Java case-insensitive map with null key

Question

Is there a Map implementation in Java that will use case-insensitive String matching for the key, but also supports the null key? I know that

new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER)

supports case-insensitive matching of String keys, but it doesn't support the null key.

If you use your own class for the key it is up to you to implement `equals` and `hashCode` so that they work case insensitive. — Henry, Jan 02 '18 at 17:15
Perhaps use `Objects.toString(key)` when inserting in the map. That will map the key to the string `null`. — Henrik Aasted Sørensen, Jan 02 '18 at 17:16
Related: [null-safe mapping Comparator using default implementations](https://stackoverflow.com/q/28499846). [How to handle nulls when using Java collection sort](https://stackoverflow.com/q/3671826). [What to do with null fields in compare()?](https://stackoverflow.com/q/128042) [comparator with null values](https://stackoverflow.com/q/2401606) — Bernhard Barker, Jan 02 '18 at 19:27

Henrik Aasted Sørensen · Answer 1 · 2018-01-02T18:02:12.877

31

If you're on Java 8, the following will create a null-compatible, case-insensitive TreeMap:

Comparator<String> cmp = Comparator.nullsFirst(String.CASE_INSENSITIVE_ORDER);
TreeMap<String, String> map = new TreeMap<>(cmp);

No external libraries needed.

edited Jan 02 '18 at 18:02

answered Jan 02 '18 at 17:23

Henrik Aasted Sørensen

6,966
11
51
60

2

Worth noting that by using a TreeMap, `get`, `put` and `remove` operations will be O(log(n)) – Michael Jan 03 '18 at 09:48

score 8 · Answer 2 · answered Jan 02 '18 at 17:15

You can use CaseInsensitiveMap for this. This will fulfill your requirement. It is case-insensitive as well as supports null keys.

From the doc

A case-insensitive Map. Before keys are added to the map or compared to other existing keys, they are converted to all lowercase in a locale-independent fashion by using information from the Unicode data file.

Null keys are supported.

The keySet() method returns all lowercase keys, or nulls.

score 5 · Answer 3 · answered Jan 02 '18 at 17:21

If you would prefer not using external libraries, you could make your own wrapper for String.CASE_INSENSITIVE_ORDER that sorts nulls in a predictable way:

 NavigableMap<String,String> m = new TreeMap(
    new Comparator<String>() {
        public int compare(String s1, String s2) {
            if (s1 == null && s2 == null) return 0;
            if (s1 != null && s2 != null) {
                return String.CASE_INSENSITIVE_ORDER.compare(s1, s2);
            }
            return s1 == null ? -1 : 1;
        }
    }
 );

Demo.

Java case-insensitive map with null key

3 Answers3