How to print keys of specific values in HashMap?

Question

The question is how to pritn the books that have value = 1 in the HashMap, using the printWhiteRavens(ArrayList<String> whiteRavens) method? In the code ive deleted the array with the books so the code could be shorter.

 public static HashMap<String, Integer> createBooksCounter() {
        HashMap<String,Integer> createBooksCounter = new HashMap<>();
        createBooksCounter.put("Chicken Abroad",5);
        createBooksCounter.put("Lord With Sins",3);
        createBooksCounter.put("Fish And Horses",4);
        createBooksCounter.put("Mistress Of Devotion",2);
        createBooksCounter.put("Guardians And Gangsters",3);
        createBooksCounter.put("Bandit Of The Void",1);
        createBooksCounter.put("Lions Of Dread",1);
        createBooksCounter.put("Future Of Fire",2);
        createBooksCounter.put("Driving Into The Depths",1);
        createBooksCounter.put("Starting The Demons",1);
        createBooksCounter.put("Maid With Blue Eyes",2);
        createBooksCounter.put("Lovers In The Forest",1);
        createBooksCounter.put("Destruction Of The Faceless Ones",1);

        return null;
    }

    public static void countBook(HashMap<String, Integer> booksCounter, String book) {

    }

    public static ArrayList<String> findWhiteRavens(HashMap<String, Integer> booksCounter) {
        return null;    
    }

    public static void printWhiteRavens(ArrayList<String> whiteRavens) {
        return null;
    }

    public static void main(String[] args) {
    }

Answer 1

Using Stream:

yourMap.entrySet().stream().filter(e -> e.getValue() == 1)
                .forEach(element -> System.out.println(element.getKey()));

Answer 2

Filter all entries of the map, create a list from the map's keys, these are your book titles. Pass that list to printWhiteRavens which then prints the titles:

public static void main(String[] args) {
  Map<String,Integer> map = createBooksCounter();
  List<String> list = map.entrySet().stream().
                         filter(e->e.getValue()==1).
                         map(e->e.getKey()).
                         collect(Collectors.toList());
  printWhiteRavens(list);
}

public static void printWhiteRavens(List<String> whiteRavens) {
  whiteRavens.stream().forEach(System.out::println);
}

Also you have to edit your createBooksCounter method to return the map using

return createBooksCounter;

Answer 3

Your method createBooksCounter should probably return the map instead of null . Note that usually you declare members as the highest common interface offering the contract you need. So even though you might instantiate HashMap, you declare the field as a Map. The same holds for method parameters and return types. Regarding the name, I would name it bookCounts, since the map is not counting anything, just representing books and their 'counts'.

public static Map<String, Integer> createBookCounts() {
    Map<String, Integer> bookCounts = new HashMap<>();
    bookCounts .put("Chicken Abroad",5);
    // ...
    return bookCounts;
}

The method findWhiteRavens needs to look up all books in the map which have count==1. This is contrary to the normal usage of a Map, where you look things up by their key rather than their value. I suggest either to use a map where count is the key, and the values are lists of books (having that count) OR to use a List with Book objects with the fields title and count. The latter approach is in my opinion the simplest and most object-oriented design (using maps to collect related fields is a code smell):

class Book {

    String title;
    int count;

    Book(String title, int count) {
        this.title = title;
        this.count = count;
    }

    // accessors to be added
}

// ...

List<Book> books = new ArrayList<>();
books.add(new Book("Chicken Abroad", 5));

You could print books with count==1 in one go like this:

books.stream()
    .filter(book -> book.getCount() == 1)
    .map(book -> book.getTitle());
    forEach(book -> System.out.println(book));

If you prefer to use a map, then this would make more sense:

private static Map<Integer, List<String>> bookTitlesByCount = new HashMap<>();

public static void main(String[] args) {
    addBookTitle("Chicken Abroad", 5);
    List<String> whiteRavens = bookTitlesByCount.get(1);
    // print them
}

private static void addBookTitle(String title, int count) {
    List<String> bookTitles = bookTitlesByCount.get(count);
    if (bookTitles == null) {
        bookTitles = new ArrayList<>();
        bookTitlesByCount.put(count, bookTitles);
    }
    bookTitles.add(title);
}