How to make java 8 Stream map continuously with null check

Question

I have this piece of code

Coverage mainCoverage = illus.getLifes().stream()
    .filter(Life::isIsmain)
    .findFirst()
    .orElseThrow(() -> new ServiceInvalidAgurmentGeneraliException(env.getProperty("MSG_002")))
    .getCoverages()  
    .stream() // <==may cause null here if list coverage is null
    .filter(Coverage::isMainplan)
    .findFirst()
    .orElseThrow(() -> new ServiceInvalidAgurmentGeneraliException(env.getProperty("MSG_002")));

which is totally work fine but I think It's a little bit messy and not cover all the null pointer exception possible (see the comment).

I try to refactor this code into

Coverage mainCoverage1 = illus.getLifes().stream()
    .filter(Life::isIsmain)
    .map(Life::getCoverages)
    .filter(Coverage::isMainplan) //<== cannot filter from list coverage to one main coverage
    ...

Seem after I map the life to coverage it is not a list of coverage anymore. So the question is how I can refactor the first section into null safe and maybe shorten it?

Answer 1

Life::getCoverages returns a collection hence the filter Coverage::isMainplan will not work, instead you should flatMap the sequences returned after .map(Life::getCoverages) then apply the filter operation on the Coverage:

Coverage mainCoverage = 
          illus.getLifes()
               .stream()
               .filter(Life::isIsmain)               
               .map(Life::getCoverages)
               //.filter(Objects::nonNull) uncomment if there can be null lists
               .flatMap(Collection::stream) // <--- collapse the nested sequences
               //.filter(Objects::nonNull) // uncomment if there can be null Coverage
               .filter(Coverage::isMainplan)
               .findFirst().orElse(...);

I've added a few things to your code:

1. I've added .filter(Objects::nonNull) after .map(Life::getCoverages) which you can uncomment given the elements returned could potentially be null.
2. I've added .flatMap(Collection::stream) which returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.
3. I've added another .filter(Objects::nonNull) which you can uncomment given the elements returned after flatMap could potentially be null.
4. We're then at a stage in which we can apply .filter(Coverage::isMainplan) and finally, retrieve the first object meeting the criteria via findFirst and if none then provide a default value via orElse.

I'd suggest having a look at the following blogs to get familiar with the flatMap method:

• Java 8 flatMap example
• Understanding flatMap
• A Guide to Streams in Java 8: In-Depth Tutorial with Examples