Add to all Strings in an arraylist different letters

December 11, 2022

I have an issue I know I can solve just using an array but that makes a later part of the program more challenging.
Im reading off of a datafile a bunch of letters spaced out. i want to sort them into columns, the data file shows this:

[J]             [F] [M]            
[Z] [F]     [G] [Q] [F]            
[G] [P]     [H] [Z] [S] [Q]        
[V] [W] [Z] [P] [D] [G] [P]        
[T] [D] [S] [Z] [N] [W] [B] [N]    
[D] [M] [R] [J] [J] [P] [V] [P] [J]
[B] [R] [C] [T] [C] [V] [C] [B] [P]
[N] [S] [V] [R] [T] [N] [G] [Z] [W]

I keep on getting an error on line 18. "

   java -classpath .:target/dependency/* Main
[J]             [F] [M]            
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
    at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
    at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
    at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266)
    at java.base/java.util.Objects.checkIndex(Objects.java:359)
    at java.base/java.util.ArrayList.get(ArrayList.java:427)
    at Main.main(Main.java:18)

1 import java.util.*;
2 import java.io.*;
3 public class Main {
4 public static void main(String [] args) throws FileNotFoundException{
5    String line; 
6    ArrayList<String> C = new ArrayList<>();
7    String a = "";
8    Scanner input = new Scanner(new File("dat.dat"));
9    while (input.hasNextLine()) {
10        line = input.nextLine();
11      if (line.indexOf("[") != -1)
12      {
13        System.out.println(line);
14        a = "";
15        for(int lcv = 1; lcv < line.length() - 1; lcv+= 4)
16          {
17            a = line.substring(lcv, lcv+1);
18            C.set(lcv/4, C.get(lcv/4) + a);
19          }
20      }
21      }

how would i go about doing this?
I don’t want to use a regular array because i’m going to be shuffling these strings around later on.

>Solution :

You’re trying to modify the existing element in your ArrayList instead of adding a new element. That’s why you’re getting the error on line 18.

First, you must add the element in the ArrayList and then you can update it at the index you want.

Also, you can initialize your ArrayList by adding empty elements based on the length of line.

C.add(a);
C.set((lcv / 4), C.get((lcv / 4)) + a);

Your end result should look something like this:

import java.util.*;
import java.io.*;

public class Main {
    public static void main(String [] args) throws FileNotFoundException{
        String line; 
        ArrayList<String> C = new ArrayList<>();
        String a = "";
        Scanner input = new Scanner(new File("dat.dat"));

        while (input.hasNextLine()) {
            line = input.nextLine();
            if (line.indexOf("[") != -1) {
                System.out.println(line);
                a = "";

                // Init the ArrayList
                for (int i = 0; i < line.length() / 4; i++) {
                    C.add("");
                }

                for(int lcv = 1; lcv < line.length() - 1; lcv+= 4) {
                    a = line.substring(lcv, lcv+1);
                    // Add the new element into the ArrayList
                    // and update it later
                    C.add(a);
                    C.set((lcv / 4), C.get((lcv / 4)) + a);
                }
            }
        }
    }
}