I need to check whether adding value already exists in Map and add values to list of integers in Map

Question

I need to create a Map with string as a key and list of integers as a value. I have created the Map, I created tokenization of words which I get from txt file. Right now I am trying to put keys and values into map, but I am still receving errors. I am wondering that maybe my Map has been created wrong, but I don't see any other way to create this map.

I've been trying to add this map with "put" function, in this way I cannot even compile this program

Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();
        //Map<String, Integer> map = new HashMap<String, Integer>();    
        int numberOfFiles;
        File directory = new File("C:/Users/User/Desktop/documents java project/");
        numberOfFiles = directory.list().length;


        for(int i = 1; i <= numberOfFiles; i++) {
            //tokenization of string
            String filePath = "C:/Users/User/Desktop/documents java project/document "+ i +".txt";
            String fileData = readFile(filePath);
            String delimiter = " ";
            String fileWords [] = fileData.split(delimiter);
            //words.add(fileWords);
            int numberOfWords = fileWords.length;

            for(int j = 0; j < numberOfWords; j++) {

                map.put(fileWords[j], i);

            }
        }
}

Answer 1

Your Map is expecting the value to be a List<Integer> but you are trying to put an int into it instead:

map.put(fileWords[j], i);

Instead, you need to create a List<Integer> and put that as the value, like:

if (!map.containsKey(fileWords[j])) {
    map.put(fileWords[j], new ArrayList<>());
}
map.get(fileWords[j]).add(i);

This will add a new empty List to your Map if it doesn't already exist, and then it will add the value of i to the list. If it does already exist, it will just add i to the list that is already there.

This can also be done with Map.computeIfAbsent() if the above is too wordy for your tastes:

map.computeIfAbsent(fileWords[j], k -> new ArrayList<>()).add(i);

If you don't want to store duplicates, you can use a Set instead. Change your declaration to:

Map<String, Set<Integer>> map = new HashMap<>();

And later, to add items:

map.computeIfAbsent(fileWords[j], k -> new HashSet<>()).add(i);