src/java.base/share/classes/java/lang/StringBuffer.java

*** 425,434 ****
--- 425,441 ----
          toStringCache = null;
          super.append(d);
          return this;
      }
  
+     @Override
+     public synchronized StringBuffer appendN(char c, int n) {
+         toStringCache = null;
+         super.appendN(c, n);
+         return this;
+     }
+ 
      /**
       * @throws StringIndexOutOfBoundsException {@inheritDoc}
       * @since      1.2
       */
      @Override