Trying to create an array to find occurrence of character in a given String. Only allowed to use methods length and charAt from the class String

Trying to create an array to find the occurrence of a character in a given String.

public static void main(String[] args)
{
  String s1 = "java";
  char ch = 'a';
 
System.out.println(indexOf(s1, ch));
}

public static int indexOf(String s, char ch)
{
  int count = 0;
  
  while (count < s.length())
  {
     if (s.charAt(count) == ch)
        return count;
        
     else
        count += 1;
  }
  return -1;
}

So the result is supposed to be 1 3

To get all the indexes where a char occurs in a string, you need a value that marks the position in the string, and the other the nulber of matches, to know the position in the result array, and at the end, cut the array to remove the leading unused boxes

public static int[] indexOf(String s, char ch) {
    int count = 0;
    int[] result = new int[s.length()];
    for (int i = 0; i < result.length; i++) {
        if (s.charAt(i) == ch) {
            result[count] = i;
            count += 1;
        }
    }
    return Arrays.copyOf(result, count); // cut to keep indexes only
}

Demo

System.out.println(Arrays.toString(indexOf("java", 'a')));        // [1, 3]
System.out.println(Arrays.toString(indexOf("abahauayata", 'a'))); // [0, 2, 4, 6, 8, 10]
System.out.println(Arrays.toString(indexOf("aaaaaaabbbb", 'a'))); // [0, 1, 2, 3, 4, 5, 6]

Because of the restrictions you have, you will have to first create an array with the size equal to the length of the string (assuming a condition when all chars are same). Then, iterate the characters of the given string and store the index where a match with the given character is found. Finally, copy the initialised positions of this array to another array and return the same. In case, no match is found, simply return new int[] { -1 }.

Demo:

import java.util.Arrays;

public class Main {
	public static void main(String[] args) {
		// Test
		String s1 = "java";
		char ch = 'a';

		int[] array = indexOf(s1, ch);
		System.out.println(Arrays.toString(array));

		array = indexOf(s1, 'x');
		System.out.println(Arrays.toString(array));
	}

	public static int[] indexOf(String s, char ch) {
		// The maximum size can be the length of the string i.e. when all chars are same
		int[] array = new int[s.length()];
		int count = 0;
		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) == ch) {
				array[count++] = i;
			}
		}
		if (count == 0) {
			return new int[] { -1 };
		} else {
			// Create an array result[], of the size = count and copy elements from array[]
			// to it
			int[] result = new int[count];
			for (int i = 0; i < count; i++) {
				result[i] = array[i];
			}
			return result;
		}
	}
}

Output:

[1, 3]
[-1]

public static int[] indexOf(String s, char ch) {
    int total = 0;

    for (int i = 0; i < s.length(); i++)
        if (s.charAt(i) == ch)
            total++;

    if (total == 0)
        return new int[0];

    int[] res = new int[total];

    for (int i = 0, j = 0; i < s.length(); i++)
        if (s.charAt(i) == ch)
            res[j++] = i;

    return res;
}