Comparing SKU numbers
Since you state that you're using Java 8, you should use the variety of new Comparator static methods to extract the relevant fields for comparison:

public int compare(Sku o1, Sku o2) {
    Comparator&lt;Sku&gt; c1 = Comparator.nullsLast(null);
    return c1.thenComparing(Sku::getSkuNumber,
            Comparator.nullsLast(Comparator.naturalOrder())).compare(o1, o2);
}



When Comparator.nullsLast(Comparator) is given a null argument, it considers two non-null argument to be equal. This is the basis of our first comparator.
We then need to compare by getSkuNumber(), so we pass the method reference Sku::getSkuNumber to Comparator.thenComparing(Function, Comparator) as our second comparator. However, since SKU numbers may be null, we use a nested (or tertiary?) null-friendly comparator, Comparator.nullsLast(Comparator) again, as the second argument. The final comparator in use is just Comparator.naturalOrder(), which performs the final straightforward integer comparison if both SKU numbers are non-null.


edit: @Simon mentioned a good point that since your existing compare(Sku, Sku) method is likely to be the implementation for implementing Comparable&lt;Sku&gt;, you should consider making the Comparator I suggested above a public static final field, so that it doesn't have to be created every time, e.g.:

public static final Comparator&lt;Sku&gt; COMPARATOR = 
                        Comparator.nullsLast((Comparator&lt;Sku&gt;) null)
                            .thenComparing(Sku::getSkuNumber,
                                Comparator.nullsLast(Comparator.naturalOrder()));