获取四位验证码

 StringBuffer strbf = new StringBuffer("");///
        StringBuffer Vcode = new StringBuffer("");//

        for (int i=48;i<=122;i++){
            if (i>=48&&i<=57||i>=97&&i<=122||i>=65&&i<=90){
                strbf.append((char)i);
            }
        }

        for (int i=0;i<=3;i++) {
            int index = (int) ((Math.random() * strbf.length()));
            Vcode.append(strbf.substring(index, index + 1));
        }
        String str = Vcode.toString();