Building strings by character

String building follows the accumulator machine with two dialects:

Filter — keep matching characters.

String out = "";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (Character.isLetter(c)) {
        out += c;
    }
}

Map — transform every character (e.g. every letter to uppercase by hand, without toUpperCase):

String out = "";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (c >= 'a' && c <= 'z') {
        out += (char) (c - 'a' + 'A');
    } else {
        out += c;
    }
}

The (char) cast matters: c - 'a' is an int arithmetic distance (0–25); adding 'A' converts the distance back to a code. Trace 'c': 'c' - 'a' = 2, 'A' + 2 = 'C'.

Split-by-index — substring inside a loop.

// first vowel run, or the whole word if none
int at = -1;
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
        at = i;
        break;
    }
}
String head = (at == -1) ? s : s.substring(0, at);

Common trap: charAt(i) after the loop ends — the index is dead there. Store the answer inside the loop (like at), never index past the end.