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examples.stream().collect(
Collectors.groupingBy(Example::getK1,
Collectors.mapping(Example::getK2,
Collectors.toList())));
It returns a list of value of k2 not a map;
I'm most of the time with javascript and it's easy to do such kind of work because it's dynamic I think.
Map<String,Map<String,Boolean>> map =
examples.stream()
.collect(Collectors.groupingBy(Example::getK1,
Collectors.toMap(Example::getK2,v->true,(v1,v2)->v1)));
This is assuming both getK1() and getK2() return a String.
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