How to select only one image from a folder?

Question

Java

I am writing a gallery for the application. Here, with the help of this function, I recursively go through all the directories and look for pictures there

private ArrayList<Album> getAlbums(File root){
        ArrayList<Album> albums = new ArrayList<>();
        String androidDataFolder = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Android/data";
        File[] fileList = root.listFiles();
        String exts = "jpg png jpeg";
        String file_ext;

        for(File f: fileList){
            if(f.isDirectory() && !f.isHidden()
                    && !f.getAbsolutePath().equals(androidDataFolder)) {
                    albums.addAll(getAlbums(f));
            }
            else {
                file_ext = f.getName().substring(f.getName().lastIndexOf(".") + 1);
                if(exts.contains(file_ext)) {
                    albums.add(new Album(f.getParent(), f));
                }
            }
        }
        return albums;
    }

Everything works, it finds all the pictures, but I want to sort the pictures by albums, that is, in fact, display only the first picture from the folder on the screen. That is, I do not need to search for all the photos in a specific folder. How to make sure that there are no several pictures from the same folder? Of course, you can get a list with all the pictures and then clean up the duplicates with this function

private ArrayList<Album> clearAlbums(ArrayList<Album> albums){
        boolean isUnique = true;
        ArrayList<Album> cleanData = new ArrayList<>();
        for (Album a: albums){
            for (Album b: cleanData){
                isUnique &= !a.getAlbumTitle().equals(b.getAlbumTitle());
            }
            if(isUnique)
                cleanData.add(a);
            isUnique = true;
        }
        return cleanData;
    }

But I want to discard duplicates at the data collection stage (in the getAlbums method), since there recursion and discarding duplicates reduced the method's running time.