Java 8 FlatMap - Add items in a nested form and retrieve object using FlatMap

Question

I have an object Foo which contains a list of Bar. The classes are described as follows:

class Foo {
    String name;
    List<Bar> bars = new ArrayList<Bar>();

    Foo(String name){
        this.name = name;
    }
}

class Bar {
    String name;

    Bar(String name){
        this.name = name;
    }
}

Now, i am creating a list of Foo objects, each containing a list of Bar objects as follows:

IntStream
        .range(1, 4)
        .forEach(i -> foos.add(new Foo("Foo" + i)));

foos.forEach(f ->
            IntStream.range(1,4)
                    .forEach(i -> f.bars.add(new Bar("Bar"+i+" -> "+f.name))));

And then using flatMap on a Stream as follows:

foos.stream()
        .flatMap(f -> f.bars.stream())
        .forEach(i -> System.out.println("Bar Name : "+i.name));

How can do all these things in a single execution using Java Stream and lambdas? Is there any other way to do such kind of things with Java 8 style?

score 4 · Accepted Answer · answered Dec 01 '16 at 10:02

If all you’re going to do, is to print the names of the Bar instances, the entire construction and collection of Foo and Bar instances is obsolete. You can generate and print the names directly:

IntStream.range(1, 4)
         .mapToObj(i -> "Foo" + i)
         .flatMap(name -> IntStream.range(1, 4)
            .mapToObj(i -> "Bar" + i + "->" + name))
         .forEach(System.out::println);

Eran · Answer 2 · 2016-12-01T10:00:09.320

You can generate the Foo instances in mapToObj instead of forEach :

IntStream.range(1, 4)
         .mapToObj (i -> {Foo foo = new Foo("Foo" + i);
                          foo.bars =
                              IntStream.range(1,4)
                                       .mapToObj(j -> new Bar("Bar"+j+" -> "+foo.name))
                                       .collect(Collectors.toList());
                          return foo;
                    })
         .flatMap (f -> f.bars.stream())
         .forEach(i -> System.out.println("Bar Name : "+i.name));

This, however, won't store the created Foo instances in a List. If you want to store them in a List, either split the operation to 2 Stream pipelines (the first ending in collect(Collectors.toList())) or use peek(f->foos.add(f)) to add the Foo instances into the foos List (which will require that you instantiate the foos List before running the pipeline).

EDIT:

Fixed some typos and tested the code :

Bar Name : Bar1 -> Foo1
Bar Name : Bar2 -> Foo1
Bar Name : Bar3 -> Foo1
Bar Name : Bar1 -> Foo2
Bar Name : Bar2 -> Foo2
Bar Name : Bar3 -> Foo2
Bar Name : Bar1 -> Foo3
Bar Name : Bar2 -> Foo3
Bar Name : Bar3 -> Foo3

I got the following compile error using your answer: The method mapToObj(IntFunction extends U>) in the type IntStream is not applicable for the arguments (( i) -> {}) — KayV, Dec 01 '16 at 09:55

KayV · Answer 3 · 2016-12-01T10:01:06.310

0

I also got the resolution from following pipeline operations using peek and mapToObject...

IntStream.range(1, 4)
            .mapToObj(i -> new Foo("Foo"+i))
            .peek(f -> IntStream.range(1, 4)
                    .mapToObj(i -> new Bar("Bar"+i+"->"+f.name))
                    .forEach(f.bars::add))
            .flatMap(f -> f.bars.stream())
            .forEach(b -> System.out.println("Bar : "+b.name));

edited Dec 01 '16 at 10:01

answered Dec 01 '16 at 09:53

KayV

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1

Using `peek` for that is not recommended. – Holger Dec 01 '16 at 10:00
What is the problem with peek? – KayV Dec 01 '16 at 10:03
2

It’s explicitly declared to be a function mainly for debugging. See also [“In Java streams is peek really only for debugging?”](http://stackoverflow.com/q/33635717/2711488) – Holger Dec 01 '16 at 10:05
Got it now @Holger – KayV Dec 01 '16 at 10:10

Java 8 FlatMap - Add items in a nested form and retrieve object using FlatMap

3 Answers3