convert from Map<Object,Set<Object>> to Map<String,Set<String>>

Question

I have a map Map<String, Set<String>>

Map<String, Set<String> result = map.entrySet().parallelStream().collect(
            Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));

I want to convert it to Map<String, Set<String>> . by grouping the values and swapping the places of key and value.

But this line gives me

Type mismatch: cannot convert from Map<Object,Set<Object>> to Map<String,Set<String>>

Your question title does not match your problem description. Further, you say you want to convert the map to Map<Set<String>, Set<String>>, but declare the result variable with an entirely different type (and there’s a > missing). So it’s not surprising that you get a compiler error. — Holger
– Holger, Commented Sep 18, 2018 at 12:07
@Holger maybe Type mismatch: cannot convert from Map<Object,Set<Object>> to Map<String,Set<String>> is the crux of it? agreed though there is clutter in the question. — Naman
– Naman, Commented Sep 18, 2018 at 12:09
@nullpointer as long as the OP doesn’t decide about what to convert to what, there is no point in thinking about the compiler error. There are three entirely different problem statements, the question’s title, the question’s text, and the code example. When I change the code example to match what the OP wrote in the textual description, there is no compiler error. — Holger
– Holger, Commented Sep 18, 2018 at 12:14

Andy Turner · Accepted Answer · 2018-09-18 16:15:22Z

4

The problem that you've got here is the type of the map you are creating is:

Map<Set<String>, Set<String>>

not Map<String, Set<String>>.

As such, you need to expand the map's values first, for example:

Map<String, Set<String>> collect = map.entrySet()
    .parallelStream()
    // Expand (k, {v1, v2, v3}) to [(v1, k), (v2, k), (v3, k)]
    .flatMap(e -> e.getValue().stream().map(ee -> new SimpleEntry<>(ee, e.getKey())))
    .collect(
        Collectors.groupingBy(
            Map.Entry::getKey,
            Collectors.mapping(Map.Entry::getValue, Collectors.toSet())));

Unless you really need the parallel processing, I think it would be much easier to use loops:

Map<String, Set<String>> collect = new HashSet<>();
for (Map.Entry<String, Set<String>> entry : map.entrySet()) {
  for (String v : entry.values()) {
      collect.computeIfAbsent(v -> new HashSet<>())
              .add(entry.getKey()));
  }
}

edited Sep 18, 2018 at 16:15

answered Sep 18, 2018 at 11:08

Andy Turner

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user10367961 · Accepted Answer · 2018-09-18 11:16:18Z

1

Here is an example considering your initial Map is Object to Object. Adapt as needed.

Map<Object,Object> map = new HashMap<>();
Map<String, Set<String>> result = map
            .entrySet()
            .parallelStream()
            .collect(Collectors.groupingBy(entry -> (String) entry.getKey(), 
                     Collectors.mapping(entry -> (String) entry.getKey(), Collectors.toSet())));

The problem with your code is that Map.Entry::getKey returns an Object, not a String.

answered Sep 18, 2018 at 11:16

user10367961

3 Comments

Dave Over a year ago

I personally prefer entry.getKey().toString() rather than casting to a String, but have an upvote regardless. :) Of course, my method leaves you succeptable to NullPointerException (since some implementations of Map can have a null key, and obviously you can have a null value).

Andy Turner Over a year ago

Note that you're not doing what OP wants (you're not using the map values at all).

Sorter Over a year ago

Hi @alexrolea, your answer helped me. But like @AndyTurner mentioned It will generate Map<Set<String>, String<String>>

Sorter · Accepted Answer · 2018-09-19 05:38:48Z

Just to avoid the confusion, I'm answering my question. Thanks to @AndyTurner @alexrolea for pointing out the solution.

Map<Set<String>, Set<String>> result = map.entrySet().parallelStream()
                .collect(
                Collectors.groupingBy(entry -> (Set<String>) entry.getValue(), 
                        Collectors.mapping(entry -> entry.getKey(), Collectors.toSet())));

I had to replace Map.Entry::getValue with entry -> (Set<String>) entry.getValue() and the other one too.

This helped me group the map by values and use them as keys. Thanks @nullpointer

In fact, this also works. The problem is I was not returning the right datatype.

Map<Set<String>, Set<String>> result = map.entrySet().parallelStream()
                .collect(
                Collectors.groupingBy(Map.Entry::getValue, 
                        Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));

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