diff --git a/Author.java b/Author.java
new file mode 100644
index 0000000..9694c0e
--- /dev/null
+++ b/Author.java
@@ -0,0 +1 @@
+public record Author(String Name) {}
diff --git a/Book.java b/Book.java
new file mode 100644
index 0000000..19ad242
--- /dev/null
+++ b/Book.java
@@ -0,0 +1 @@
+public record Book(String title) {}
diff --git a/BookCollection.java b/BookCollection.java
new file mode 100644
index 0000000..688804d
--- /dev/null
+++ b/BookCollection.java
@@ -0,0 +1,42 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map.Entry;
+
+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 Book getBookByTitle(String title) {
+    for (List<Book> books : collection.values()) {
+      for (Book b : books) {
+        if (b.title().equals(title)) {
+          return b;
+        }
+      }
+    }
+    return null;
+  }
+
+  public 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 mostDiligentAuthor;
+  }
+}
diff --git a/DuplicateKeyException.java b/DuplicateKeyException.java
new file mode 100644
index 0000000..f6b572c
--- /dev/null
+++ b/DuplicateKeyException.java
@@ -0,0 +1 @@
+public class DuplicateKeyException extends Exception {}
diff --git a/Exercise.java b/Exercise.java
index 3c092f9..14125d0 100644
--- a/Exercise.java
+++ b/Exercise.java
@@ -1,6 +1,27 @@
+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());
   }
 }