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:

instead of (my expected output):

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

s is a reference to an object. When you add a copy of this reference it just adds that reference, not a copy of the object referenced. — Peter Lawrey
– Peter Lawrey, Commented Jun 6, 2015 at 14:10

Lrrr · Accepted Answer · 2015-06-06 13:22:23Z

8

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);
}

answered Jun 6, 2015 at 13:22

Lrrr

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2 Comments

PatLas Over a year ago

Such stupid mistake.. Thanks guys for help, I've forgotten that add method doesn't make a copy of an object.

Lrrr Over a year ago

@PatLas glad to be helpful :)

Eran · Accepted Answer · 2015-06-06 13:22:45Z

2

You are adding the same inner ArrayList reference to the outer ArrayList multiple times. Therefore all the inner ArrayLists 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);
}

answered Jun 6, 2015 at 13:22

Eran

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Comments

Sathish · Accepted Answer · 2015-06-06 13:29:34Z

0

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("-----------------");
    }

answered Jun 6, 2015 at 13:29

Sathish

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