Java, How to add values to Array List used as value in HashMap

String courseID = "Comp-101";
List<String> scores = new ArrayList<String> ();
scores.add("100");
scores.add("90");
scores.add("80");
scores.add("97");

Map<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
myMap.put(courseID, scores);

Hope this helps!

You could either use the Google Guava library, which has implementations for Multi-Value-Maps (Apache Commons Collections has also implementations, but without generics).

However, if you don't want to use an external lib, then you would do something like this:

if (map.get(id) == null) { //gets the value for an id)
    map.put(id, new ArrayList<String>()); //no ArrayList assigned, create new ArrayList

map.get(id).add(value); //adds value to list.