1. Under Windows, when we press the Enter key on the keyboard, we actually enter two characters: carriage return and line feed: rn, and the ASCII codes are 13和10

2. By default, the input obtained by Scanner does not contain carriage returns and line feeds. For example, if you enter ____one_tworn, it will only get the output from ____one_two (of course, it may have to be obtained through multiple next()s), and automatically filter out carriage returns and line feeds

3. But when we force carriage return or line feed as the delimiter, it will not automatically filter the carriage return and line feed. For example, if you use the carriage return character n as the separator, entering the above content will result in ____one_twor, which is why the returned length is 12. You can use the following program to verify it. You can find that the ASCII code of the last character in the obtained string is 13, which means it is a carriage return character r:

// 输入`____one_two\r\n`来测试
Scanner scanner = new Scanner(System.in);
scanner.useDelimiter("\n");
String str = scanner.next();
// 打印获取到的内容和其长度，以及最后一个字符的ASCII码
System.out.println(str + ":" + str.length()
        + " (" + str.codePointAt(str.length()-1) + ")");

Solution: Use rn as the delimiter and it will be normal

Enter under Windows is rn