Finding the mechanism isn't too hard.
scala> new java.util.HashMap[Long, Long]().get(1L)
res0: Long = 0
scala> :javap -prv -
Binary file res0 contains $line3.$read$$iw$$iw$
[snip]
17: invokestatic #30 // Method scala/runtime/BoxesRunTime.boxToLong:(J)Ljava/lang/Long;
20: invokevirtual #34 // Method java/util/HashMap.get:(Ljava/lang/Object;)Ljava/lang/Object;
23: invokestatic #38 // Method scala/runtime/BoxesRunTime.unboxToLong:(Ljava/lang/Object;)J
and
public static long unboxToLong(Object l) {
return l == null ? 0 : ((java.lang.Long)l).longValue();
}