Fix: Add null checking to GenericAttribute extraction

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/datastructure/GenericAttribute.java

@@ -9,6 +9,8 @@ import java.io.Serial;
 import java.io.Serializable;
 
 import java.util.List;
+import java.util.StringJoiner;
+import java.util.function.Supplier;
 
 
 public class GenericAttribute implements Serializable {
@@ -17,6 +19,7 @@ public class GenericAttribute implements Serializable {
     private static final long serialVersionUID = -4101408947632132328L;
 
     private static final Logger logger = LogManager.getLogger(GenericAttribute.class);
+    private static final String NULL_VALUE_STRING= "NULL OR INVALID VALUE";
 
     private final AbstractGenericAttributeProperty original;
 
@@ -24,6 +27,7 @@ public class GenericAttribute implements Serializable {
     private String name;
     private String value;
 
+
     public GenericAttribute(AbstractGenericAttributeProperty attributeProperty) {
         original = attributeProperty;
         if (attributeProperty.getObject() == null) {
@@ -34,40 +38,42 @@ public class GenericAttribute implements Serializable {
 
         if (attributeProperty.getObject() instanceof StringAttribute sa) {
             type = "StringAttribute";
-            value = String.format("\"%s\"", sa.getValue());
+            value = extractValue(() -> String.format("\"%s\"", sa.getValue()));
         } else if (attributeProperty.getObject() instanceof IntAttribute ia) {
             type = "IntAttribute";
-            value = ia.getValue().toString();
+            value = extractValue(() -> ia.getValue().toString());
         } else if (attributeProperty.getObject() instanceof DoubleAttribute da) {
             type = "DoubleAttribute";
-            value = da.getValue().toString();
+            value = extractValue(() -> da.getValue().toString());
         } else if (attributeProperty.getObject() instanceof DateAttribute date) {
             type = "DateAttribute";
-            value = date.getValue().toString();
+            value = extractValue(() -> date.getValue().toString());
         } else if (attributeProperty.getObject() instanceof UriAttribute ua) {
             type = "UriAttribute";
-            value = ua.getValue();
+            value = extractValue(ua::getValue);
         } else if (attributeProperty.getObject() instanceof MeasureAttribute ma) {
             type = "MeasureAttribute";
-            value = String.format("%s %s", ma.getValue().getValue().toString(), ma.getValue().getUom());
+            value = extractValue(() ->
+                    String.format("%s %s", ma.getValue().getValue().toString(), ma.getValue().getUom()));
         } else if (attributeProperty.getObject() instanceof CodeAttribute ca) {
             type = String.format("CodeAttribute (%s)", ca.getValue().getLanguage());
-            value = String.format("'''%s''' %n CodeSpace: %s", ca.getValue().getValue(), ca.getValue().getCodeSpace());
+            value = extractValue(() ->
+                    String.format("'''%s''' %n CodeSpace: %s", ca.getValue().getValue(), ca.getValue().getCodeSpace()));
         } else if (attributeProperty.getObject() instanceof GenericAttributeSet gas) {
             type = "GenericAttributeSet";
             List<AbstractGenericAttributeProperty> attributes = gas.getValue();
-            StringBuilder sb = new StringBuilder();
-            sb.append("{");
+            if(attributes == null || attributes.isEmpty()){
+                value = NULL_VALUE_STRING;
+                return;
+            }
+            StringJoiner sj = new StringJoiner("; ","{","}");
+            sj.setEmptyValue("EMPTY_SET");
             for (AbstractGenericAttributeProperty attribute : attributes) {
                 if (attribute.getObject() != null) {
-                    sb.append(attribute.getObject().getName());
-                    sb.append(" = ");
-                    sb.append(attribute.getObject().getValue());
-                    sb.append("; ");
+                    sj.add(String.format("%1$s = %2$s",attribute.getObject().getName(), attribute.getObject().getValue()));
                 }
             }
-            sb.replace(sb.length() - 1, sb.length(), "}");
-            value = String.format("'%s'", sb);
+            value = sj.toString();
 
         } else {
             logger.warn("GenericAttribute {} is of unknown type {}", attributeProperty, attributeProperty.getObject());
@@ -77,6 +83,14 @@ public class GenericAttribute implements Serializable {
 
     }
 
+    private String extractValue(Supplier<String> supply){
+        try{
+            return supply.get();
+        }catch (NullPointerException e){
+            return NULL_VALUE_STRING;
+        }
+    }
+
     public AbstractGenericAttributeProperty getOriginal() {
         return original;
     }