[CSV-328] Fix quoted null string after disabling quote

setNullString(String) rebuilt quotedNullString by concatenating the nullable
quoteCharacter field directly, so calling setQuote(null) before setNullString(...)
produced a literal "nullNULLnull". Extract a shared setQuotedNullString() helper
that applies the default-quote fallback, so both builder orders produce the same state.

Reviewed-by: OpenAI Codex
Reviewed-by: Anthropic Claude Code

Author: OldTruckDriver

src/changes/changes.xml

@@ -53,6 +53,7 @@
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-325">CSVParser applies characterOffset to bytePosition (#604).</action>
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-326">CSVPrinter Reader printing with quote and escape can emit CSV that its parser cannot read back.</action>
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-327">CSVParser applies maxRows to record numbers instead of rows produced when setRecordNumber(...) is used.</action>
+      <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-328">CSVFormat.Builder.setNullString(String) can build an invalid quoted null string after setQuote(null).</action>
       <action type="fix" dev="ggregory" due-to="OldTruckDriver, Gary Gregory" issue="CSV-326">Escape Reader values with quote and escape (#606).</action>
       <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Clear escape delimiter buffer before peek in Lexer.isEscapeDelimiter() (#608, #611).</action>
       <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Escape quote char in printWithEscapes when QuoteMode is NONE (#609).</action>

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -780,8 +780,7 @@ public Builder setMaxRows(final long maxRows) {
          */
         public Builder setNullString(final String nullString) {
             this.nullString = nullString;
-            this.quotedNullString = quoteCharacter + nullString + quoteCharacter;
-            return this;
+            return setQuotedNullString();
         }
 
         /**
@@ -806,6 +805,10 @@ public Builder setQuote(final Character quoteCharacter) {
                 throw new IllegalArgumentException("The quoteCharacter cannot be a line break");
             }
             this.quoteCharacter = quoteCharacter;
+            return setQuotedNullString();
+        }
+
+        private Builder setQuotedNullString() {
             final Character quote = quoteCharacter != null ? quoteCharacter : Constants.DOUBLE_QUOTE_CHAR;
             this.quotedNullString = quote + nullString + quote;
             return this;

src/test/java/org/apache/commons/csv/CSVFormatTest.java

@@ -1040,6 +1040,11 @@ void testQuotedNullStringTracksQuoteCharacter() throws IOException {
         builder.setQuote((Character) null);
         builder.get().print(null, out, true);
         assertEquals("\"NULL\"", out.toString());
+        // reset, reverse setter order
+        out.setLength(0);
+        builder.setNullString(null).setQuote((Character) null).setNullString("NULL");
+        builder.get().print(null, out, true);
+        assertEquals("\"NULL\"", out.toString());
     }
 
     @Test