First i will explain why i have this structure in mind, i got a list of songs that recharted the top100 during the years.
So 'Frank Sinatra' with the song 'white christmas' appears 3 times for example, 1944, 1945, 1946.
(all this is to draw a visual graphic in the end, in case it matters).

I have a class Song, it holds the title and the artist name and a class array charted which holds the unique information for every year the song recharted.

1. class Song {
     String title;
     String artist;
     Charted[] charted = new Charted[1];
   
     Song(String title, String artist) {
       this.title = title;
       this.artist = artist;
   
     }
   }
   

The class charted:

1. class Charted {
     int weeksCharted;
     Date dateEntered;
      
     // empty constructor
     Charted () {
      
     }
    
     Charted(int weeksCharted, Date dateEntered) {
       this.weeksCharted = weeksCharted;
       this.dateEntered = dateEntered;
      
     }
    
   }
   

Now here everything gets together, it works apart for the if line,
There the charted info needs to be added.

In the else i use this:
s.charted[0] = new Charted(weeksCharted, dateEntered);

which works fine but afcose only works once.
For the if part i tried to append the array but i doens work with the syntax i'm trying.

1. public void setupSongs() {
     for (int i = 1; i < lines.length; i++) { //skip first, first one are the column names
       String[] tokens = splitTokens(lines[i], "~");
   
       String title = tokens[TITLE];
       String artist = tokens[ARTIST];
       String combined = artist+title;
       int weeksCharted = int(tokens[CH]);
       Date dateEntered = stringToDate(tokens[DATE_ENTERED]);
   
       if (songs_hm.containsKey(combined)) {
         // Get the object and add the charted info
         Song s = (Song) songs_hm.get(combined);
2.       //s.charted = append(s.charted, new Charted(weeksCharted, dateEntered)); //not working
       }
       else {
         // Otherwise make a new song and add the charted info
         Song s = new Song(title, artist);
         s.charted[0] = new Charted(weeksCharted, dateEntered);
         //s.charted = append(s.charted, new Charted(weeksCharted, dateEntered)); //not working
         // add to the HashMap
         songs_hm.put(combined,s);
       }
     }
   }