A DESCRIPTION OF THE PROBLEM :
java.util.Currency.getInstance method throw IllegalArgumentException without any error message. As an enhancement a more meaningful error message could be produce.
Before :
            boolean found = false;
            if (currencyCode.length() != 3) {
                throw new IllegalArgumentException();
            }

After :
            boolean found = false;
            if (currencyCode.length() != 3) {
                throw new IllegalArgumentException("Currency code not 3 characters length");
            }
Before :
            if (!found) {
                OtherCurrencyEntry ocEntry = OtherCurrencyEntry.findEntry(currencyCode);
                if (ocEntry == null) {
                    throw new IllegalArgumentException("Currency code not found");
                }
                ...
            }
After :

            if (!found) {
                OtherCurrencyEntry ocEntry = OtherCurrencyEntry.findEntry(currencyCode);
                if (ocEntry == null) {
                    throw new IllegalArgumentException("Currency code not found");
                }
                ...
            }

As a comparable in java.time.format.DateTimeFormatter some error message are added in the exception message :

    public TemporalAccessor parseBest(CharSequence text, TemporalQuery<?>... queries) {
        Objects.requireNonNull(text, "text");
        Objects.requireNonNull(queries, "queries");
        if (queries.length < 2) {
            throw new IllegalArgumentException("At least two queries must be specified");
        }