cannot unbox from bound of type variable.

==========$ cat -n Main.java
     1 class Main {
     2 <U extends Integer, V extends Double> Number cond1(boolean z, U x1, V x2) {
     3 return (z? x1 : x2);
     4 }
     5 }
==========$ newjavac Main.java
+ exec /usr/j2sdk1.5.0/bin/javac -J-esa -source 1.5 -target 1.5 -XDdev Main.java
Main.java:3: cannot find symbol
symbol : method intValue()
location: bound of type variable U
        return (z? x1 : x2);
                   ^
Fatal Error: Unable to find method intValue
==========$