Question

I want to be able to sort files based on what day they were entered into a Map. The key is the day, and the value is a List of n Files added on the respective day. However, I need to be able to add the files to the List one at a time – but I’m stuck on the syntax. How do I call List.add() from within Map.put()? Here’s my code:

public static NavigableMap<String, List<File>> myFiles = new TreeMap<>();
String today = new Date().toString();
File currentFile;
myFiles.put(today, currentFile);  //problem here adding currentFile

Answer 1

It’s because of type safetyp (using generics). today points to a list of files and you want to just add one file to that list.

I would recommend you to first check if today is in the map and if it isn’t add the backing list, then, add the currentFile to that list:

if (!myFiles.containsKey(today))
    myFiles.put(today, new ArrayList<File>());

myFiles.get(today).add(currentFile);