+ existsBook

+ getCategories

src/main/java/de/hsel/itech/db/Database.java

@@ -193,6 +193,34 @@ public class Database {
         return category;
     }
 
+    @Nullable
+    public List<Category> getCategories() {
+        List<Long> ids = getIds(category);
+        List<Category> categories = new ArrayList<>();
+        for (long id : ids) {
+            categories.add(getCategory(id));
+        }
+        return categories;
+    }
+
+    private List<Long> getIds(String table) {
+        Map.Entry<ResultSet, Connection> entry = getResultSets(table, "id");
+        assert entry != null;
+
+
+        List<Long> ids = new ArrayList<>();
+        try {
+            ResultSet rs = entry.getKey();
+            while (rs.next()) {
+                ids.add(rs.getLong("id"));
+            }
+            entry.getValue().close();
+        } catch (SQLException ex) {
+            ex.printStackTrace();
+        }
+        return ids;
+    }
+
     /**
      * gets publisher by id.
      *
@@ -275,14 +303,30 @@ public class Database {
     public Book getBook(long id) {
         Map.Entry<ResultSet, Connection> entry = getResultSet(id, book);
         assert entry != null;
+        return getBook(entry);
+    }
 
 
+    private Book getBook(@NotNull Map.Entry<ResultSet, Connection> entry) {
         ResultSet rs = entry.getKey();
+
+        Book book = null;
+        try {
+            book = buildBook(rs);
+            entry.getValue().close();
+        } catch (SQLException e) {
+            e.printStackTrace();
+        }
+        return book;
+    }
+
+    @Nullable
+    private Book buildBook(ResultSet rs) {
         Book book = null;
         try {
             Category category = getCategory(rs.getLong(this.category));
             Publisher publisher = getPublisher(rs.getLong(this.publisher));
-            List<Integer> authorIds = getAuthors(id);
+            List<Integer> authorIds = getAuthors(rs.getLong("id"));
             assert category != null;
             assert publisher != null;
             assert authorIds != null;
@@ -296,7 +340,7 @@ public class Database {
             book = new Book(rs.getLong("id"), rs.getLong("isbn"), authors, publisher, category, rs.getString("title"),
                     Year.of(rs.getInt("year")), rs.getInt("price"), rs.getString("description"),
                     rs.getString("image"));
-            entry.getValue().close();
+
         } catch (SQLException e) {
             e.printStackTrace();
         }
@@ -310,19 +354,7 @@ public class Database {
      */
     @Nullable
     public List<Book> getBooks() {
-        Map.Entry<ResultSet, Connection> entry = getResultSets(book, "id");
-        assert entry != null;
-
-        List<Long> ids = new ArrayList<>();
-        try {
-            ResultSet rs = entry.getKey();
-            while (rs.next()) {
-                ids.add(rs.getLong("id"));
-            }
-            entry.getValue().close();
-        } catch (SQLException ex) {
-            ex.printStackTrace();
-        }
+        List<Long> ids = getIds(book);
         List<Book> books = new ArrayList<>();
         for (long id : ids) {
             books.add(getBook(id));
@@ -359,6 +391,19 @@ public class Database {
         return books;
     }
 
+    public boolean existsBook(long isbn) {
+        Map.Entry<ResultSet, Connection> entry = getResultSet(book, "isbn", isbn);
+        try {
+            if (entry != null && entry.getKey() != null) {
+                entry.getValue().close();
+                return true;
+            }
+        } catch (SQLException ex) {
+            ex.printStackTrace();
+        }
+        return false;
+    }
+
     /**
      * get books by category.
      *
@@ -621,11 +666,16 @@ public class Database {
      */
     @Nullable
     private Map.Entry<ResultSet, Connection> getResultSet(long id, String table) {
+        return getResultSet(table, "id", id);
+    }
+
+    @Nullable
+    private Map.Entry<ResultSet, Connection> getResultSet(String table, String column, long value) {
         Connection connection = getConnection();
         try {
             assert connection != null;
-            PreparedStatement statement = connection.prepareStatement("SELECT * FROM " + table + " WHERE id = ?");
-            statement.setLong(1, id);
+            PreparedStatement statement = connection.prepareStatement("SELECT * FROM " + table + " WHERE " + column + " = ?");
+            statement.setLong(1, value);
             ResultSet resultSet = statement.executeQuery();
             if (resultSet.next()) {
                 return new AbstractMap.SimpleEntry<>(resultSet, connection);

src/main/java/de/hsel/itech/servlet/BookList.java

@@ -74,20 +74,9 @@ public class BookList extends HttpServlet {
                 String image = req.getParameter("image");
 
                 //check isbn
-                List<Book> books = db.getBooks();
-                boolean found = false;
-                if (books != null) {
-                    for(Book b : books){
-                        if(b.getIsbn() == isbn){
-                            found = true;
-                            break;
-                        }
-                    }
-                }
-
-                if (found) {
+                if (db.existsBook(isbn)) {
                     out.println("<aside class =\"m-note m-danger\">");
-                    out.println("<h3>Es gibt breits ein buch mit dieser ISBN </h3>");
+                    out.println("<h3>Es gibt bereits ein Buch mit dieser ISBN </h3>");
                     out.println("</aside>");
                 } else {
                     ArrayList<Author> authors = new ArrayList<>();