java 随机生成数字和密码

public String getInvitationCode(int length) {

StringBuffer sbf = new StringBuffer();
Random random = new Random();

int length = 6;//准备生成几位随机数

//参数length，表示生成几位随机数
for(int i = 0; i < length; i++) {

String flag = random.nextInt(2) % 2 == 0 ? "char" : "num";
//输出字母还是数字
if( "char".equalsIgnoreCase(flag) ) {
//输出是大写字母还是小写字母
int temp = random.nextInt(2) % 2 == 0 ? 65 : 97;
sbf.append((char)(random.nextInt(26) + temp));
} else if( "num".equalsIgnoreCase(flag) ) {
sbf.append(String.valueOf(random.nextInt(10)));
}
}
return sbf.toString();
}