public class ParseData implements Comparable<ParseData> {
   
    private File inDir = new File("C:\\Users\\User\\Desktop\\tmp"); 
    private File outDir = new File("C:\\Users\\User\\Desktop\\tmp2");
    private File[] files;
    private Scanner sc;
    private String[] stringsBuffer;
    private Date unixDate = new Date();
    private long tmpdate;
    private SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy");
    private String formattedDate;

    //private ArrayList<ParseData> tmplist = new ArrayList<>();
   // private ArrayList<ParseData> dataList = new ArrayList<>();
    private int avgTime = 0;
    //private StringBuilder sb = new StringBuilder();
    private int counter = 0;
    private HashMap<String, String> map = new HashMap<>();

}

public void parseLogData() {
        
        try {
            if (inDir.exists() && inDir.isDirectory() && inDir.canRead()) {
                files = inDir.listFiles();
                for (File f : files) {
                    if (f.getName().endsWith(".csv")) {
                        sc = new Scanner(f);
                        while (sc.hasNext()) {
                            String next = sc.nextLine();
                            stringsBuffer = next.split(",");
                            tmpdate = (long) (Long.parseLong(stringsBuffer[0])) * 1000;
                            unixDate.setTime(tmpdate);
                            formattedDate = sdf.format(unixDate);
                            for (int i = 0; i < stringsBuffer.length; i++) {
                                stringsBuffer[0] = formattedDate;
                                map.put(stringsBuffer[0], "\n"+stringsBuffer[1]+"\n"+stringsBuffer[2]+"\n"+stringsBuffer[3]+"\n");
                            }
                            
//                          tmplist.add(new ParseData(stringsBuffer[0], stringsBuffer[1], 
//                                                    stringsBuffer[2], stringsBuffer[3]));
                        }                        
                    }                    
                    Set set = map.entrySet();
                    Iterator it = set.iterator();
                    while (it.hasNext()) {
                        Map.Entry me = (Map.Entry) it.next();
                        System.out.println(me.getKey()+""+ me.getValue());                     
                    }
                }
            } else {
                System.err.println("The directory does not exist or you do not have access");
            }
        } catch (FileNotFoundException ex) {
            System.err.println(ex.getMessage());
        }        
    }

class MyData {
    String date;
    String userName;
    String url;
}

while ( ! eof ) {
   String s = readLine();
   MyData data = parse( s );  в этой строке имеется ввиду получение массива строк?
   // Группировка
   String k = data.date;
   ArrayList<MyData> v = map.get( k );
   if ( v==null ) {
      v = new ArrayList<MyData>();
      map.put( k, v );
   }
   v.add( data );
}