Convert to Lambda expression using java?

Question

I am newly learning lambda expression. I am trying to calculate values.

Here is example:

double sendersCount = 0.0;
double reciversCount = 0.0;
for(RecordDTO record : records){
   if("1".equals(record.getSendersId())) {
     sendersCount += record.getCount().doubleValue();
   } else {
     reciversCount +=  record.getCount().doubleValue();
   }
}

class Record{

private String id;
private BigDecimal count;
private String senderId;

//setters
//getters
}

How we can achieve using Streams and maps?

IMHO your current approach would be better than any stream approach. you're better off sticking with the current approach. — Ousmane D.
– Ousmane D., Commented Jun 12, 2020 at 12:20
Record is a bad name for a class. It's used in Java 14 by java.lang.Record. — ZhekaKozlov
– ZhekaKozlov, Commented Jun 12, 2020 at 15:59

Naman · Accepted Answer · 2020-06-12 12:26:10Z

6

In one way, you can possibly partition the list and then sum the appropriate attribute as -

Map<Boolean, List<Record>> partitioned = records.stream()
        .collect(Collectors.partitioningBy(rec -> rec.getSendersId().equals("1")));

double sendersCount = partitioned.get(Boolean.TRUE).stream()
        .mapToDouble(Record::getSendersCount).sum();
double receiversCount = partitioned.get(Boolean.FALSE).stream()
        .mapToDouble(Record::getReceiversCount).sum();

or as Ousmane has pointed out, you can use downstream summingDouble with a condition to map such as:

Map<Boolean, Double> summingValue = records.stream()
        .collect(Collectors.partitioningBy(r -> "1".equals(r.getSendersId()),
                Collectors.summingDouble(r -> "1".equals(r.getSendersId()) ?
                        r.getSendersCount() : r.getReceiversCount())));

edited Jun 12, 2020 at 12:26

answered Jun 12, 2020 at 12:06

Naman

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