Java生成指定长度的随机数

char[] str = {'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'};

int[] num = {1, 2, 3, 4, 5, 6, 7, 8, 9};

String ALLCHAR = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

    @Test
    public void getRamdonString() {

        final int maxNum = 36;
        int i; // 生成的随机数
        int count = 0; // 生成的密码的长度

        char[] str = {'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'};
        StringBuffer pwd = new StringBuffer("");
        Random r = new Random();
        while (count < maxNum) {
            // 生成随机数，取绝对值，防止生成负数，
            i = Math.abs(r.nextInt(50)); // 生成的数最大为36-1
            if (i >= 0 && i < str.length) {
                pwd.append(str[i]);
                count++;
            }
        }
        System.out.println(pwd);
    }

    @Test
    public void getRamdonString2() {
        StringBuffer sb = new StringBuffer();
        String ALLCHAR = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        Random random = new Random();
        for (int i = 0; i < 20; i++) {
            sb.append(ALLCHAR.charAt(random.nextInt(ALLCHAR.length())));
        }

        System.out.println(sb.toString());
    }