Retrieving List<String> from List<Object> containing List<String> with java stream

Question

I have this situation:

Class Employee{
//some attributes
List<String> idContract

//getter, setter
}

And I need to retrieve a List idContracts from a list using java 8 stream.

I was trying something like that:

lst.stream().filter(o->!o.getLstDipFuoriSoglia().isEmpty())
      .map(ResultOdg::getLstDipFuoriSoglia)
      .collect(Collectors.toList());

but that, of course, returns a List< List < String >>, so how can I achieve that goal?

Thanks for your answers

Use flatMap. docs.oracle.com/javase/8/docs/api/java/util/stream/… — kaya3
– kaya3, Commented Jan 20, 2020 at 16:14
Leave the .map as it is, and add .flatMap(List::stream). — kaya3
– kaya3, Commented Jan 20, 2020 at 16:17
What is getLstDipFuoriSoglia in the code? Is lst a List<Employee>? How is ResultOdg defined? — Naman
– Naman, Commented Jan 21, 2020 at 2:32

Ryuzaki L · Accepted Answer · 2020-01-20 16:32:18Z

7

Just add flatMap operation for converting List to Stream

lst.stream().filter(o->!o.getLstDipFuoriSoglia().isEmpty)
            .map(ResultOdg::getLstDipFuoriSoglia)
            .flatMap(List::stream)
            .collect(Collectors.toList());

Or you can have one flatMap operation

lst.stream().filter(o->!o.getLstDipFuoriSoglia().isEmpty)
            .flatMap(list->list.getLstDipFuoriSoglia().stream())
            .collect(Collectors.toList());

edited Jan 20, 2020 at 16:32

answered Jan 20, 2020 at 16:17

Ryuzaki L

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