CODEWITHSUNDEEP
javacollectionsjava-8java-stream
gstackoverflow
gstackoverflow

Reputation: 36984

Transform List to List<List> according the source List content

I have following object DataPoint:

public class DataPoint {
   private String name:
   private int uploadIndex;
   ...
}

I have List:

{uploadIndex:1, name:"name_1"}
{uploadIndex:5, name:"name_2"}
{uploadIndex:3, name:"name_3"}
{uploadIndex:3, name:"name_4"}
{uploadIndex:4, name:"name_5"}
{uploadIndex:5, name:"name_6"}
{uploadIndex:2, name:"name_7"}
{uploadIndex:4, name:"name_8"}

I want to transform it to the List<List<DataPoint>> sorted by uploadIndex.
Result should be(ordering inside the internal list is not important):

{
   {uploadIndex:1, name:"name_1"}
} 
{
   {uploadIndex:2, name:"name_7"}
} 
{
   {uploadIndex:3, name:"name_3"}
   {uploadIndex:3, name:"name_4"}
} 
{
   {uploadIndex:4, name:"name_5"}
   {uploadIndex:4, name:"name_8"}
} 
{    
   {uploadIndex:5, name:"name_2"}
   {uploadIndex:5, name:"name_6"}
}

I wrote following code:

List<DataPoint> dataPoints = getFromDb();
Map<Integer, List<DataPoint>> groupedDataPoints = dataPoints.stream()
                    .collect(Collectors.groupingBy(DataPoint::getUploadIndex, TreeMap::new, Collectors.toList()));
List<List<DataPoint>> groupedList = new ArrayList<>(groupedDataPoints.values());

1. Does this code guarantee the ordering?
2. Is there more concise way to implement it?

Upvotes: 3

Views: 78

Answers (1)

Eugene
Eugene

Reputation: 120858

 dataPoints.stream()
           .collect(
                 Collectors.collectingAndThen(
                      Collectors.groupingBy(
                         DataPoint::getUploadIndex, 
                         TreeMap::new, 
                         Collectors.toList()
                      )
                 ),
                      map -> new ArrayList(map.values())
           );

You could use Collectors.collectingAndThen; other than that your solution looks fine to me.

Upvotes: 4

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