Java 8 Stream + Map + ForEach + Collect

Question

I am reading values from a File and get List. like below -

List<String> fields = Utils.readFile("myText.txt", true);

Once I get this List<String> I want to iterate through this list and pass each value to a function which uses this value, queries into DB and prepares a List<MyCustomObject> and returns the same.

Implementing in Java 7 is quite easy but I'm unable to get it in Java 8 using Stream + Map/forEach + Collect operations.

I have tried the below -

Utils.readFile("myText-"+".txt", true)
        .stream()
        .map(str -> getListOfCustomObjForEachStr(str, pstmt))
        .collect(Collectors.toList());

But it returns a List<List<MyCustomObject>> instead of merged List<MyCustomObject>

getListOfCustomObjForEachStr() method returns a List for each input str passed in the above code. And I need a merged List

Can someone please guide me what can be done here to get a merged list? (I'm new to Java 8 so it might seem a silly question but any help is appreciated)

Edit 1 Got this implemented like below using FlatMap - Kindly suggest if any other better way to do it. Hope it helps someone somewhere someday :) Keep sharing.

List<MyCustomObjects> = Utils.readFile("myText-"+".txt", true)
    .stream()
    .map(str -> getListOfCustomObjForEachStr(str, pstmt))
    .flatMap(Collection::stream)
    .collect(Collectors.toList());

Answer 1

Using flatMap is the canonical way to get a flat list. You may use a single step instead of map+flatMap:

List<MyCustomObjects> list = Utils.readFile("myText-"+".txt", true)
    .stream()
    .flatMap(str -> getListOfCustomObjForEachStr(str, pstmt).stream())
    .collect(Collectors.toList());

You may think about an alternative to getListOfCustomObjForEachStr returning a Stream in the first place. Further, not the existence of Files.lines, which returns a stream of line strings without filling a temporary List first.

In this specific case, there is an alternative. Considering that .collect(Collectors.toList()) is equivalent to .collect(ArrayList::new, List::add, List::addAll)¹, you also use

List<MyCustomObjects> list = Utils.readFile("myText-"+".txt", true)
    .stream()
    .map(str -> getListOfCustomObjForEachStr(str, pstmt))
    .collect(ArrayList::new, List::addAll, List::addAll);

replacing add with addAll, which is already sufficient to add all sub-List<MyCustomObjects>s to the result List<MyCustomObjects>.

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¹ That’s how toList() works now, but since it not specified to do this, it might be different in other versions