Populate list with arrays of strings

Question

I'm trying to populate a list with string arrays but the only array that is added to the list is the last added. And this seems to be added to all list elements.

What am I doing wrong?

String[] indata = new String[2];
List<String[]> ls = new ArrayList<String[]>();
indata[0]="test1";
indata[1]="test2";
ls.add(indata);
indata[0]="test3";
indata[1]="test4";
ls.add(indata);
for(int index=0; index < ls.size(); index++)
        System.out.println("ZZZZZZZZZZZZ----->>  " + index + "     " + Arrays.toString(ls.get(index)));

Expected output:

ZZZZZZZZZZZZ----->>  0     [test1, test2]
ZZZZZZZZZZZZ----->>  1     [test3, test4]

Actual output:

ZZZZZZZZZZZZ----->>  0     [test3, test4]
ZZZZZZZZZZZZ----->>  1     [test3, test4]

You need to create new String array for every array added into the List — nhahtdh
– nhahtdh, Commented Feb 14, 2013 at 9:17
Shorthand way to write, if you are adding literal string. ideone.com/bonDwJ — nhahtdh
– nhahtdh, Commented Feb 14, 2013 at 9:24

Mikhail Vladimirov · Accepted Answer · 2013-02-14 09:17:26Z

3

Should be:

String [] indata = new String [2];
List <String []> ls = new ArrayList <String []> ();
indata [0] = "test1";
indata [1] = "test2";
ls.add (indata);
indata = new String [2]; // This line added
indata [0] = "test3";
indata [1] = "test4";
ls.add (indata);

When you do ls.add (indata) a reference to your array is stored in the list, but not the copy of the array. So after this both, ls.get (0) and indata points to the same instance of array, and modifying first element this array with indata [0] = "test3" will change what you see using ls.get (0)[0].

answered Feb 14, 2013 at 9:17

Mikhail Vladimirov

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Comments

Renjith · Accepted Answer · 2013-02-14 09:22:43Z

0

When you create an object and add it to a list, the object is not getting copied to list.Instead a reference of existing object is getting added to list.So when you do this:

String[] indata = new String[2];
List<String[]> ls = new ArrayList<String[]>();
indata[0]="test1";
indata[1]="test2";
ls.add(indata);

Both indata and ls.get(0) are pointing to the same object.

Here you are again modifying the data pointed to by indata to add second array.So ls.get(0) also point to the updated data.

Inorder to add new array, you need to create new array and add it to list.

answered Feb 14, 2013 at 9:22

Renjith

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