Implement Solution

Author: SteffenLm

Author.java

@@ -0,0 +1,3 @@
+public record Author(String Name) {
+
+}

Book.java

@@ -0,0 +1,4 @@
+public record Book(String title) {
+    
+
+}

BookCollection.java

@@ -0,0 +1,43 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map.Entry;
+import java.util.Optional;
+
+public record BookCollection(HashMap<Author, List<Book>> collection) {
+
+    public void addAuthor(Author author) throws DuplicateKeyException {
+        if (collection.containsKey(author)) {
+            throw new DuplicateKeyException();
+        }
+
+        collection.put(author, new ArrayList<>());
+    }
+
+    public void addBook(Author author, Book book) {
+        collection.get(author).add(book);
+    }
+
+    public Optional<Book> getBookByTitle(String title) {
+        for (List<Book> books : collection.values()) {
+            for (Book b : books) {
+                if (b.title().equals(title)) {
+                    return Optional.of(b);
+                }
+            }
+        }
+        return Optional.empty();
+    }
+
+    public Optional<Author> getMostDiligentAuthor() {
+        Author mostDiligentAuthor = null;
+        int mostBooks = 0;
+        for (Entry<Author, List<Book>> entry : collection.entrySet()) {
+            if (entry.getValue().size() > mostBooks) {
+                mostDiligentAuthor = entry.getKey();
+                mostBooks = entry.getValue().size();
+            }
+        }
+        return Optional.ofNullable(mostDiligentAuthor);
+    }
+}

DuplicateKeyException.java

@@ -0,0 +1,5 @@
+public class DuplicateKeyException extends Exception {
+
+    private static final long serialVersionUID = 1L;
+
+}

Exercise.java

@@ -1,6 +1,29 @@
+import java.util.HashMap;
+
 public class Exercise {
 
   public static void main(String[] args) {
-    // implement exercise here
+    BookCollection collection = new BookCollection(new HashMap<>());
+
+    try {
+      collection.addAuthor(new Author("Stephen King"));
+      collection.addAuthor(new Author("George RR Martin"));
+    } catch (DuplicateKeyException e) {
+      e.printStackTrace();
+    }
+
+    collection.addBook(new Author("Stephen King"), new Book("Es"));
+    collection.addBook(new Author("Stephen King"), new Book("Sie"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 1"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 2"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 3"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 4"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 5"));
+    collection.addBook(new Author("George RR Martin"), new Book("Das Lied von Eis und Feuer 6"));
+
+    System.out.println(collection.getBookByTitle("Das Lied von Eis und Feuer 5"));
+    System.out.println(collection.getMostDiligentAuthor());
+
   }
+
 }