Reading a csv file into a HashMap<String, ArrayList<Integer>>

Question

I've been trying to make a java program in which a tab delimited csv file is read line by line and the first column (which is a string) is added as a key to a hash map and the second column (integer) is it's value.

In the input file, there are duplicate keys but with different values so I was going to add the value to the existing key to form an ArrayList of values.

I can't figure out the best way of doing this and was wondering if anyone could help?

Thanks

EDIT: sorry guys, heres where i've got to with the code so far: I should add the first column is the value and the second column is the key.

public class WordNet {

    private final HashMap<String, ArrayList<Integer>> words;
    private final static String LEXICAL_UNITS_FILE = "wordnet_data/wn_s.csv";

    public WordNet() throws FileNotFoundException, IOException {

        words = new HashMap<>();
        readLexicalUnitsFile();
    }

    private void readLexicalUnitsFile() throws FileNotFoundException, IOException{

        BufferedReader in = new BufferedReader(new FileReader(LEXICAL_UNITS_FILE));
        String line;

        while ((line = in.readLine()) != null) {
            String columns[] = line.split("\t");
            if (!words.containsKey(columns[1])) {
                words.put(columns[1], new ArrayList<>());
            }

        }
        in.close();

    }

Answer 1

You are close

String columns[] = line.split("\t");
if (!words.containsKey(columns[1])) {
    words.put(columns[1], new ArrayList<>());
}

should be

String columns[] = line.split("\t");
String key = columns[0];                // enhance readability of code below
List<Integer> list = words.get(key);    // try to fetch the list
if (list == null)                       // check if the key is defined
{                                       //   if not
    list = new ArrayList<>();           //      create a new list
    words.put(key,list);                //      and add it to the map
}
list.add(new Integer(columns[1]));      // in either case, add the value to the list

In response to the OP's comment/question

... the final line just adds the integer to the list but not to the hashmap, does something need to be added after that?

After the statement

List<Integer> list = words.get(key);

there are two possibilities. If list is non-null, then it is a reference to (not a copy of) the list that is already in the map.

If list is null, then we know the map does not contain the given key. In that case we create a new empty list, set the variable list as a reference to the newly created list, and then add the list to the map for the key.

In either case, when we reach

list.add(new Integer(columns[1]));

the variable list contains a reference to an ArrayList that is already in the map, either the one that was there before, or one we just creatd and added. We just add the value to it.