fix: keep trailing comments separate from appended properties

Fixes #90

Author: quintesse

src/main/java/org/codejive/properties/Properties.java

@@ -513,16 +513,37 @@ private Cursor addNewKeyValue(
             pos = afterLastProperty();
         }
 
+        boolean afterFinalComment = isAfterFinalComment(pos);
+
         // Add a newline whitespace token if necessary
         pos = prepareNewLine(pos);
 
+        if (afterFinalComment) {
+            // Keep a trailing comment free rather than attaching it to the new key.
+            pos.addEol(eolType);
+        }
+
         // Add tokens for key, separator and value
         pos.add(new PropertiesParser.Token(PropertiesParser.Type.KEY, rawKey, key));
         pos.add(new PropertiesParser.Token(PropertiesParser.Type.SEPARATOR, "="));
         pos.add(new PropertiesParser.Token(PropertiesParser.Type.VALUE, rawValue, value));
         return pos;
     }
 
+    // Returns true only at the end of the token list when the last token is a comment,
+    // optionally followed by one EOL. An existing blank line after the comment returns false.
+    // Uses a copy so the insertion cursor is not moved.
+    private boolean isAfterFinalComment(Cursor pos) {
+        if (!pos.atEnd()) {
+            return false;
+        }
+        Cursor previous = pos.copy().prev();
+        if (previous.isEol()) {
+            previous.prev();
+        }
+        return previous.isType(PropertiesParser.Type.COMMENT);
+    }
+
     // Prepare the list for adding a new line at the given position. It will make sure the cursor
     // is at a valid insertion point and that any required EOLs are added.
     private Cursor prepareNewLine(Cursor pos) {

src/test/java/org/codejive/properties/TestProperties.java

@@ -1394,6 +1394,24 @@ void testPutFirstLineHeaderOnlyCrLf() throws IOException {
         expectStoreText(p, "put=putVal\r\n  # header\r\n");
     }
 
+    @Test
+    void testPutLastLine() throws IOException {
+        final String given = "key=val\n" + "# trailer comment";
+        final String expected = "key=val\n" + "# trailer comment\n" + "\n" + "put=putVal";
+        Properties p = Properties.loadProperties(new StringReader(given));
+        p.put(p.last().next(), "put", "putVal");
+        expectStoreText(p, expected);
+        assertThat(p.getComment("put")).isEmpty();
+    }
+
+    @Test
+    void testPutLastLineWithFinalEol() throws IOException {
+        Properties p = Properties.loadProperties(new StringReader("key=val\r\n  # trailer\r\n"));
+        p.put(p.last().next(), "put", "putVal");
+        expectStoreText(p, "key=val\r\n  # trailer\r\n\r\nput=putVal\r\n");
+        assertThat(p.getComment("put")).isEmpty();
+    }
+
     private void expectStoreText(Properties props, String expectedText) throws IOException {
         StringWriter sw = new StringWriter();
         props.store(sw);