Using Streams / Collecting to Transform a Map
I am trying to convert a map to another map where the new key is just the original toString () key. With the streams API, I do it like this:
mapMap.entrySet().stream().collect(Collectors.toMap(
(Map.Entry entry) -> entry.getKey().toString(),
(Map.Entry entry) -> entry.getValue()
));
The problem is that this does not preserve the internal type of the card. I don't mind returning a TreeMap if the original map is a HashMap, but the other way around is a problem since the sorting of the items is removed. I have cheated with variations of the above code to do this, but I don't seem to go very far. Right now, I wrote it without threads, like this:
TreeMap<String, Integer> stringMap = new TreeMap<>();
for (OriginalType t: originalMap.keySet()) {
stringMap.put(t.toString(), originalMap.get(t));
}
Can anyone put me in the right direction to do this with streams? Thanks to
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There is an overload Collectors.toMap
that will allow you to specify what type of card you want to return.
mapMap.entrySet().stream().collect(Collectors.toMap(
(Map.Entry entry) -> entry.getKey().toString(),
(Map.Entry entry) -> entry.getValue(),
(val1, val2) -> { throw new RuntimeException("Not expecting duplicate keys"); },
() -> new TreeMap<>()
));
(A note on the third argument: it is intended as a function that concatenates two values together with the same key. When I don't expect this to happen, I prefer the exception.)
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