IndexOutofBoundsException: I cant solve it

Question

Im trying to do a basic java task, there are ships and i have to give the coordinates of them using an import file in which there are characters representing the movements of the ships. (N = +1 to North, S= +1 to South, E= +1 to East, W= +1 to West)

I want to count the coordinates in a two dimensional Array, in which the first column represent the Vertical, the second column represents the Horizontal directions.

Get the following problem:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
    at robotok.main(robotok.java:36)

I marked line 36 in the source code.

ArrayList<String> control = new ArrayList<String>();
int coords[][] = new int[control.size()][2]; // index = robot sorszáma / [] Hosszúság / [] Szélesség

try {
    Scanner scan = new Scanner(robotok);

    while (scan.hasNextLine()) {
        String line = scan.nextLine();
        control.add(line);
    }
} catch(Exception e){
}

for (int i = 0;i<control.size();i++) {
    char[] directions = control.get(i).toCharArray();

    for (int j =0;j<directions.length;j++) {
        if (directions[j] == 'N') {
            coords[i][0] --;
        }else if (directions[j] == 'S'){
            coords[i][0] ++;
        }else if (directions[j] == 'W'){
            coords[i][1] --;
        }else if (directions[j] == 'E'){
            coords[i][1] ++;                     /////THIS IS 36///// 
        }
    }
}

for (int i =0;i<coords.length;i++) {
     System.out.print("The "+i+". ship's coords: "+coords[i][0]+" ; "+coords[i][1]);
}

Answer 1

Instead of

ArrayList<String> control = new ArrayList<String>();
int coords[][] = new int[control.size()][2]; // index = robot sorszáma / [] Hosszúság / [] Szélesség

try {
    Scanner scan = new Scanner(robotok);

    while (scan.hasNextLine()) {
        String line = scan.nextLine();
        control.add(line);
    }
} catch(Exception e){
}

for (int i = 0;i<control.size();i++) {
    char[] directions = control.get(i).toCharArray();

    for (int j =0;j<directions.length;j++) {
        if (directions[j] == 'N') {
            coords[i][0] --;
        }else if (directions[j] == 'S'){
            coords[i][0] ++;
        }else if (directions[j] == 'W'){
            coords[i][1] --;
        }else if (directions[j] == 'E'){
            coords[i][1] ++;                     /////THIS IS 36///// 
        }
    }
}

for (int i =0;i<coords.length;i++) {
     System.out.print("The "+i+". ship's coords: "+coords[i][0]+" ; "+coords[i][1]);
}

use

ArrayList<String> control = new ArrayList<String>();

try {
    Scanner scan = new Scanner(robotok);

    while (scan.hasNextLine()) {
        String line = scan.nextLine();
        control.add(line);
    }
} catch(Exception e){
}

int coords[][] = new int[control.size()][2]; // index = robot sorszáma / [] Hosszúság / [] Szélesség

for (int i = 0;i<control.size();i++) {
    char[] directions = control.get(i).toCharArray();

    for (int j =0;j<directions.length;j++) {
        if (directions[j] == 'N') {
            coords[i][0] --;
        }else if (directions[j] == 'S'){
            coords[i][0] ++;
        }else if (directions[j] == 'W'){
            coords[i][1] --;
        }else if (directions[j] == 'E'){
            coords[i][1] ++;                     /////THIS IS 36///// 
        }
    }
}

for (int i =0;i<coords.length;i++) {
     System.out.print("The "+i+". ship's coords: "+coords[i][0]+" ; "+coords[i][1]);
}

The problem was that you intended to initialize coords based on control, which has a size of 0 before you add the elements. Later on, after the elements are added, control becomes larger and you try to reference coords' element at certain indexes above its size. Hence the exception.

Answer 2

You create the coords array before you have added the elements to control, while it is still size 0. You need delay creating it until the list has reached its final size.

Answer 3

When you create the control ArrayList its initial length is 0 and does not insert any element in it before accessing its length Due to which the Array length of the coords is also zero and you are trying to access the array beyond its length so, it is advised to insert the value to control before assigning the value to the lenght