sign_in_user method implemented

pom.xml

@@ -28,6 +28,31 @@
             <artifactId>json</artifactId>
             <version>20210307</version>
         </dependency>
+        <dependency>
+            <groupId>commons-codec</groupId>
+            <artifactId>commons-codec</artifactId>
+            <version>1.17.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.poi</groupId>
+            <artifactId>poi</artifactId>
+            <version>3.9</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.poi</groupId>
+            <artifactId>poi-ooxml</artifactId>
+            <version>5.2.3</version>
+        </dependency>
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>2.7</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.logging.log4j</groupId>
+            <artifactId>log4j-core</artifactId>
+            <version>2.20.0</version>
+        </dependency>
     </dependencies>
     <build>
         <resources>

src/main/java/de/hs_mannheim/domain/System.java

@@ -94,8 +94,18 @@ public class System {
         return all_users;
     }
 
-    public boolean sign_in_user(String username, String password){
+    public boolean sign_in_user(String username, String password) throws IOException {
-        return true;
+        ArrayList<User> mem = new ArrayList<>(get_all_user());
+
+        for(User user : mem){
+            if(user.getUsername().equals(username) && user.getPassword().equals(password)){
+                current_user = new User(user.getUsername(), user.getPassword(),
+                        user.getHometown(), user.getZip(), user.getCar_name(),
+                        user.getCar_l_100km(), user.getCar_avg_kmh(), user.getBike_avg_kmh());
+                return true;
+            }
+        }
+        return false;
     }
 
     public boolean sign_up_user(String username, String password, String hometown, int zip,