Using a stream, how to sort a list of objects by field (in my case ,componentCode) that has the maximum number of duplicates, and then find distinct
I tried something like this, but how to add the size of the duplicates when sorting.
List<String> conflictingComponentsCode = componentWarnings.stream()
.sorted(Comparator.comparing(ComponentErrorDetail::getComponentCode))
.map(ComponentErrorDetail::getComponentCode)
.distinct()
.collect(Collectors.toList());
Very similar to @nafas option:
List<String> conflictingComponentsCode = componentWarnings.stream()
.map(ComponentErrorDetail::getComponentCode)
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
.entrySet()
.stream()
.sorted(Map.Entry.<String, Long>comparingByValue().reversed())
.map(Map.Entry::getKey)
.collect(Collectors.toList());
You can check this question for another example of grouping by count: Group by counting in Java 8 stream API
.map(ComponentErrorDetail::getComponentCode) .collect(Collectors.groupingBy(Function.identity(), Collectors.counting())), you can simply use .collect(Collectors.groupingBy(ComponentErrorDetail::getComponentCode, Collectors.counting()))the idea is to use Collectors.groupingBy function to make a map (value to count), then sort the map in reverse order then map back to list again.
here a rough implementation:
List<String> conflictingComponentsCode =
componentWarnings.stream()
.map(ComponentErrorDetail::getComponentCode).collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting())
)
//sort map
.entrySet().stream()
.sorted(Map.Entry.<String, Long>comparingByValue()
.reversed())
//map to list
.map(entry -> entry.key()).collect(Collectors.toList());
;
entrySet the second time.
collect(...) is too and that's fine), the problem is that forEachOrdered(...) has a void return type.collect returns a value - the reason it seems ok here is that all further operations are called on the return value. In truth, the stream generated by componentWarnings::stream can no longer be operated on, the moment that collect is called.forEachOrdered is terminal, but that it is void. I used collect as an example that a terminal operation is fine in a method chain like this. It's having a void method that breaks the chain, not having a terminal operation.void return type of forEachOrdered and the finalMap appearing out of nowhere. Besides the point that this collecting this sorted stream into a map, just to stream again over it, before collecting into the final list, is entirely obsolete.