The best way to test private methods is to use Reflection.
There are many ways, but I would simple do this;
@Test
public void testConstructorIsPrivate() throws Exception {
Constructor constructor = Product.class.getDeclaredConstructor();
assertTrue(Modifier.isPrivate(constructor.getModifiers()));
constructor.setAccessible(true);
constructor.newInstance();
}
This will cover the constructor when running the coverage tool emma.