convert comma separated string to list without intermediate container

Question

I need to convert comma separated string to list of integers. For example if I have following string

String numbersArray = \"1, 2, 3, 5, 7, 9,\";
         


        

Answer 1

This works, as long as the String ends in a comma, like your example.

 String numbersArray = "1, 2, 3, 14, 5,";
 List list = new ArrayList();
 for (int i = 0, j, n = numbersArray.length(); i < n; i = j + 1) {
     j = numbersArray.indexOf(",", i);
     list.add(Integer.parseInt(numbersArray.substring(i, j).trim()));
 }

However, it's pretty useless, as on my machine it's about 2 times slower than the original.

This next solution, on the other hand, is surprisingly fast. I tested it on lists of 50000 integers obtained using Math.abs(random.nextInt()) and it was about 4 times faster than the original.

List list = new ArrayList();
for (int i = 0, a = 0, n = numbersArray.length(); i < n; i++) {
    char c = numbersArray.charAt(i);
    if (c == ',') {
        list.add(a);
        a = 0;
    } else if (c != ' ') {
        a = a * 10 + (c - '0');
    }
}

And this is about twice as fast again:

List list = new ArrayList();
for (int i = 0, a = 0, n = numbersArray.length(); i < n; i++) {
    switch(numbersArray.charAt(i)) {
        case ',': list.add(a); a = 0; break;
        case ' ': break;
        case '0': a = a * 10; break;
        case '1': a = a * 10 + 1; break;
        case '2': a = a * 10 + 2; break;
        case '3': a = a * 10 + 3; break;
        case '4': a = a * 10 + 4; break;
        case '5': a = a * 10 + 5; break;
        case '6': a = a * 10 + 6; break;
        case '7': a = a * 10 + 7; break;
        case '8': a = a * 10 + 8; break;
        case '9': a = a * 10 + 9; break;
        default: throw new AssertionError();
    }
}