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

Instead of using null, you could use some version of the "null object" design pattern. For example:

public class Family {
    private final PersonList people;
    public Family(PersonList people) {
        this.people = people;
    }

    public PersonList getPeople() {
        if (people == null) {
            return PersonList.NULL;
        }
        return people;
    }

    public boolean isNull() {
        return false;
    }

    public static Family NULL = new Family(PersonList.NULL) {
        @Override
        public boolean isNull() {
            return true;
        }
    };
}


import java.util.ArrayList;

public class PersonList extends ArrayList {
    @Override
    public Person get(int index) {
        Person person = null;
        try {
            person = super.get(index);
        } catch (ArrayIndexOutOfBoundsException e) {
            return Person.NULL;
        }
        if (person == null) {
            return Person.NULL;
        } else {
            return person;
        }
    }
    //... more List methods go here ...

    public boolean isNull() {
        return false;
    }

    public static PersonList NULL = new PersonList() {
        @Override
        public boolean isNull() {
            return true;
        }
    };
}

public class Person {
    private Address address;

    public Person(Address address) {
        this.address = address;
    }

    public Address getAddress() {
        if (address == null) {
            return Address.NULL;
        }
        return address;
    }
    public boolean isNull() {
        return false;
    }

    public static Person NULL = new Person(Address.NULL) {
        @Override
        public boolean isNull() {
            return true;
        }
    };
}

etc etc etc

Then your if statement can become:

if (!family.getPeople().get(0).getAddress().getPostalCode.isNull()) {...}

It's suboptimal since:

• You're stuck making NULL objects for every class,
• It's hard to make these objects generic, so you're stuck making a null-object version of each List, Map, etc that you want to use, and
• There are potentially some funny issues with subclassing and which NULL to use.

But if you really hate your == nulls, this is a way out.