String#toCharArray allocates a new array. Calling charAt is more efficient, because it avoids creating a new array with a copy of the character data.

That is, prefer this:

boolean isDigits(String string) {
  for (int i = 0; i < string.length(); i++) {
    char c = string.charAt(i);
    if (!Character.isDigit(c)) {
      return false;
    }
  }
  return true;
}

to this:

boolean isDigits(String string) {
  // this allocates a new char[]
  for (char c : string.toCharArray()) {
    if (!Character.isDigit(c)) {
      return false;
    }
  }
  return true;
}

Note that many loops over characters can be expressed using streams with String#chars or String#codePoints, for example:

boolean isDigits(String string) {
  string.codePoints().allMatch(Character::isDigit);
}