How to Lowercase or Uppercase Map Keys in Java

Published May 29, 2022  ∙  Updated Jul 2, 2022

How can we lowercase or uppercase all map keys in Java?

Suppose we’re working with a simple map.

Map<String, Object> map;

1. Lowercase using toLowerCase()

We can lowercase all the map keys by creating a new map.

map.entrySet().stream().collect(
   Collectors.toMap(
      entry -> entry.getKey().toLowerCase(),
      entry -> entry.getValue()
   )
);

This will not mutate the original map, so we’ll have to assign this to a new map to use the updated keys.

2. Uppercase using toUpperCase()

We can run through the exact same stream using toUpperCase() instead of toLowerCase().

map.entrySet().stream().collect(
   Collectors.toMap(
      entry -> entry.getKey().toUpperCase(),
      entry -> entry.getValue()
   )
);

3. Lowercase/Uppercase nested map

Suppose we’re working with a nested map:

Map<String, Map<String, Object>> nestedMap;

We can simply run the same logic in a nested fashion.

map.entrySet().stream().collect(
   Collectors.toMap(
      e1 -> e1.getKey().toLowerCase(),
      e1 -> e1.getValue().entrySet().stream().collect(
         Collectors.toMap(
            e2 -> e2.getKey().toLowerCase(),
            e2 -> e2.getValue()
         )
      )
   )
);

And of course, we can call toUpperCase() instead of toLowerCase().

Remember to assign the output to a new map in order to use the lowercase or uppercase keys.