How to find an index of the ArrayList from the starting index in Java?

Question

I want to search for an index of the element in the ArrayList but I want to start searching from starting index different than 0.

I tried like this:

import java.util.*;

public class Test{
    public static void main(String[] args) {
        ArrayList<String> bricks = new ArrayList<String>(List.of("BBBB","CCCC","DDDD"));
        System.out.println(bricks.subList(1, bricks.size()).indexOf("CCCC"));
    }
}

Output:

0

Expected output:

1

I want to start searching for "CCCC" in "bricks" from the starting index "1" not from "0"

Answer 1

Your code finds the index within the sublist.

To find the index within the original list, add the index used to create the sublist to the result:

System.out.println(bricks.subList(1, bricks.size()).indexOf("CCCC") + 1);

Some refactoring makes this clearer:

public static <T> int indexOfAfter(List<T> list, T item, int from) {
    int result = list.subList(from, list.size()).indexOf(item);
    return result == -1 ? -1 : (from + result);
}

Answer 2

import java.util.*;

public class Test{
    public static void main(String[] args) {
        int startIndex = 1;
        ArrayList<String> bricks = new ArrayList<String>(List.of("BBBB","CCCC","DDDD"));
        int index = bricks.subList(startIndex, bricks.size()).indexOf("CCCC");
        if(index != -1){
            index = index + startIndex;
        }
        System.out.println(index);
    }
}

Answer 3

You could write your own helper:

static <T> int indexOf(List<T> list, int offset, T element) {
    for (int i = offset; i < list.size(); i++) {
        if (Objects.equals(list.get(i), element)) {
            return i;
        }
    }
    return -1;
}

Note that this may be inefficient for non-random-access lists like LinkedList. It also handles the lower bound differently from the upper bound, but that can be fixed with an additional check.