生成百万4位字符串算法

生成100万条不重复的4位数字或字母（比如X0Au）
1：要求至少有一位小写字母
2：要求至少有一位大写字母
3：必须是【大写字母】开头
4：不允许出现【一位大写三位小写】或者【一位小写三位大写】（比如Abcd/ABCd等）
5 : 4位必须不同（大小写不敏感，比如X0Aa，只包含x,0,a三位，所以不允许）
6 : 具备不可枚举性

 private void calculate() {
        String[] charactor = new String[]{"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"};
        boolean[] select = new boolean[62];
        int randomInt;
        int hasLowerCount = 0;
        int hasUpperCount = 0;
        StringBuilder resultBuilder = new StringBuilder();
        for (int m = 0; m < 1000000; m++) {
            for (int i = 0; i < 4; i++) {
                if (i == 0) {
                    randomInt = (int) (Math.random() * 26);
                } else {
                    randomInt = (int) (Math.random() * 62);
                    while (select[randomInt]
                            || (randomInt + 26 < 52 && select[randomInt + 26])
                            || (randomInt - 26 >= 0 && select[randomInt - 26])
                            || (hasLowerCount > 1 && randomInt > 25 && randomInt < 52)
                            || (hasUpperCount > 1 && randomInt < 26)) {
                        randomInt = (int) (Math.random() * 62);
                    }
                }
                if (randomInt < 26) {
                    hasUpperCount++;
                }
                if (randomInt >= 26 && randomInt < 52) {
                    hasLowerCount++;
                }
                select[randomInt] = true;
            }
            resultBuilder.append(m+1).append(",");
            for (int i = 0; i < 62; i++) {
                if (select[i]) {
                    select[i] = false;
                    resultBuilder.append(charactor[i]);
                }
            }
            hasLowerCount = 0;
            hasUpperCount = 0;
            resultBuilder.append("\n");
        }
        writeToTxt(resultBuilder.toString());
    }

private void writeToTxt(String result) {
        File file = new File(getExternalCacheDir(), "T.txt");
        if (!file.exists()) {
            try {
                file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        BufferedWriter out = null;
        try {
            FileOutputStream fileOutputStream = new FileOutputStream(file.getAbsoluteFile(),true);
            OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream);
            out = new BufferedWriter(outputStreamWriter);
            out.write(result);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }