Adding items of ArrayList to other ArrayList of ArrayList

Question

I have added an item to list a and then added list a to list b and did the same thing again.

My question is if I print b.get(0) and b.get(1), I am getting the same list that is both the items "One" and "Two", why is it so?

At b.get(0) I want to get only one item I added that is a.add("One").

After adding a.add("Two"), if I print b.get(1) I should get both "One" and "Two"?

Is there any solution or any changes to manage this?

List<String> a= new ArrayList<String>();
List<List<String>> b= new ArrayList<List<String>>();

a.add("One");
b.add(a);

a.add("Two");
b.add(a);

System.out.println("b="+b.get(0));
System.out.println("b="+b.get(1));

output:

b=[One, Two]
b=[One, Two]

you are Still working on the same instance of the ArrayList, hence the List b at the index one and two refers to the same instance of the ArrayList — SomeJavaGuy
– SomeJavaGuy, Commented May 13, 2015 at 9:59

Eran · Accepted Answer · 2015-05-13 09:59:22Z

5

You are adding the same List twice, so you see the same elements for both indices of the outer List.

In order to add two different Lists, you must create a new ArrayList before adding each element to the outer List :

a.add("One");
b.add(a);

a = new ArrayList<>(a); // assuming you want the second list to contain both "One" and "Two"
a.add("Two");
b.add(a);

answered May 13, 2015 at 9:59

Eran

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Fran Montero · Accepted Answer · 2015-05-13 10:02:03Z

0

You are adding the same reference in b[0] and b[1]. If you want to have differente lists at diferent index on list b you have to create a new List object

        List<String> a= new ArrayList<String>();
    List<String> c= new ArrayList<String>();
    List<List<String>> b= new ArrayList<List<String>>();
    a.add("One");
    b.add(a);

    c= new ArrayList<String>();
    c.addAll(a);
    c.add("Two");
    b.add(c);

    System.out.println("b="+b.get(0));
    System.out.println("b="+b.get(1));

answered May 13, 2015 at 10:02

Fran Montero

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Comments

cheyiliu · Accepted Answer · 2015-05-13 10:04:10Z

0

the reason is in your code, the b.get(0) and b.get(1) point to the same List a, so the output is same.

use this code can achieve what you want, List a1= new ArrayList(); List a2= new ArrayList(); List> b= new ArrayList>();

    a1.add("One");
    b.add(a1);

    a2.add("Two");
    b.add(a2);

    System.out.println("b="+b.get(0));
    System.out.println("b="+b.get(1));

output is, b=[One] 100 b=[Two]

answered May 13, 2015 at 10:04

cheyiliu

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