. Your case is similar. (Hint : temporalRow.clear();) I think maybe you would like to familiarize yourself with the concept of deep copy vs shallow copy.

Creating List of Lists

2 Answers2