Unexpected behavior of ArrayList inside ArrayList

Question

I am trying to use ArrayList<ArrayList<String>> to parse some fragment from web page but something goes wrong within my code:

ArrayList<ArrayList<String>> x = new ArrayList<ArrayList<String>>();
ArrayList<String> s = new ArrayList<String>();

int z, y, w = 0;
for(z=0; z<3;z++)
{
    s.clear(); 
    for(y=0;y<4;y++)
    {
        s.add(""+w++);
    }
    x.add(s);
}

for(ArrayList<String>  tr : x)
{
    for( String t : tr)
    {
        System.out.println(t);
    }
    System.out.println("-----------------");
}

The output is:

8
9
10
11

8
9
10
11

8
9
10
11

instead of ( my expected output ):

0
1
2
3

4
5
6
7

8
9
10
11

Can somebody explain to me why is this happening? Thanks.

Answer 1

It is expected behavior of Object inside loops , Because s is a same object in every iteration, you need to create new object in your loop.

change your code like this:

for(z=0; z<3;z++)
{
    s = new ArrayList<String>();
    for(y=0;y<4;y++)
    {
        s.add(""+w++);
    }
    x.add(s);
}

Answer 2

You are adding the same inner ArrayList reference to the outer ArrayList multiple times. Therefore all the inner ArrayList s are the same object, and contain the same elements.

You must create a new instance for each iteration of the outer loop :

for(z=0; z<3;z++)
{
    s = new ArrayList<String>(); 
    for(y=0;y<4;y++)
    {
        s.add(""+w++);
    }
    x.add(s);
}

Answer 3

The above program reuses the same ArrayList (inner list: s) over and over and hence you get only the last set of values,

Move the line 2 (inner Arraylist initialization) to inside the loop as follows to get the desired result,

    ArrayList<ArrayList<String>> x = new ArrayList<ArrayList<String>>();
    int z,y,w=0;
    for(z=0; z<3;z++)
    {
        ArrayList<String> s = new ArrayList<String>(); 
        for(y=0;y<4;y++)
        {
            s.add(""+w++);
        }
        x.add(s);
    }

    for(ArrayList<String>  tr : x)
    {
        for( String t : tr)
        {
            System.out.println(t);
        }
        System.out.println("-----------------");
    }