java - 使用 LinkedHashSet 时计算出现次数

Question

我有一个 XML 文件，我需要在其中查找并计算年份标签的出现次数。例如:

Found year 2020 10 times.
Found year 2017 1 times.
Found year 2019 2 times. 
(...)

为了避免重复年份我使用了HashSet。代码:

public class Publications {
    public static void main(String[] args) throws IOException {
        Set<String> publicationYears = new LinkedHashSet<>();
        try (BufferedReader reader = Files.newBufferedReader(Paths.get("dblp-2020-04-01.xml"))) {
            Pattern pattern = Pattern.compile("<year>(.+?)</year>", Pattern.DOTALL);
            for (String line; (line = reader.readLine()) != null; ) {
                Matcher matcher = pattern.matcher(line);
                if (matcher.find()) {
                    String year = matcher.group(1);
                    publicationYears.add(year);
                }
            }
        }

结果:

2010
2002
1992
1994
1993
2006(...)

但是现在我找不到一个高效的代码来统计每年的出现次数。创建一个多维数组然后搜索会非常慢。有什么建议吗？

Answer 1

试试这个:

• 我用 map 替换了套装。
• 执行此操作的语句是

        count.compute(year, (k,v)->v == null ? 1 : v + 1); 

• 如果是第一次遇到的年份，则简单地为该年份添加 1，否则为该年份加 1。

   Map<String, Integer> count = new LinkedHashMap<>();
        try (BufferedReader reader = Files.newBufferedReader(Paths.get("dblp-2020-04-01.xml"))) {
            Pattern pattern = Pattern.compile("<year>(.+?)</year>", Pattern.DOTALL);
            for (String line; (line = reader.readLine()) != null; ) {
                Matcher matcher = pattern.matcher(line);
                if (matcher.find()) {
                    String year = matcher.group(1);
                    count.compute(year, (k,v)->v == null ? 1 : v + 1);
                }
            }
        }
    }

要打印它们，请执行以下操作

count.entrySet().forEach(System.out::println);