Java: avoid checking for null in nested classes (Deep Null checking)

Question

Imagine I have a class Family. It contains a List of Person. Each (class) Person contains a (class) Address. Each (class) Address contains a (class) PostalCode. Any "i

Answer 1

Although this post is almost five years old, I might have another solution to the age old question of how to handle NullPointerExceptions.

In a nutshell:

end: {
   List people = family.getPeople();            if(people == null || people.isEmpty()) break end;
   People person = people.get(0);                       if(person == null) break end;
   Address address = person.getAddress();               if(address == null) break end;
   PostalCode postalCode = address.getPostalCode();     if(postalCode == null) break end;

   System.out.println("Do stuff");
}

Since there is a lot of legacy code still in use, using Java 8 and Optional isn't always an option.

Whenever there are deeply nested classes involved (JAXB, SOAP, JSON, you name it...) and Law of Demeter isn't applied, you basically have to check everything and see if there are possible NPEs lurking around.

My proposed solution strives for readibility and shouldn't be used if there aren't at least 3 or more nested classes involved (when I say nested, I don't mean Nested classes in the formal context). Since code is read more than it is written, a quick glance to the left part of the code will make its meaning more clear than using deeply nested if-else statements.

If you need the else part, you can use this pattern:

boolean prematureEnd = true;

end: {
   List people = family.getPeople();            if(people == null || people.isEmpty()) break end;
   People person = people.get(0);                       if(person == null) break end;
   Address address = person.getAddress();               if(address == null) break end;
   PostalCode postalCode = address.getPostalCode();     if(postalCode == null) break end;

   System.out.println("Do stuff");
   prematureEnd = false;
}

if(prematureEnd) {
    System.out.println("The else part");
}

Certain IDEs will break this formatting, unless you instruct them not to (see this question).

Your conditionals must be inverted - you tell the code when it should break, not when it should continue.

One more thing - your code is still prone to breakage. You must use if(family.getPeople() != null && !family.getPeople().isEmpty()) as the first line in your code, otherwise an empty list will throw a NPE.