Following methods in j.l.Character throws IllegalArgumentException without any message.

    Character.UnicodeBlock.of(cp)
    Character.UnicodeScript.of(cp)
    Character.toChars(cp, char[], int)
    Character.toChars(cp)
    Character.getName(cp)

Providing more diagnostic messages along with them would be helpful.

(cf. http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-March/051887.html)