使用Java流将字符串列表转换为地图列表

Question

I have a list of strings in the following pattern

String test ="name=john,age=28;name=paul,age=30;name=adam,age=50";
List<String> listOfStrings = Arrays.asList(test.split(";"));

I want to convert the above list of strings to a list of map of key value pairs (like shown below).

[{name=john, age=28}, {name=paul, age=30}, {name=adam, age=50}]

Each entry in the above list a map with keys as name and age and values as their corresponding values.

This is what I have done to achieve the result.

listOfStrings.stream()
  .map(record -> Arrays.asList(record.split(",")).stream().map(field -> field.split("="))
  .collect(Collectors.toMap(keyValue -> keyValue[0].trim(), keyValue -> keyValue[1].trim())))
  .collect(Collectors.toList());

I would like to know if that is efficient or if there is a better way to do it using Java streams.

Answer 1

Here is another alternative using pattern matching, not as fast as a for loop but much faster than the original stream solution in my measurements.

public static void main(String[] args) {
    String test ="name=john,age=28;name=paul,age=30;name=adam,age=50";
    String patternString = "(name)=(\\w*),(age)=(\\d*)[;]?";
    Pattern pattern = Pattern.compile(patternString);
    Matcher matcher = pattern.matcher(test);
    List<Map<String, String>> list = new ArrayList<>();

    while (matcher.find()) {
        Map<String, String> map = new HashMap<>();
        map.put(matcher.group(1), matcher.group(2));
        map.put(matcher.group(3), matcher.group(4));
        list.add(map);
    }
}

A slight performance improvement could be had by not matching against the keys (name & age) and instead hardcoding them when creating the map elements.

Answer 2

If you are out for performace, ditch the Stream API. Especially streams with substreams are really bad for writing highly performant applications.

Here is a comparison of your Stream API version vs. a plain old for-loop:

public static void main(String[] args) {
  final String test = "name=john,age=28;name=paul,age=30;name=adam,age=50";

  final List<Map<String, String>> result1 = loop(test);
  final List<Map<String, String>> result2 = stream(test);

  System.out.println(result1);
  System.out.println(result2);
}


private static List<Map<String, String>> loop(String str) {
  long start = System.nanoTime();

  List<Map<String, String>> result = new ArrayList<>();
  String[] persons = str.split(";");

  for (String person : persons) {
    String[] attributes = person.split(",");
    Map<String, String> attributeMapping = new HashMap<>();

    for (String attribute : attributes) {
      String[] attributeParts = attribute.split("=");

      attributeMapping.put(attributeParts[0], attributeParts[1]);
    }

    result.add(attributeMapping);
  }

  long end = System.nanoTime();
  System.out.printf("%d nano seconds\n", (end - start));

  return result;
}

private static List<Map<String, String>> stream(final String str) {
  long start = System.nanoTime();

  List<String> listOfStrings = Arrays.asList(str.split(";"));
  List<Map<String, String>> result = listOfStrings.stream()
    .map(record -> Arrays.asList(record.split(",")).stream().map(field -> field.split("="))
    .collect(Collectors.toMap(keyValue -> keyValue[0].trim(), keyValue -> keyValue[1].trim())))
    .collect(Collectors.toList());

  long end = System.nanoTime();

  System.out.printf("%d nano seconds\n", (end - start));

  return result;
}

Outpout:

183887 nano seconds

53722108 nano seconds

[{name=john, age=28}, {name=paul, age=30}, {name=adam, age=50}]

[{name=john, age=28}, {name=paul, age=30}, {name=adam, age=50}]