Optimize method to iterate to over Map<String, List<Object>>

Question

I have a Map<String, List<Object>> multiFieldMap and I need to iterate over its value set and add the value to multiFieldsList:

public List<Object> fetchMultiFieldsList() {
    List<Object> multiFieldsList = new ArrayList<>();
    for (Entry<String, List<Object>> entry : multiFieldMap.entrySet()) {
        String entityName = entry.getKey();
        List<Object> ids = entry.getValue();
        for (Object id : ids) {
            Object entity = queryService.query(entityName, queryService.property("id").eq(id));
            multiFieldsList.add(entity);
        }
    }
    return multiFieldsList;
}

Can this method be simplified further?

Alexis King · Accepted Answer · 2015-04-27 08:49:37Z

You can accomplish this using Java 8 streams.

public List<Object> fetchMultiFieldsList() {
    return multiFieldMap.entrySet().stream()
      .flatMap(e -> {
          String entityName = e.getKey();
          List<Object> ids = e.getValue();
          return ids.stream()
            .map(id -> queryService.query(entityName, queryService.property("id").eq(id)));
      }).collect(Collectors.toList());
}

I think the code itself is relatively self-explanatory. The important bit is the usage of flatMap instead of map for the first mapping operation so that the resulting streams are concatenated, ending up with a flat list of objects rather than a list of lists.

It is unfortunate that Java does not support destructuring assignment, which would allow the first lambda expression to be a single expression of the form (k, v) -> ..., but it does not, so I've left the statements in for clarity.

If you are willing to omit the inner declarations, the above code can be simplified further.

public List<Object> fetchMultiFieldsList() {
    return multiFieldMap.entrySet().stream()
      .flatMap(e -> e.getValue().stream()
          .map(id -> queryService.query(e.getKey(), queryService.property("id").eq(id)))
      ).collect(Collectors.toList());
}

Whichever one you prefer is a matter of personal preference. The former has more descriptive variable names, while the latter is simpler and more functional.

Christoph Walesch · Accepted Answer · 2015-04-27 11:15:44Z

1

You could replace the map with a multimap. Using Google's Guava Multimap, you don't need the nested loop / flatMap():

private Multimap<String, Object> multiFieldMap = ArrayListMultimap.create();

public List<Object> fetchMultiFieldsList() {
    return multiFieldMap.entries().stream()
        .map(e -> queryService.query(e.getKey(),
                      queryService.property("id").eq(e.getValue())))
        .collect(Collectors.toList());
}

answered Apr 27, 2015 at 11:15

Christoph Walesch

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