复杂密码生成器

import java.util.*;

public class Main {
    private static final String []range = new String[5];
    private static int[] sum = new int[10];
    private static void init() {
        range[0] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        range[1] = "abcdefghijklmnopqrstuvwxyz";
        range[2] = "0123456789";
        range[3] = "~!@#$%^&*()_+/-=[]{};:'<>?.";
    }

    private static int nextInt(int len) {
        return Math.abs(new Random().nextInt(len));
    }

    private static char getChar(int op) {
        return range[op].charAt(nextInt(range[op].length()));
    }
    private static char getRand() {
        int op = nextInt(4);
        return getChar(op);
    }

    public static void main(String[] args) {
        init();
        StringBuffer s = new StringBuffer();
        Arrays.fill(sum, 0);
        for (int i = 0; i < 16; i++) {
            s.append(getRand());
        }
        for (int i = 0; i < 4; i++) {
            if (sum[i] == 0) {
                s.append(getChar(i));
            }
        }
        System.out.println(s);
    }
}