refactor: Guarantee non-null errors and depth-sort on raw pointers

Author: thced

src/main/java/com/retailsvc/http/internal/ProblemDetail.java

@@ -18,6 +18,12 @@
 public record ProblemDetail(
     String type, String title, int status, String detail, List<Entry> errors) {
 
+  public ProblemDetail {
+    if (errors == null) {
+      errors = List.of();
+    }
+  }
+
   /** One validation failure: its body location, the failed keyword, and a human-readable detail. */
   public record Entry(String pointer, String keyword, String detail) {}
 
@@ -38,20 +44,20 @@ public static ProblemDetail forBadRequest(BadRequestException e) {
 
   /**
    * Flattens a validation error into ordered {@code errors} entries: the failed branches of a
-   * combinator (one each), or the single leaf otherwise. Multi-entry results are sorted deepest
+   * combinator (one each), or the single leaf otherwise. Multiple sources are sorted deepest
    * pointer first (most-likely-intended branch) and de-duplicated on exact equality.
    */
   private static List<Entry> entriesOf(ValidationError e) {
     List<ValidationError> sources = e.branches().isEmpty() ? List.of(e) : e.branches();
-    List<Entry> entries = new ArrayList<>(sources.size());
+    if (sources.size() > 1) {
+      sources = new ArrayList<>(sources);
+      sources.sort(Comparator.comparingInt((ValidationError s) -> depth(s.pointer())).reversed());
+    }
+    LinkedHashSet<Entry> entries = new LinkedHashSet<>();
     for (ValidationError s : sources) {
       entries.add(new Entry(fragment(s.pointer()), s.keyword(), s.message()));
     }
-    if (entries.size() <= 1) {
-      return entries;
-    }
-    entries.sort(Comparator.comparingInt((Entry en) -> depth(en.pointer())).reversed());
-    return new ArrayList<>(new LinkedHashSet<>(entries));
+    return new ArrayList<>(entries);
   }
 
   private static String fragment(String pointer) {

src/main/java/com/retailsvc/http/internal/ProblemDetailRenderer.java

@@ -33,7 +33,7 @@ public static byte[] renderJson(ProblemDetail pd) {
 
   private static void appendErrors(
       StringBuilder out, boolean first, List<ProblemDetail.Entry> errors) {
-    if (errors == null || errors.isEmpty()) {
+    if (errors.isEmpty()) {
       return;
     }
     if (!first) {