[BUG][JAVA] Handling default values leads to wrong defaults or not compilable code fixed

[JanLubenow]

PR fixes Issue #15835

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@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

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[wing328]

tested locally to confirm the fix:

@@ -53,19 +53,19 @@ import org.openapitools.client.JSON;
 /**
  * DefaultValuesType
  */
-@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2023-06-14T11:15:48.062201+08:00[Asia/Hong_Kong]")
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2023-06-14T11:16:28.074870+08:00[Asia/Hong_Kong]")
 public class DefaultValuesType {
   public static final String SERIALIZED_NAME_STRING_DEFAULT = "stringDefault";
   @SerializedName(SERIALIZED_NAME_STRING_DEFAULT)
-  private List<String> stringDefault = new ArrayList<>();
+  private List<String> stringDefault = new ArrayList<>(Arrays.asList("[]"));
 
   public static final String SERIALIZED_NAME_STRING_DEFAULT2 = "stringDefault2";
   @SerializedName(SERIALIZED_NAME_STRING_DEFAULT2)
-  private List<String> stringDefault2 = new ArrayList<>(Arrays.asList("Hallo", "Huhu"));
+  private List<String> stringDefault2 = new ArrayList<>(Arrays.asList("[Hallo, Huhu]"));
 
   public static final String SERIALIZED_NAME_OBJECT_DEFAULT = "objectDefault";
   @SerializedName(SERIALIZED_NAME_OBJECT_DEFAULT)
-  private List<ObjectType> objectDefault = new ArrayList<>();
+  private List<ObjectType> objectDefault = new ArrayList<>(Arrays.asList([]));
 
   public DefaultValuesType() {
   }
@@ -78,7 +78,7 @@ public class DefaultValuesType {
 
   public DefaultValuesType addStringDefaultItem(String stringDefaultItem) {
     if (this.stringDefault == null) {
-      this.stringDefault = new ArrayList<>();
+      this.stringDefault = new ArrayList<>(Arrays.asList("[]"));
     }
     this.stringDefault.add(stringDefaultItem);
     return this;
@@ -107,7 +107,7 @@ public class DefaultValuesType {
 
   public DefaultValuesType addStringDefault2Item(String stringDefault2Item) {
     if (this.stringDefault2 == null) {
-      this.stringDefault2 = new ArrayList<>(Arrays.asList("Hallo", "Huhu"));
+      this.stringDefault2 = new ArrayList<>(Arrays.asList("[Hallo, Huhu]"));
     }
     this.stringDefault2.add(stringDefault2Item);
     return this;
@@ -136,7 +136,7 @@ public class DefaultValuesType {
 
   public DefaultValuesType addObjectDefaultItem(ObjectType objectDefaultItem) {
     if (this.objectDefault == null) {
-      this.objectDefault = new ArrayList<>();
+      this.objectDefault = new ArrayList<>(Arrays.asList([]));
     }
     this.objectDefault.add(objectDefaultItem);
     return this;

(changes were using current master)

thanks for the PR.