Flatten Map<String,Set<String>> to List<String> (CSV conversion) with streams

Question

Given input (Map key:value)

"a":["e","c","d"]
"b":["c"]

Desired sorted output:

"a,c"
"a,d"
"a,e"
"b,c"

It could really be as simple as : map.forEach((k,v) -> v.forEach( val -> list.add(k + "," +val))) . Give it a try once! — Naman
– Naman, Commented Mar 14, 2019 at 17:32
@hey_you alright, what is not functional in it? could you elaborate bad here in terms of what? — Naman
– Naman, Commented Mar 14, 2019 at 17:34
@Naman the fact that you're using foreach everywhere, might as well write a normal for instead. — Pavel Orekhov
– Pavel Orekhov, Commented Mar 14, 2019 at 17:35

Naman · Accepted Answer · 2019-03-14 18:22:57Z

1

What you could possibly be looking for is using flatMap with a sort of values tweaked to compare converted integer values as :

return map.entrySet().stream()
        .flatMap(e -> e.getValue().stream()
                .sorted(Comparator.comparingInt(Integer::parseInt)) // or just .sorted()
                .map(v -> e.getKey() + "," + v))
        .collect(Collectors.toList());

edited Mar 14, 2019 at 18:22

answered Mar 14, 2019 at 18:17

Naman

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Comments

Costi Ciudatu · Accepted Answer · 2019-03-14 19:03:34Z

1

To keep this readable, it's best to split the code in two methods, just so you don't inline the flatMap logic:

public List<String> toPairs(Map<String, Set<String> map) {
    return map.entrySet().stream()
        .flatMap(this::entryPairs)
        .sorted() // if you want the whole output to be sorted
        .collect(Collectors.toList())
}

private Stream<String> entryPairs(Map.Entry<String, Set<String>> entry) {
    return entry.getValue().stream()
        // .sorted() // you can sort the values for each key here, but this is useless if you're sorting above
        .map(v -> entry.getKey() + ',' + v)
}

edited Mar 14, 2019 at 19:03

answered Mar 14, 2019 at 18:35

Costi Ciudatu

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Comments

Alexei Vinogradov · Accepted Answer · 2019-03-14 18:19:00Z

0

Not sure if it is a perfect solution but it is working.

map.entrySet().stream()
  .map(entry.getValue().stream()
     .sorted()
     .map(value->entry.getKey()+","+value)
     .collect(Collectors.joining("\n"))
  )
  .sorted()
  .collect(Collectors.joining("\n"));

answered Mar 14, 2019 at 18:19

Alexei Vinogradov

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