System.out.print删除匹配器类异常

所以我用Jsoup来抓取一些网页,这个有线问题就发生了。

// Sets the prefix for all pages to prevent navigate to unwanted pages.
String prefix = "https://handbook.unimelb.edu.au/%d/subjects";
// Postfix for search page
String searchPostfix = "(\\?page=\\d+)?$";
// Postfix for subject page
String subjectPostfix = "\\/(\\w+)(\\/.+)?$";

String root = String.format(prefix, "2019");
String pattern = root.replace("/", "\\/").replace(".", "\\.");
Pattern reg1 = Pattern.compile("^" + pattern + searchPostfix);
Pattern reg2 = Pattern.compile("^" + pattern + subjectPostfix);

使用这些正则表达式模式。我用绳子拉它

String s1 = "https://handbook.unimelb.edu.au/2019/subjects/undergraduate";

用一种方法:

private String getSubjectCode(String link) {
    System.out.println(link);
    if (isSubjectPage(link)) {
        Matcher subjectMatcher = subjectPattern.matcher(link);
        System.out.println(link);
        // System.out.println(subjectMatcher.matches());   ## Exception if commented
        System.out.println(subjectMatcher.group(0));
        System.out.println(subjectMatcher.group(1));


        return subjectMatcher.group(1);
    }
    return null;
}

但是,如果我评论这句话

Exception in thread "main" java.lang.IllegalStateException: No match found
    at java.base/java.util.regex.Matcher.group(Matcher.java:645)
    at Page.Pages.getSubjectCode(Pages.java:54)
    at Page.Pages.enqueue(Pages.java:85)
    at Crawler.Crawler.parsePage(Crawler.java:41)
    at Crawler.Crawler.crawl(Crawler.java:51)
    at Main.main(Main.java:9)

将引发上述异常,为什么打印行会影响程序的运行?

而且,没有任何评论

System.out.println(subjectMatcher.matches());   // Exception if commented
// out -> true