Solution for Optionals 01

BookCollection.java

@@ -2,6 +2,7 @@
 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) {
 
@@ -17,18 +18,18 @@ public void addBook(Author author, Book book) {
     collection.get(author).add(book);
   }
 
-  public Book getBookByTitle(String title) {
+  public Optional<Book> getBookByTitle(String title) {
     for (List<Book> books : collection.values()) {
       for (Book b : books) {
         if (b.title().equals(title)) {
-          return b;
+          return Optional.of(b);
         }
       }
     }
-    return null;
+    return Optional.empty();
   }
 
-  public Author getMostDiligentAuthor() {
+  public Optional<Author> getMostDiligentAuthor() {
     Author mostDiligentAuthor = null;
     int mostBooks = 0;
     for (Entry<Author, List<Book>> entry : collection.entrySet()) {
@@ -37,6 +38,6 @@ public Author getMostDiligentAuthor() {
         mostBooks = entry.getValue().size();
       }
     }
-    return mostDiligentAuthor;
+    return Optional.ofNullable(mostDiligentAuthor);
   }
 }

Exercise.java

@@ -21,7 +21,14 @@ public static void main(String[] args) {
     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());
+    collection
+        .getBookByTitle("Das Lied von Eis und Feuer 5")
+        .ifPresentOrElse(
+            System.out::println, () -> System.out.println("Das gesuchte Buch ist nicht vorhanden"));
+    collection
+        .getMostDiligentAuthor()
+        .ifPresentOrElse(
+            System.out::println,
+            () -> System.out.println("Es ist kein entsprechender Autor vorhanden"));
   }
 }