diff --git a/pom.xml b/pom.xml
index c1c60f2cbfe18bee1099a46af53bef762914cdd2..865c2857c3cdced6d4b8e915bb9de654bc36c6af 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,6 +26,12 @@
             <artifactId>sqlite-jdbc</artifactId>
             <version>3.28.0</version>
         </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>17.0.0</version>
+            <scope>compile</scope>
+        </dependency>
     </dependencies>
     <build>
         <plugins>
diff --git a/src/main/java/de/telefonbuch/App.java b/src/main/java/de/telefonbuch/App.java
index d870811d63514b314e441c76ae1d56578dfed0f9..c4b0fe5746121a96b74955b83053079f1f86c44a 100644
--- a/src/main/java/de/telefonbuch/App.java
+++ b/src/main/java/de/telefonbuch/App.java
@@ -25,20 +25,20 @@ public class App extends Application {
     public static int debug = 0;
 
     @Override
-    public void start(Stage stage) throws IOException {
+    public void start(@org.jetbrains.annotations.NotNull Stage stage) throws IOException {
         new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true);
 
-    	scene = new Scene(loadFXML("Telefonbuch"));
+    	scene = new Scene(loadFXML());
         stage.setScene(scene);
         stage.show();
     }
 
-    static void setRoot(String fxml) throws IOException {
-        scene.setRoot(loadFXML(fxml));
+    static void setRoot() throws IOException {
+        scene.setRoot(loadFXML());
     }
 
-    private static Parent loadFXML(String fxml) throws IOException {
-        FXMLLoader fxmlLoader = new FXMLLoader(App.class.getResource(fxml + ".fxml"));
+    private static Parent loadFXML() throws IOException {
+        FXMLLoader fxmlLoader = new FXMLLoader(App.class.getResource("Telefonbuch" + ".fxml"));
         return fxmlLoader.load();
     }
 
diff --git a/src/main/java/de/telefonbuch/SecondaryController.java b/src/main/java/de/telefonbuch/SecondaryController.java
index 9161624e203d53adfbf1a24c275952eb26fc90cf..12cf914cfe14888a53f07c3105f2890c46935046 100644
--- a/src/main/java/de/telefonbuch/SecondaryController.java
+++ b/src/main/java/de/telefonbuch/SecondaryController.java
@@ -7,6 +7,6 @@ public class SecondaryController {
 
     @FXML
     private void switchToPrimary() throws IOException {
-        App.setRoot("Telefonbuch");
+        App.setRoot();
     }
 }
\ No newline at end of file
diff --git a/src/main/java/de/telefonbuch/TelefonbuchController.java b/src/main/java/de/telefonbuch/TelefonbuchController.java
index e9c0e8a6f5328fb22d5816002b8c39a97bacb395..00b09f9cd28a7bb4ce4c5769729d6c47badbeeea 100644
--- a/src/main/java/de/telefonbuch/TelefonbuchController.java
+++ b/src/main/java/de/telefonbuch/TelefonbuchController.java
@@ -8,6 +8,8 @@ import javafx.scene.control.Alert;
 import javafx.scene.control.Button;
 import javafx.scene.control.ButtonType;
 import javafx.scene.control.TextField;
+import org.jetbrains.annotations.NotNull;
+
 import java.util.Optional;
 
 /**
@@ -392,7 +394,7 @@ public class TelefonbuchController {
 	 *
 	 * @param kontakt : Kontakt : Kontakt Objekt
 	 */
-	private void setKontaktToForm(Kontakt kontakt) {
+	private void setKontaktToForm(@NotNull Kontakt kontakt) {
 		new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true, 2);
 
 		txtVorname.setText(kontakt.getVorname());
diff --git a/src/main/java/de/telefonbuch/model/Datenbank.java b/src/main/java/de/telefonbuch/model/Datenbank.java
index 702b5197e6e1ba6306115d8f0174bbc092e5466e..fe32efa787da98ede5c699bf35021ce4d79ac72a 100644
--- a/src/main/java/de/telefonbuch/model/Datenbank.java
+++ b/src/main/java/de/telefonbuch/model/Datenbank.java
@@ -1,5 +1,7 @@
 package de.telefonbuch.model;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.sql.*;
 
 public class Datenbank {
@@ -45,7 +47,7 @@ public class Datenbank {
 		this.con = DriverManager.getConnection("jdbc:sqlite:" + datenbankdatei);
 	}
 
-	public void addKontakt(Kontakt kontakt) {
+	public void addKontakt(@NotNull Kontakt kontakt) {
 
 		new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true, 1);
 
@@ -110,7 +112,7 @@ public class Datenbank {
 		}
 	}
 
-	public Kontakt getKontakt(String telefon) {
+	public Kontakt getKontakt(@NotNull String telefon) {
 		new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true, 3);
 
 		Kontakt kontakt = new Kontakt();
diff --git a/src/main/java/de/telefonbuch/model/Kontakt.java b/src/main/java/de/telefonbuch/model/Kontakt.java
index 5bba9b80c0ce5efb04646ea998f6266c87ee4e28..a3de30fdb168ed0bdde0ec8e8ce17ff023a587f7 100644
--- a/src/main/java/de/telefonbuch/model/Kontakt.java
+++ b/src/main/java/de/telefonbuch/model/Kontakt.java
@@ -224,7 +224,7 @@ public class Kontakt {
 		return s != null && s.matches("[-+]?\\d*\\.?\\d+");
 	}
 
-	public String getId() {
+	String getId() {
 		new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true, 3);
 
 		return id;
@@ -240,15 +240,13 @@ public class Kontakt {
 	public String toString() {
 		new Fehler().debug(this.getClass().getName() + "." + new Throwable().getStackTrace()[0].getMethodName(), true, 3);
 
-		String strResult = "Vorname: " + this.getVorname() +
+		return "Vorname: " + this.getVorname() +
 				" Nachname: " + this.getNachname() +
 				" Straße: " + this.getStrasse() +
 				" Hnr: " + this.getHausnummer() +
 				" Plz: " + this.getPlz() +
 				" Ort: " + this.getOrt() +
 				" Telefon: " + this.getTelefon();
-
-		return strResult;
 	}
 	//endregion
 }
diff --git a/src/main/java/module-info.java b/src/main/java/module-info.java
index 257d39ca038f5e90cb000719edfd21d0355ba2ef..7376dd0caf38886d5e8582957f1f3fda073d1294 100644
--- a/src/main/java/module-info.java
+++ b/src/main/java/module-info.java
@@ -3,6 +3,7 @@ module de.telefonbuch {
     requires javafx.fxml;
 	requires java.desktop;
 	requires java.sql;
+	requires org.jetbrains.annotations;
 
 	opens de.telefonbuch to javafx.fxml;
     exports de.telefonbuch;