Optimizing the list traversal with stream

user_432216 0
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I have a List<BatchDTO> with the following class

public class BatchDTO {

    private String batchNumber;
    private Double quantity;
.
.
//Getters and setters
}

What I have to do is to sum up the total if the batchNumber is duplicate. I have used a LinkedHashMap to implement this, and did the iterations. But what I would like to have is a more optimized way. Can I use stream to do this in an optimized way.

private static List<BatchDTO > getBatchDTO (Map<String, BatchDTO > batchmap) {
    return batchmap.values().stream().collect(Collectors.toList());
}

private static Map<String, BatchDTO > getBatchMap(List<BatchDTO > batchList, Map<String, BatchDTO > batchMap) {
        for (BatchDTO  batchDTO  : batchList) {
            batchMap = getBatchMap(batchMap, batchDTO );
        }
    return batchMap;
}

private static Map<String, BatchDTO > getBatchMap(Map<String, BatchDTO > batchMap, BatchDTO  batchObject) {
    String batchCode = batchObject.getBatchNumber();
        if(!batchMap.containsKey(batchCode)) {
            batchMap.put(batchCode, batchObject);
        } else {
            batchObject.setQuantity(getTotalQuantity(batchMap,batchObject));
            batchMap.put(batchCode, batchObject);
        }
    return batchMap;
}

private static Double getTotalQuantity(Map<String, BatchDTO > batchmap, BatchDTO  batchObject) {
    return batchmap.get(batchObject.getBatchNumber()).getQuantity() + batchObject.getQuantity();
}
java-stream java-8 list java
2 Answers

You could try using stream api the way @Naman suggested in comments. And assuming BatchDTO has all args constructor you could return back from Map to List<BatchDTO>

List<BatchDTO> collect = list.stream()
        .collect(groupingBy(BatchDTO::getBatchNumber, summingDouble(BatchDTO::getQuantity)))
        .entrySet().stream()
        .map(entry -> new BatchDTO(entry.getKey(), entry.getValue()))
        .collect(Collectors.toList());

JavaDoc: groupingBy(), summingDouble()

user_51087370

Probably a little unreadable with comments in the code, but this is all I have time for.

// Result will be a Map where the keys are the unique 'batchNumber's, and the
// values are the sum of the 'quantities' for those with that 'batchNumber'.
public Map<String, Double> countBatchQuantities(final List<BatchDTO> batches) {
    // Stream over all the batches...
    return batches.stream()

    // Group them by 'batch number' (gives a Map<String, List<BatchDTO>>)
            .collect(Collectors.groupingBy(BatchDTO::getBatchNumber))

    // Stream over all the entries in that Map (gives Stream<Map.Entry<String, List<BatchDTO>>>)
            .entrySet().stream()

    // Build a map from the Stream of entries
    // Keys stay the same
            .collect(Collectors.toMap(Entry::getKey, 

    // Values are now the result of streaming the List<BatchDTO> and summing 'getQuantity'
                    entry -> entry.getValue().stream().mapToDouble(BatchDTO::getQuantity).sum()));
}

Note: I make no promises that this is more optimised than your existing approach... But it does the job with Streams. Note: This will throw an exception if the quantity is null for any of your BatchDTOs...

user_24783980
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