> When a function receives an object, this object might be null. That is, if you see ‘String s’ in your code, you often have no way of knowing whether ‘s’ contains an actually String unless you check at runtime. Can you guess whether programmers always check

Actually, my experience is the opposite - they check like hell, all over the place, reflexively, because a null pointer exception is a blaring emergency that has to be fixed RIGHT NOW, so I always see Java code written like this:

    if (customer != null && customer.getOrder() != null && customer.getOrder().getLineItemList() != null) {
      for (LineItem item : customer.getOrder().getLineItemList()) {
         if (item.getProduct() != null && item.getProduct().getPrice() != null && item.getProduct().getPrice().getCurrency() != null) {
           ...
         } // else don't do anything, don't even log an error, just continue on as if an empty product or price object is an ignorable error and create a much harder to diagnose problem than a null pointer exception