What structure should I use to store key and list of possible values?

Question

I want to store information about filetype and possible files extension. What I want to do is if I provide file extension and this extension is on the list I will return key for it.

Eg:

Map<Filetype, List<String>> extensionMapping= new HashMap<>();
extensionMapping.put(Filetype.IMAGE, Arrays.asList("jpg", "jpeg"));
extensionMapping.put(Filetype.WORD, Arrays.asList("doc", "docx"));
extensionMapping.put(Filetype.EXCEL, Arrays.asList("xls", "xlsx", "xlsm"));
extensionMapping.put(Filetype.PPT, Arrays.asList("ppt", "pptx", "pptm"));`

And then I want to do something like this:

return extensionMapping.get().contains("jpg");

which for string "jpg" returns me Filetype.IMAGE.

Which collection should I use?

It seems like your extensions are unique per key, so I'd opt for a `Map` where the key is the extension. — Jacob G., Feb 12 '19 at 20:19
[`EnumMap`](https://docs.oracle.com/javase/7/docs/api/java/util/EnumMap.html) is a better container for cases where the key is of a single `Enum` type. — YoYo, Apr 23 '19 at 13:31

score 2 · Answer 1 · answered Feb 13 '19 at 12:56

2

I would create a reverse map, with the keys being the extension and the values the file type:

Map<String, Filetype> byExtension = new HashMap<>();
extensionMapping.forEach((type, extensions) -> 
        extensions.forEach(ext -> byExtension.put(ext, type)));

Now, for a given extension, you simply do:

FileType result = byExtension.get("jpg"); // FileType.IMAGE

answered Feb 13 '19 at 12:56

fps

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This is probably the better answer. Here is another posting to a similar question, where the mapping is being made part of the enum through a `static Map`: https://stackoverflow.com/a/12659023/744133 – YoYo Apr 23 '19 at 13:25

Ryuzaki L · Accepted Answer · 2019-02-12T20:49:41.767

1

I mean you can do that now also by having same structure Map<Filetype, List<String>>. In Map any of values (which is List of strings contains "jpg") will return Filetype.IMAGE or else it will return Filetype.None

extensionMapping.entrySet().stream().filter(entry->entry.getValue().contains("jpg")).map(Map.Entry::getKey)
      .findFirst().orElse(Filetype.None);

edited Feb 12 '19 at 20:49

answered Feb 12 '19 at 20:25

Ryuzaki L

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What structure should I use to store key and list of possible values?

2 Answers2