The compiler generates calls to valueOf (to box) and [type]value (to unbox) as described in lovely detail at the Java Tutorials, here.

If you tell CFR not to hide sugaring (with --sugarboxing false), it's really obvious to see what's going on ... and it's obvious why null checks are important when dealing with boxed values....

Original

    public Double foo(Integer i, Double d) {
        return d + i;
    }

CFR decompiled (with --sugarboxing false)

    public Double foo(Integer i, Double d){
        return Double.valueOf((d.doubleValue() + i.intValue()));
    }