src/java.base/share/classes/java/text/MessageFormat.java

@@ -699,10 +699,14 @@
      * @param newFormat the format to use for the specified format element
      * @exception ArrayIndexOutOfBoundsException if {@code formatElementIndex} is equal to or
      *            larger than the number of format elements in the pattern string
      */
     public void setFormat(int formatElementIndex, Format newFormat) {
+
+        if (formatElementIndex > maxOffset) {
+            throw new ArrayIndexOutOfBoundsException(formatElementIndex);
+        }
         formats[formatElementIndex] = newFormat;
     }
 
     /**
      * Gets the formats used for the values passed into