Using Test-class correctly

I want to try and test this method for license-plates, but I can't seem to get the test in the main to work. Could someone help me out? The method should look like this according to my exercise instructions. This is my code:

public class RegNumber {

public static boolean isCorrectLicensPlate (String regNbr, Boolean isPrivate) {

    String generalPlate = "ABCDEFGHJKLMNOPRSTUWXYZX";
    String privatePlate = "ABCDEFGHIJKLMNOPQRSTUVWXYZÅÄÖ 123456789";


    if (isPrivate) {
        if (regNbr.length() < 2 && regNbr.length() > 7) return false;
        for(int i = 0; i < regNbr.length(); i++) {
        if(!privatePlate.contains(Character.toString(regNbr.charAt(i))) ) return false;
        }

    }
    else {
    if (regNbr.length() != 7) return false;

    if (regNbr.substring(3,4).equals(" ")) return false;

    String letters = regNbr.substring(0, 3);
    for (int i = 0; i < letters.length(); i++) {
        if(!generalPlate.contains(Character.toString(letters.charAt(i)))) return false;

    }

    String numbers = regNbr.substring(4);
    for (int i = 0; i < numbers.length(); i++) {
    if (!Character.isDigit(numbers.charAt(i))) return false;
    }


}

    return true;

}

public static void main(String[] args) {
    RegNumber asd = new RegNumber();
    asd.isCorrectLicensPlate(123, false);

}

}