How do I declare a 2D String arraylist?

Question

I want to do something like this ArrayList<String<String>> mylist

How can I create it?

How can I add to the external and internal list

and how can I convert the internal list to a simple array list?

Answer 1

You can go with

List<List<String>> ls2d = new ArrayList<List<String>>();
List<String> x = new ArrayList<String>();
x.add("Hello");
x.add("world!");
ls2d.add(x);

System.out.println(Arrays.deepToString(ls2d.toArray()));

Answer 2

The first array list isn't an array list of String, it's an ArrayList of ArrayList.

ArrayList<ArrayList<String>>

Answer 3

List<List<String>> array = new ArrayList<List<String>>();
...
array.add(new ArrayList<String>())
array.get(0).add("qqq");

array.get(0) - is a internal list

Answer 4

List<List<String>> super2dArray = new ArrayList<ArrayList<String>>()

This is an arraylist of arraylists holding strings.

Also ArrayList<String<String>> mylist makes no sense, because String is not a collection/list, but I think you understood that. Future readers might not though.

See this answer to see why I chose to have List on the left side.

Answer 5

Try this :

public class JavaTests {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {

    String[ ][ ] test2 = new String[3][3];      //3 can be replace if you add more test2[?][?] = "RandomString"

        test2[0][0] = "String1";
        test2[0][1] = "String2";
        test2[0][2] = "String3";

        test2[1][0] = "String4";
        test2[1][1] = "String5";
        test2[1][2] = "String6";

        test2[2][0] = "String7";
        test2[2][1] = "String8";
        test2[2][2] = "String9";

        for (int i = 0; i <= 2; i++){
            for (int j = 0; j <= 2; j++){
                System.out.print(test2[i][j] +"\t");
            }
            System.out.println("\n");
        }  
        System.out.println("\n");      
    }
}

Answer 6

There are two ways to achieve what you desire. I am providing code snippet for both:

1.List<List<String>> lol = new ArrayList<List<String>>();

Scanner in = new Scanner(System.in);
int size = in.nextInt();

//Declare your two dimensional ArrayList of Strings. 
List< List<String>> lol = new ArrayList<List<String>>();

//Instantiate and Populate
for (int i=0;i<size;i++){
    int internalListSize = in.nextInt(); //the size of each internal list
    lol.add(new ArrayList<String>());
    for (int j=0;j<internalListSize;j++){
        String whateverYouWanttoPut = in.nextLine();
        lol.get(i).add(whateverYouWanttoPut);
    }
}

//Access Elements 
try {
    System.out.println(lol.get(0).get(4));
    System.out.println(lol.get(1).get(2));
    System.out.println(lol.get(3).get(2));
} catch (Exception e){
    System.out.println("ERROR!");
}     

2. ArrayList[] set = new ArrayList[n];

Scanner in = new Scanner(System.in);
int size = in.nextInt();
//Declare your two dimensional ArrayList of Strings.
ArrayList[] set = new ArrayList[size];

//Populate it. 
for(int i=0;i<size;i++){
    int innerSize = in.nextInt();
    set[i] = new ArrayList<String>();
    for(int j=0;j<innerSize;j++){  
        set[i].add(in.nextLine());                
    }
}        
try{
    System.out.println(set[0].get(1));
    System.out.println(set[1].get(2));
} catch(Exception e){
    System.out.println("ERROR!");
}

Answer 7

Create 2D array like Below -

for primitive data types -

int[][] arr = new int[10][10];

for Custom Classes -

public class MyClass {
    int x;
    MyClass() { 
        this.x = 0
    }
}

MyClass[][] arr = new MyClass[10][10];
for (int i = 0; i < 10; ++i) {
    for (int j = 0; j < 10; ++j) {
        arr[i][j] = new MyClass();
    }
}