자바 랜덤 문자열 생성

class RandomString {
    private static String pattern = "[a-zA-Z0-9]+$";
    private static StringBuffer returnString = null;

    public static void main(String[] args) {
        RandomString rndStr = new RandomString();

        System.out.println("[랜덤 문자열]");
        System.out.println("숫자 (50자리) : " + rndStr.getString(50,"1"));
        System.out.println("대문자 (16자리) : " + rndStr.getString(16,"A"));
        System.out.println("소문자 (16자리) : " + rndStr.getString(16,"a"));
        System.out.println("대문자 + 숫자 (32자리) : " + rndStr.getString(32,"A1"));
        System.out.println("소문자 + 숫자 (32자리) : " + rndStr.getString(32,"a1"));
        System.out.println("대문자 + 소문자 + 숫자 (32자리) : " + rndStr.getString(32,""));
    }

    //Overload Constructor
    public RandomString() { returnString = new StringBuffer(); }
    public RandomString(int len) { returnString = new StringBuffer(); this.setLength(len); }
    public RandomString(String type) { returnString = new StringBuffer(); this.setType(type); }
    public RandomString(int len, String type) { returnString = new StringBuffer(); this.setLength(len); this.setType(type); }
    public RandomString(String type, int len) { returnString = new StringBuffer(); this.setType(type); this.setLength(len); }

    //Get Random Character
    private static void getRndChar() {
        int r = (int)Math.round(Math.random() * 1000);

        java.util.Random random = new java.util.Random();
        int rnd = random.nextInt(1000);

        if(test(pattern, String.valueOf((char)rnd))) {
            returnString.append((char)rnd);
        } else {
            getRndChar();
        }
    }

    //Regular Express Check Function
    private static boolean test(String pattern, String value) {
        java.util.regex.Pattern p = java.util.regex.Pattern.compile(pattern);
        java.util.regex.Matcher m = p.matcher(value);
        boolean r = m.matches();

        return r;
    }

    //Set String Length
    public void setLength(int len) {
        returnString.setLength(0);
        for(int i=0; i<Math.abs(len); i++) {
            getRndChar();
        }
    }

    //Set String Pattern Type
    public void setType(String type) {
        if(type.equals("1")) pattern = "[0-9]+$";
        else if(type.equals("A")) pattern = "[A-Z]+$";
        else if(type.equals("a")) pattern = "[a-z]+$";
        else if(type.equals("A1")) pattern = "[A-Z0-9]+$";
        else if(type.equals("a1")) pattern = "[a-z0-9]+$";
        else pattern = "[a-zA-Z0-9]+$";
    }

    //Get Random String
    public String getString() {
        return returnString.toString();
    }

    //Overload getString()
    public String getString(int len, String type) {
        this.setType(type);
        this.setLength(len);
        return this.getString();
    }
}