Case Insensitive map in Java

Question

I have a code that writes to a text file and then prints out how many times each word in the text file is found. It works fine, but it needs to be case insensitive. And I'm stuck. May you can suggest how can it be done?

    Map<String, Integer> map = new LinkedHashMap<>();
    for (String w : data) {
        Integer n = map.get(w.);
        n = (n == null) ? 1 : ++n;
        map.put(w, n);
    }
    System.out.println(map);

Here's the code. Thank you for your time.

Answer 1

As others have said, you need to lowercase (or uppercase) the keys before putting entries into the map. For counting words, you can use Map.merge :

Map<String, Integer> frequencies = new LinkedHashMap<>();
data.forEach(w -> frequencies.merge(w.toLowerCase(), 1, Integer::sum);

Alternately, you could do it with streams:

Map<String, Long> frequencies = data.stream()
    collect(Collectors.groupingBy(
            String::toLowerCase,
            Collectors.counting()));

Answer 2

只需在for循环内将.toLowerCase()用于w

Answer 3

You can convert the strings into lower case while putting into the map.

Map<String, Integer> map = new LinkedHashMap<>();
for (String w : data) {
    Integer n = map.get(w.toLowerCase());
    n = (n == null) ? 1 : ++n;
    map.put(w.toLowerCase(), n);
}
System.out.println(map);

Answer 4

You can use CaseInsensitiveMap from Apache Commons Collections.

Others have suggested changing the keys to lowercase but this may not be an option since other parts of the program could also have access to the map and expect the keys to be in their original format. So you need to tread carefully depending on the use case.