Can someone help me with this "functional programm...

# codereview

[ecampolo]

Can someone help me with this "functional programming style" problem? I've spent several hours with no luck

class Foo(val bars: List<String> = ArrayList())

class FooBar(val foo: Foo, val bar: String?)

fun main(args: Array<String>) {

    val foo1 = Foo(listOf("a", "b", "c"))
    val foo2 = Foo()
    val foo3 = Foo(listOf("d"))
    val foo4 = Foo()

    val foos = listOf(foo1, foo2, foo3, foo4)

    // How can I flat map into a list of:
    
    // FooBar(foo1, "a")
    // FooBar(foo1, "b")
    // FooBar(foo1, "c")
    // FooBar(foo2, null)
    // FooBar(foo3, "d")
    // FooBar(foo4, null)

    // This is actually working but I would like to avoid creating the list a priori and
    // remove the conditional
   val fooBars: MutableList<FooBar> = ArrayList()
   foos.forEach { foo ->
        if (foo.bars.isEmpty()) {
            fooBars.add(FooBar(foo, null))
        } else {
            foo.bars.forEach { bar -> fooBars.add(FooBar(foo, bar)) }
        }
    }
}