How to map List of objects to Map of values from the given object using java 8

Question

How to map List<File> into Map<String, List<String>> without creating any additional classes using Java 8 ?

Key should be parent file name, and list should contain all children file names.

This would return Map<String, List<File>> so it doesn't compile.

Function<File, String> parentName = (f) -> f.getParent();
List<File> files = new ArrayList<>();

Map<String, List<String>> var = files
        .stream()
        .collect(Collectors.groupingBy(parentName));

Does this help stackoverflow.com/questions/23213891/…?

dzieciou
– dzieciou

2016-01-29 13:47:05 +00:00
Commented Jan 29, 2016 at 13:47 — dzieciou
– dzieciou, Commented Jan 29, 2016 at 13:47
what is parent file? parent directory?

Gavriel
– Gavriel

2016-01-29 13:51:41 +00:00
Commented Jan 29, 2016 at 13:51 — Gavriel
– Gavriel, Commented Jan 29, 2016 at 13:51

Tunaki · Accepted Answer · 2016-01-29 13:47:52Z

4

You need to add a mapping downstream collector to the group by operation that maps the File Stream element into its name and collects those name into a List.

Map<String, List<String>> var = 
    files.stream()
         .collect(Collectors.groupingBy(
             File::getParent,
             Collectors.mapping(File::getName, Collectors.toList())
         ));

answered Jan 29, 2016 at 13:47

Tunaki

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