So I have a map that takes a Long as a key and a list as a value. I want both the map and the list in each entry to be unmodifiable.
Map<Long, List<VariantValueDto>> variantValues = productVariantValues.stream()
.map(p -> new VariantValueDto(
p.getProductVariantId(), p.getOptionId(), p.getOptionName(), p.getOptionValueId(), p.getValueName()
))
.collect(Collectors.groupingBy(VariantValueDto::getProductVariantId));
how to return such results?
For the unmodifiable list, groupingBy takes a collector as an optional 2nd argument, so you can pass Collectors.toUnmodifiableList(), rather than the implicit default toList().
For the unmodifiable map, there is a function collectingAndThen which takes a collector and a function to be applied at the end, and returns a new collector. So you can wrap the groupingBy in that and call unmodifiableMap on it.
Map<Long, List<VariantValueDto>> variantValues = productVariantValues.stream()
.map(p -> new VariantValueDto(...))
.collect(
Collectors.collectingAndThen(
Collectors.groupingBy(
VariantValueDto::getProductVariantId,
Collectors.toUnmodifiableList()
),
Collections::unmodifiableMap
)
);