java beginner programming, putting strings in char array to filter input

java beginner programming, putting strings in char array to filter input

in this code I've been trying to filter the characters in the array with a
for-loop to a second array, but am unable to. Could anyone tell me what is
exactly wrong with my code?
public class Deel1 {
public static void main(String[] args) {
String zinInvoer = getInput();
String zinUitvoer = filterZin(zinInvoer);
}
static String getInput() {
Scanner scan = new Scanner(System.in);
String zinInvoer = "";
System.out.println("Voer een zin in: ");
if (scan.hasNextLine()) {
zinInvoer = scan.nextLine().trim();
}
if (zinInvoer.equals("")) {
System.out.println("Geen invoer!");
System.exit(0);
}
return zinInvoer;
}
static String filterZin(String zinInvoer) {
String zinUitvoer = "";
char ongefilterd[] = zinInvoer.toCharArray();
String nogFilteren = new String(ongefilterd);
char a = nogFilteren.charAt(97);
for (a = 97; a <= 122; a++) {
a = a += 32;
char gefilterd[] = //second array to be printed
}
System.out.println("Gefilterd: " + zinUitvoer);
return zinInvoer;
}
}