The method in the title checks for input indices larger than the internal format array length and throws a new AIOOBE if so. The method does not check negative values however which leads to the default AIOOBE being thrown for that case which causes a confusing error message.

For example,

jshell> var fmt = new MessageFormat("The date was {0,date,full}");
fmt ==> java.text.MessageFormat@98462a7a

jshell> fmt.setFormat(-1, new DecimalFormat())
| Exception java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 10
| at MessageFormat.setFormat (MessageFormat.java:708)
| at (#11:1)

While the MessageFormat is only composed of a singular format/argument, the error message alludes to there being 10 formats/arguments. The code should be updated to throw a new AIOOBE as to not give off the misleading error.

This fix should also take the opportunity to include any cleanup required in similar methods.