Java add a string in inner list of an array

Question

Im trying to add string values inside a 2D list. Was able to add the 1st layer using this:

List<List<String>> twodaray = new ArrayList<List<String>>();
    List<String> x = new ArrayList<String>();


   for ( int i = 0, m = 1; i < attribteList.size(); i = i+2, m = m+2) {
       String attributeName = attribteList.get(i);
       x.add(attributeName);
   }
   twodaray.add(x);

   for(List<String> ls : twodaray) {
       System.out.println(Arrays.deepToString(ls.toArray()));
   }

Was wondering how can I access the inner layer of the ArrayList like print it out (sysout) or even adding values to it? Something like this.

    for(List<String> ls : twodaray) {
        System.out.println(Arrays.deepToString(ls.toArray()));
        for(List<List<String>> ls2d : ls) {
           if (ls = something) {
               ls2d.add(something);
           }
        }
    }

Kinda new to using this 2 dimensional list and any help is greatly appreciated. ty...

Tried this for(List ls2d : ls) but eclipse error. keeps asking me to change ls2d to string for(String ls2d : ls) — user3736748, Jul 01 '14 at 00:58
For that part, Ricky's answer below is close (one layer of `List<>` needs to be taken off). You're supposed to be getting closer to `String`, not further away from it! — saiarcot895, Jul 01 '14 at 01:00
For the printing to `stdout`, though, just doing `twodarray.toString()` should work (no for-loops or anything). — saiarcot895, Jul 01 '14 at 01:00

score 0 · Accepted Answer · edited May 23 '17 at 10:26

It's not apparent what exactly you're trying to do, but, this code is close other than a couple of mistakes (= should be .equals(), and the inner and outer foreach loops were somewhat mixed up ):

List<String> a = new List<String>();
a.add("Hello");

List<String> b = new List<String>();
b.add("World");

List<List<String>> ls = new List<List<String>>();
ls.add(a);
ls.add(b);

//The outer part (ie. one List of a List of strings
for(List<String> ls : twodaray) {

    //The inner part (ie. one string for each list of strings)
    for(String str : ls) {

       //Check two things for equality (.equals() for strings)
       if (str.equals("Hello")) {
           //Adding "something" to the list (that str is in);
           ls.add("something");
       }
    }
}

This question is pretty similar to this.

If I recall my Java correctly, shouldn't there be one less layer of `List<>` in the for-loop declaration? (`List ls` instead of `List> ls`) — saiarcot895, Jul 01 '14 at 01:01
@saiarcot895 sorry, you're absolutely right. I was mistaken, and will edit accordingly. — Ricky Mutschlechner, Jul 01 '14 at 01:14

ironwood · Answer 2 · 2014-07-01T01:47:59.357

I think it should be like this.

for(List<String> ls : twodray) { // twodaray is a List<List<String>>
        System.out.println(ls); // Yes you can simple sysout
        for(String str : ls) { // ls is a List<String>
           if (str.equals("something")) {
               // do something.
           }
        }
    }

I think its better to understand the structure of this 2D ArrayList. The 2D ArrayList's elements are 1D ArrayLists. 1D ArrayList's elements are the Strings.

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If you are modifying the same arrayList (specially removes values) while reading it better to use CopyOnWriteArrayList.

Java add a string in inner list of an array

2 Answers2