Challenges

Remember the rules for this are

• Try to use only the information given up to this point in this book.
• Try not to give up until you've given it a solid attempt

Challenge 1.

Make a Library class. It should have four exposed methods

• add which takes a Book and adds it to the library.
• list which returns an ArrayList<String> of all the Books in the library. The Strings in this list should be the ISBN of the book.
• find which takes a String representing the ISBN and returns a full Book record.
• remove which takes a String representing the ISBN and removes that Book from the library.

Use a HashMap for storing this info in the Library.

record Book(String isbn, String title, String author) {}

class Library {
    // CODE HERE
}

class Main {
    void main() {
        Library publicLibrary = new Library();
        publicLibrary.add(new Book("978-0-8041-3902-1", "The Martian", "Andy Weir"));
        publicLibrary.add(new Book("978-0062060624", "The Song of Achilles", "Madeline Miller"));

        IO.println(publicLibrary.list());

        Book b1 = publicLibrary.find("978-0062060624");
        IO.println(b1); // The Song of Achilles

        Book b2 = publicLibrary.find("123");
        IO.println(b2); // null

        publicLibrary.remove("978-0062060624");

        Book b3 = publicLibrary.find("978-0062060624");
        IO.println(b3); // null

        IO.println(publicLibrary.list());
    }
}

Challenge 2.

Write a method which takes as an argument an ArrayList<String> and returns a HashMap<String, Integer> where each key is a String from the original ArrayList and each value is the number of times it appeared in that ArrayList.

class Main {
    HashMap<String, Integer> count(ArrayList<String> words) {
        // CODE HERE
    }

    void main() {
        ArrayList<String> w = new ArrayList<>();
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("goose");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("duck");
        w.add("goose");
        w.add("zebra");

        // {duck=11,goose=2,zebra=1}
        IO.println(
            count(w)
        );
    }
}

If you feel like this is hard to do with just .put and .get, you can check for other methods on HashMap that can help you.

Challenge 3.

Without calling any methods on the HashMap, make it so that map.get(person) returns null.

class Person {
    int age;

    Person(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person[age=" + age + "]";
    }

    @Override
    public boolean equals(Object o) {
        if (o instanceof Person p) {
            return age == p.age;
        }
        else {
            return false;
        }
    }

    @Override
    public int hashCode() {
        return Objects.hash(age);
    }
}

class Main {
    void main() {
        var person = new Person("Patrocolus");
        var map = new HashMap<Person, String>();
        map.put(person, "Achilles");

        // -------------

        // CODE HERE
        // Do not directly touch "map"

        // -------------

        // Should output `null`
        IO.println(map.get(person));
    }
}