Java Properties配置文件读取到Map<String, List<String>>的方法(lambda)

Properties配置文件中内容如下：

A=groupA1
A=groupA2
A=groupA3
B=groupB1
B=groupB2

1、通过Map和collect来实现

File reqFile = new File("test.config");
try (Stream<String> stream = Files.lines(reqFile.toPath())) {
  Map<String, List<String>> conf = stream
  .map(s -> Arrays.asList(s.split("=")))
  .collect(HashMap::new,
      (map, item) -> map.computeIfAbsent(item.get(0), k -> new ArrayList<>()).add(item.get(1)),
      HashMap::putAll);
  for (Map.Entry<String, List<String>> entry: conf.entrySet()) {
    System.out.println("KEY: " + entry.getKey());
    for (String value : entry.getValue()) {
      System.out.println("VALUE: " + value);
    }
  }
} catch (Exception e) {
  e.printStackTrace();
}

2、通过map和group by来实现

File reqFile = new File("test.config");
try (Stream<String> stream = Files.lines(reqFile.toPath())) {
  Map<String, List<String>> conf = stream
    .map(s -> Arrays.asList(s.split("=")))
    .collect(Collectors.groupingBy(s -> s.get(0), Collectors.mapping(v->v.get(1), Collectors.toList())));
  for (Map.Entry<String, List<String>> entry: conf.entrySet()) {
    System.out.println("KEY: " + entry.getKey());
    for (String value : entry.getValue()) {
      System.out.println("VALUE: " + value);
    }
  }
} catch (Exception e) {
  e.printStackTrace();
}

相关文档：https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#collect-java.util.function.Supplier-java.util.function.BiConsumer-java.util.function.BiConsumer-