Lowercase all HashMap keys

Instead of using HashMap, you could try using a TreeMap with case-insensitive ordering. This would avoid the need to create a lower-case version of each key:

Map<String, Long> map = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
map.putAll(myMap);

Once you've constructed this map, put() and get() will behave case-insensitively, so you can save and fetch values using all-lowercase keys. Iterating over keys will return them in their original, possibly upper-case forms.

Here are some similar questions:

• Case insensitive string as HashMap key
• Is there a good way to have a Map<String, ?> get and put ignoring case?

You cannot remove the entry while iterating over the map. You will have a ConcurentModificationException if you try to do this.

As the issue is an OutOfMemoryError, not a performance error, using parallel stream will not help either.

Despite some task on the Stream API will be done lately, this will still lead to have two maps in memory at some point so you will still have the issue.

To workaround it, I only saw two ways :

• Give more memory to your process (by increasing -Xmx on the Java command line). Memory is cheap these days ;)
• Split the map and work in chunks : for example you divide the size of the map by ten and you process one chunck at a time and delete the processed entries before processing the new chunk. By this instead of having two times the map in memory you will just have 1.1 times the map.

For the split algorithm, you can try someting like this using the Stream API :

Map<String, String> toMap = new HashMap<>();            
int chunk = fromMap.size() / 10;
for(int i = 1; i<= 10; i++){
    //process the chunk
    List<Entry<String, String>> subEntries = fromMap.entrySet().stream().limit(chunk)
        .collect(Collectors.toList());  

    for(Entry<String, String> entry : subEntries){
        toMap.put(entry.getKey().toLowerCase(), entry.getValue());
        fromMap.remove(entry.getKey());
    }
}