diff --git a/src/main/java/de/joethei/hs/java2/praktikum/praktikum1/GrosseZahl.java b/src/main/java/de/joethei/hs/java2/praktikum/praktikum1/GrosseZahl.java
index 0e585fb..8dd116c 100644
--- a/src/main/java/de/joethei/hs/java2/praktikum/praktikum1/GrosseZahl.java
+++ b/src/main/java/de/joethei/hs/java2/praktikum/praktikum1/GrosseZahl.java
@@ -2,7 +2,7 @@ package de.joethei.hs.java2.praktikum.praktikum1;
 
 public class GrosseZahl {
 
-    private int zahl[];
+    private int[] zahl;
 
     public GrosseZahl(String d) {
         Boolean korrekteEingabe=true;
@@ -37,7 +37,8 @@ public class GrosseZahl {
 
     public GrosseZahl add(GrosseZahl b) {
         int uebertrag = 0;
-        GrosseZahl rueckgabe, operand;
+        GrosseZahl rueckgabe;
+        GrosseZahl operand;
         if(this.less(b)) {
             rueckgabe = new GrosseZahl(b.toString());
             operand = new GrosseZahl(this.toString());
@@ -65,9 +66,7 @@ public class GrosseZahl {
     public GrosseZahl sub(GrosseZahl b) {
         int uebertrag = 0;
         StringBuilder x;
-        if(this.equals(b)) {
-            return new GrosseZahl(0);
-        } else if(this.less(b)) {
+        if(this.equals(b) || this.less(b)) {
             return new GrosseZahl(0);
         }
         GrosseZahl rueckgabe = new GrosseZahl(this.toString());
@@ -94,8 +93,11 @@ public class GrosseZahl {
     }
 
     public GrosseZahl mult(GrosseZahl b) {
-        GrosseZahl rueckgabe, rueckgabe_b, operand, operand2;
-        rueckgabe_b = new GrosseZahl(0);
+        GrosseZahl rueckgabe;
+        GrosseZahl result;
+        GrosseZahl operand;
+        GrosseZahl operand2;
+        result = new GrosseZahl(0);
         if(this.less(b)) {
             operand2 = b;
             operand = this;
@@ -108,12 +110,12 @@ public class GrosseZahl {
             for(int w = 0; w<operand.zahl[i]; w++) {
                 rueckgabe = rueckgabe.add(operand2);
             }
-            rueckgabe_b = rueckgabe_b.add(rueckgabe);
+            result = result.add(rueckgabe);
             if(i != operand.zahl.length-1) {
-                rueckgabe_b = new GrosseZahl(rueckgabe_b.toString()+"0");
+                result = new GrosseZahl(result.toString()+"0");
             }
         }
-        return rueckgabe_b;
+        return result;
     }
 
 
@@ -156,4 +158,8 @@ public class GrosseZahl {
         return false;
     }
 
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
 }
diff --git a/src/main/java/de/joethei/hs/java2/praktikum/praktikum2/CharCollection.java b/src/main/java/de/joethei/hs/java2/praktikum/praktikum2/CharCollection.java
index cfc1c03..00d1699 100644
--- a/src/main/java/de/joethei/hs/java2/praktikum/praktikum2/CharCollection.java
+++ b/src/main/java/de/joethei/hs/java2/praktikum/praktikum2/CharCollection.java
@@ -63,7 +63,7 @@ public class CharCollection implements Cloneable{
     }
 
     public CharCollection except(CharCollection cc) {
-        ArrayList<Character> list= new ArrayList<>(this.list);
+        ArrayList<Character> list = new ArrayList<>(this.list);
         boolean removeCharacterOnce = false;
         for(int i=0; i<list.size(); i++){
             for(int w=0; w<cc.list.size(); w++) {
@@ -104,4 +104,8 @@ public class CharCollection implements Cloneable{
         }return false;
     }
 
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
 }
\ No newline at end of file
diff --git a/src/main/java/de/joethei/hs/java2/praktikum/praktikum4/KlausurenServer.java b/src/main/java/de/joethei/hs/java2/praktikum/praktikum4/KlausurenServer.java
index 9b4ae90..f1eed2c 100644
--- a/src/main/java/de/joethei/hs/java2/praktikum/praktikum4/KlausurenServer.java
+++ b/src/main/java/de/joethei/hs/java2/praktikum/praktikum4/KlausurenServer.java
@@ -4,6 +4,7 @@ import java.io.*;
 import java.net.ServerSocket;
 import java.util.*;
 import java.util.concurrent.*;
+import java.util.logging.Logger;
 
 public class KlausurenServer {
 
@@ -11,11 +12,17 @@ public class KlausurenServer {
     private ExecutorService executorService = Executors.newCachedThreadPool();
     private ServerSocket serverSocket;
 
+    private static Logger logger = Logger.getLogger("KlausurenServer");
     private static Map<String, CommandHandler> commands = new HashMap<>();
+
     static Map<String, CommandHandler> getCommands() {
         return commands;
     }
 
+    public static Logger getLogger() {
+        return logger;
+    }
+
     private boolean running;
 
     public KlausurenServer(int port) {
@@ -23,8 +30,8 @@ public class KlausurenServer {
 
         commands.put("test", ((writer, args) -> writer.write("Hallo Welt")));
         commands.put("delall", (writer, args) -> {
-           data.clear();
-           save();
+            data.clear();
+            save();
         });
         commands.put("put", (writer, args) -> {
             if (args.size() >= 3) {
@@ -52,9 +59,8 @@ public class KlausurenServer {
                 Set<TreeSet<Integer>> result = new HashSet<>();
 
                 set.forEach((key, value) -> set.forEach((s, integers) -> {
-                    if(!value.equals(integers)) {
-                        if (integers.containsAll(value))
-                            set.remove(key);
+                    if (!value.equals(integers) && integers.containsAll(value)) {
+                        set.remove(key);
                     }
                 }));
 
@@ -62,7 +68,7 @@ public class KlausurenServer {
 
                 result.removeIf(TreeSet::isEmpty);
 
-                writer.write(result.toString().substring(1, result.toString().length()-1).replace(" ", ""));
+                writer.write(result.toString().substring(1, result.toString().length() - 1).replace(" ", ""));
             } else {
                 writer.write("0");
             }
@@ -91,7 +97,7 @@ public class KlausurenServer {
         Runnable runnable = () -> {
             try {
                 serverSocket = new ServerSocket(port);
-                System.out.println("waiting for clients");
+                logger.info("waiting for clients");
                 while (running) {
                     executorService.submit(new ClientRunnable(serverSocket.accept()));
                 }
@@ -103,10 +109,10 @@ public class KlausurenServer {
                 }
 
                 serverSocket.close();
-                System.out.println("server shutdown");
+                logger.info("server shutdown");
                 Thread.currentThread().interrupt();
             } catch (IOException e) {
-                e.printStackTrace();
+                logger.throwing("KlausurenServer", "Constructor", e.getCause());
             }
 
         };
@@ -114,12 +120,12 @@ public class KlausurenServer {
         thread.start();
     }
 
-    synchronized void load() {
+    private synchronized void load() {
         FileReader fileReader = null;
         try {
-            fileReader = new FileReader(getClass().getClassLoader().getResource("praktikum4/data.txt").getFile());
+            fileReader = new FileReader(Objects.requireNonNull(getClass().getClassLoader().getResource("praktikum4/data.txt")).getFile());
         } catch (FileNotFoundException e) {
-            e.printStackTrace();
+            logger.throwing("KlausurenServer", "load", e.getCause());
         }
         try (BufferedReader br = new BufferedReader(Objects.requireNonNull(fileReader))) {
             for (String line = br.readLine(); line != null; line = br.readLine()) {
@@ -131,15 +137,15 @@ public class KlausurenServer {
                 data.put(array[0], set);
             }
         } catch (IOException e) {
-            e.printStackTrace();
+            logger.throwing("KlausurenServer", "load", e.getCause());
         }
     }
 
-    synchronized void save() {
+    private synchronized void save() {
         FileOutputStream fileOutputStream;
         PrintWriter printWriter = null;
         try {
-            fileOutputStream = new FileOutputStream(getClass().getClassLoader().getResource("praktikum4/data.txt").getFile());
+            fileOutputStream = new FileOutputStream(Objects.requireNonNull(getClass().getClassLoader().getResource("praktikum4/data.txt")).getFile());
             printWriter = new PrintWriter(fileOutputStream);
 
             for (Map.Entry<String, TreeSet<Integer>> entry : data.entrySet()) {
@@ -151,7 +157,7 @@ public class KlausurenServer {
 
             }
         } catch (IOException e) {
-            e.printStackTrace();
+            logger.throwing("KlausurenServer", "save", e.getCause());
         } finally {
             assert printWriter != null;
             printWriter.close();