Initialisation of object using streams

menteith :

I have a Long value that depending whether a certain list has a null as one of its fields, should be set to null and when no null is found should be set to sum of these fields of the list.

Please see my (working code) that achieves this:

Long usedBytes = bdList.stream()
                       .anyMatch(bd -> bd.getBytes() == null) ? 
                 null :
                 bdList.stream()
                       .mapToLong(Backup::getBytes)
                       .sum();

I'm wondering why similiar and much simplier code won't set Long usedBytes to null when there is a null as one of its fields. In this scenario Long totalUsedSpace is set to 0L.

Long totalUsedSpace = bdList.stream().filter(bd -> bd.bd.getBytes() != null)
                                      .mapToLong(Backup::getBytese)
                                      .sum();

Eran :

long java.util.stream.LongStream.sum()

As you can see, LongStream's sum method returns a long, not a Long, so it can never return null. It returns 0 when the LongStream is empty.

Besides, if there is null only in some of the fields, the second snippet would return the sum of the values of the non-null fields.

You can achieve your logic with a single Stream pipeline using reduce:

Long usedBytes = bdList.stream ()
                       .reduce (Long.valueOf (0),
                                (sum,bd) -> sum == null || bd.getBytes() == null ? null : sum + bd.getBytes(),
                                (sum1,sum2) -> sum1 == null || sum2 == null ? null : sum1 + sum2);

I couldn't test this code, but I tested similar code that computes the sum of String lengths and returns null if any of the Strings are null:

List<String> strlist = Arrays.asList ("One","Two","Three");
Long sumOfLengths = strlist.stream ()
                           .reduce (Long.valueOf (0),
                                    (sum,s) -> sum == null || s == null ? null : sum + s.length(),
                                    (sum1,sum2) -> sum1 == null || sum2 == null ? null : sum1 + sum2);
System.out.println (sumOfLengths);

This prints

And if you change the input List to

List<String> strlist = Arrays.asList ("One",null,"Two","Three");

it prints

null

Initialisation of object using streams

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