replace String.toCharArray with String.charAt

[yeikel]

final String helloWorld = "Hello world";
char[] chars = helloWorld.toCharArray(); // creates a copy using `System.arraycopy`

for (char c : chars) {
System.out.println(c);
}

final String helloWorld = "Hello world";

// iterates over the array backing the String

for (int i = 0 ; i < helloWorld.length();i++){
 System.out.println(helloWorld.charAt(i));
}