Java 8 using stream, flatMap and lambda

Question

I have this piece of code and I want to return a list of postCodes:

List<String> postcodes = new ArrayList<>();
List<Entry> entries = x.getEntry(); //getEntry() returns a list of Entry class
for (Entry entry : entries) {
    if (entry != null) {
       Properties properties = entry.getContent().getProperties();
       postcodes.addAll(Arrays.asList(properties.getPostcodes().split(",")));
   }
} 
return postcodes;

Here's my attempt to use stream() method and the following chained methods:

...some other block of code
List<Entry> entries = x.getEntry.stream()
    .filter(entry -> recordEntry != null)
    .flatMap(entry -> {
        Properties properties = recordEntry.getContent().getProperties();
        postCodes.addAll(Arrays.asList(properties.getPostcodes().split(",")));
});

first: filter(entry -> recordEntry != null) i guess this is wrong — IEE1394
– IEE1394, Commented Dec 19, 2017 at 21:44
@AniketSahrawat Missing return statement is the smallest of his problems :-) — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Dec 19, 2017 at 21:49

Ousmane D. · Accepted Answer · 2017-12-20 13:05:52Z

5

you've got several issues with your code i.e:

postCodes.addAll is a side-effect and therefore you should avoid doing that otherwise when the code is executed in parallel you'll receive non-deterministic results.
flatMap expects a stream, not a boolean; which is what your code currently attempts to pass to flatMap.
flatMap in this case consumes a function that also consumes a value and returns a value back and considering you've decide to use a lambda statement block then you must include a return statement within the lambda statement block specifying the value to return. this is not the case within your code.
stream pipelines are driven by terminal operations which are operations that turn a stream into a non-stream value and your code currently will not execute at all as you've just set up the ingredients but not actually asked for a result from the stream.
the receiver type of your query should be List<String> not List<Entry> as within your current code the call to Arrays.asList(properties.getPostcodes().split(",")) returns a List<String> which you then add to an accumulator with the call addAll.
thanks to Holger for pointing it out, you're constantly failing to decide whether the variable is named entry or recordEntry.

That said here's how I'd rewrite your code:

List<String> entries = x.getEntry.stream()
        .filter(Objects::nonNull)
        .map(Entry::getContent)
        .map(Content::getProperties)
        .map(Properties::getPostcodes‌)
        .flatMap(Pattern.co‌mpile(",")::splitAsS‌tream)
        .collect(Collectors.toList());

and you may want to use Collectors.toCollection to specify a specific implementation of the list returned if deemed appropriate.

edit:

with a couple of good suggestions from shmosel we can actually use method references throughout the stream pipelines and therefore enabling better intent of the code and a lot easier to follow.

or you could proceed with the approach:

List<String> entries = x.getEntry.stream()
       .filter(e -> e != null)
       .flatMap(e -> Arrays.asList(
         e.getContent().getProperties().getPostcodes().split(",")).stream()
       )
       .collect(Collectors.toList());

if it's more comfortable to you.

edited Dec 20, 2017 at 13:05

answered Dec 19, 2017 at 22:55

Ousmane D.

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6 Comments

shmosel Over a year ago

You can replace Arrays.asList(...).stream() with Arrays.stream(...) or Stream.of(...).

Ousmane D. Over a year ago

@shmosel true, very much appreciated for the suggestion.

shmosel Over a year ago

If you prefer method references, you can also do

.map(Entry::getContent).map(Content::getProperties).map(Properties::getPostcodes).flatMap(Pattern.compile(",")::splitAsStream)

.

Ousmane D. Over a year ago

@shmosel once again very much appreciated. :)

Holger Over a year ago

You don’t need Arrays.asList(…).stream(); use Arrays.stream(…) to get a stream in the first place. And the list of issues is incomplete. Another issue of the original code is that the OP constantly fails to decide whether the variable is named entry or recordEntry.

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