List of String lists in Java

Question

I have a list of String lists initialised to object1 as mentioned below:

List<List<String>> object1 = Arrays.asList(Arrays.asList("A", "B", "C"),
                                           Arrays.asList("D", "E", "F"), 
                                           Arrays.asList("G", "H", "I"));

Now I want to create a new list of String lists object let be object 2 as mentioned below:

List<List<String>> object2 = new ArrayList<>();

I am not sure whether above written syntax is correct while creating new object "object2" which is same as "object1".

I want to iterate each and every element present in object1 and store in object2 using two for loops (outer and inner loop).

I tried to refer lot of examples but not able to get proper implementation for it.

Ashish Patil · Accepted Answer · 2022-08-01 10:33:28Z

1

Though you might need to update where you are facing issue, but in meantime this is what my workable attempt is:

        List<List<String>> object1 = Arrays.asList(Arrays.asList("A", "B", "C"), Arrays.asList("D", "E", "F"),
                Arrays.asList("G", "H", "I"));

        List<List<String>> object2 = new ArrayList<>();

        for (int i = 0; i < object1.size(); i++) {
            List<String> ls = new ArrayList<String>();
            for (int j = 0; j < object1.get(i).size(); j++) {
                ls.add(object1.get(i).get(j));
            }
            object2.add(ls);
        }
    System.out.println(object2);

Output:

[[A, B, C], [D, E, F], [G, H, I]]

answered Aug 1, 2022 at 10:33

Ashish Patil

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Romeo Sheshi · Accepted Answer · 2022-08-01 10:32:14Z

1

The syntax is correct to iterate each element of object1 and add to object2 just do it this way

        List<List<String>> object1 = Arrays.asList(Arrays.asList("A", "B", "C"),
                Arrays.asList("D", "E", "F"),
                Arrays.asList("G", "H", "I"));
        List<List<String>> object2 = new ArrayList<>();

        for(List<String> list : object1){
            List<String> copyList  =  new ArrayList<>(); 
            for(String value : list){
                copyList.add(value);
            }
            object2.add(copyList);
        }

answered Aug 1, 2022 at 10:32

Romeo Sheshi

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Pshemo · Accepted Answer · 2022-08-01 12:01:58Z

1

If you don't really need to explicitly use nested loops, instead you can use

copy constructor which each list provides,
streams and collect its elements with toList() to separate list.

So your code can look like:

List<List<String>> object2 = new ArrayList<>();
for (List<String> row : object1){
    object2.add(new ArrayList<>(row)); //adds separate list containing copy of *row* elements
}

OR with streams:

List<List<String>> object2 = object1.stream()
                                    .map(row -> row.stream().toList())//maps to copy of row
                                    .toList();

answered Aug 1, 2022 at 12:01

Pshemo

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