Count files recursive. Show file count per extension for folder and sub-folders

CountFilesRecursive.java

import java.io.File;
import java.util.HashMap;
import java.util.stream.Collectors;

/**
 * Execute via `java --source 11 "CountFilesRecursive.java" "path/to/folder"`
 */
public class CountFilesRecursive {

    private static final String DEFAULT_FOLDER_PATH = "C:/Users/andi/Desktop";

    public static void main(String[] args) {
        if (args.length < 1)
        {
            args = new String[] { DEFAULT_FOLDER_PATH };
        }

        File folder = new File(args[0]);
        if(!folder.exists())
        {
            throw new RuntimeException("Folder does not exist: "+ args[0]);
        }

        HashMap<String, Integer> fileExtensionToCount = countFilesRecursive(folder);
        printFileCount(folder, fileExtensionToCount);
    }

    private static void printFileCount(File file, HashMap<String, Integer> fileExtensionToCount)
    {
        String fileExtensionAndCountCsv = fileExtensionToCount.entrySet().stream()
            .map(entry -> entry.getKey() + ": "+ entry.getValue())
            .collect(Collectors.joining(", "));
        System.out.println(file.getAbsolutePath() + ": " + fileExtensionAndCountCsv);
    }

    private static HashMap<String, Integer> countFilesRecursive(File file)
    {
        HashMap<String, Integer> fileExtensionToCount = new HashMap<>();
        for(File subFile : file.listFiles())
        {
            if(subFile.isDirectory())
            {
                HashMap<String, Integer> subFileExtensionToCount = countFilesRecursive(subFile);
                printFileCount(subFile, subFileExtensionToCount);
                mergeHashMaps(subFileExtensionToCount, fileExtensionToCount);
            }
            else if(subFile.isFile())
            {
                String extension = getFileExtension(subFile.getName());
                Integer count = fileExtensionToCount.get(extension);
                if (count == null)
                {
                    count = 0;
                }
                count++;
                fileExtensionToCount.put(extension, count);
            }
        }
        return fileExtensionToCount;
    }

    private static String getFileExtension(String filePath)
    {
        int i = filePath.indexOf(".");
        if (i >= 0)
        {
            return filePath.substring(i);
        }
        return "";
    }

    private static void mergeHashMaps(HashMap<String, Integer> from, HashMap<String, Integer> to)
    {
        for(var fromEntry : from.entrySet())
        {
            Integer toValue = to.get(fromEntry.getKey());
            if (toValue == null)
            {
                to.put(fromEntry.getKey(), fromEntry.getValue());
            }
            else
            {
                Integer newToValue = toValue + fromEntry.getValue();
                to.put(fromEntry.getKey(), newToValue);
            }
        }
    }
}