Refactoring

src/main/java/de/gecheckt/asv/domain/model/InputFile.java

@@ -38,6 +38,8 @@ public record InputFile(String sourceFileName, List<Message> messages) {
         if (messages == null) {
             throw new IllegalArgumentException("Messages must not be null");
         }
+        // Defensive copy to ensure immutability
+        messages = List.copyOf(messages);
     }
     
     /**

src/main/java/de/gecheckt/asv/domain/model/Message.java

@@ -40,6 +40,8 @@ public record Message(int messagePosition, List<Segment> segments) {
         if (segments == null) {
             throw new IllegalArgumentException("Segments must not be null");
         }
+        // Defensive copy to ensure immutability
+        segments = List.copyOf(segments);
     }
     
     /**

src/main/java/de/gecheckt/asv/domain/model/Segment.java

@@ -4,7 +4,6 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
-import java.util.stream.Collectors;
 
 /**
  * Represents a segment in a message from an input file.
@@ -46,6 +45,8 @@ public record Segment(String segmentName, int segmentPosition, List<Field> field
         if (fields == null) {
             throw new IllegalArgumentException("Fields must not be null");
         }
+        // Defensive copy to ensure immutability
+        fields = List.copyOf(fields);
     }
     
     /**

src/main/java/de/gecheckt/asv/validation/field/DefaultFieldValidator.java

@@ -5,8 +5,6 @@ import java.util.List;
 
 import de.gecheckt.asv.domain.model.Field;
 import de.gecheckt.asv.domain.model.InputFile;
-import de.gecheckt.asv.domain.model.Message;
-import de.gecheckt.asv.domain.model.Segment;
 import de.gecheckt.asv.validation.model.ValidationError;
 import de.gecheckt.asv.validation.model.ValidationResult;
 import de.gecheckt.asv.validation.model.ValidationSeverity;

src/main/java/de/gecheckt/asv/validation/model/ValidationResult.java

@@ -1,10 +1,7 @@
 package de.gecheckt.asv.validation.model;
 
-import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
-import java.util.stream.Collectors;
 
 /**
  * Repräsentiert das Ergebnis einer Validierung mit allen gefundenen Fehlern, Warnungen und Infos.

src/main/java/de/gecheckt/asv/validation/structure/DefaultStructureValidator.java

@@ -3,7 +3,6 @@ package de.gecheckt.asv.validation.structure;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
-import java.util.Set;
 
 import de.gecheckt.asv.domain.model.Field;
 import de.gecheckt.asv.domain.model.InputFile;