Ooh. Good catch! I've updated the errata with "given that a and b are of type Integer" to "given that a and b are classes that follow the spec". You are correct about the answer being wrong for Integer. The intent wasn't to find out if people happen to know what Integer's hash code method returns. Have a cow for noticing this detail!
Note: this was in exam 1. (If you report any others, can you say which of the three exams it is in so I know where to look)