StackOverflow has a good discussion about this exact topic in this Q&A. In the top rated question, kronoz notes:

Returning null is usually the best idea if you intend to indicate that no data is available.

An empty object implies data has been returned, whereas returning null clearly indicates that nothing has been returned.

Additionally, returning a null will result in a null exception if you attempt to access members in the object, which can be useful for highlighting buggy code - attempting to access a member of nothing makes no sense. Accessing members of an empty object will not fail meaning bugs can go undiscovered.

Personally, I like to return empty strings for functions that return strings to minimize the amount of error handling that needs to be put in place. However, you'll need to make sure that the group that your working with will follow the same convention - otherwise the benefits of this decision won't be achieved.

However, as the poster in the SO answer noted, nulls should probably be returned if an object is expected so that there is no doubt about whether data is being returned.

In the end, there's no single best way of doing things. Building a team consensus will ultimately drive your team's best practices.

A solution such as Option<T> is not a so bad idea. In fact, it has been successfully introduced in Haskell as a Maybe (but I don't know if this is the first time this solution has been used). Maybe monads are common in Haskell programs. Since, other languages have adopted the same system. For instance, Scala has an Option type.

If you don't want to become dependent of Guava, you could use Java 8, which introduces the Optional class.

However, Option/Maybe/… types are more relevant in a functional environment, because what you typically want to do is to get a behaviour/a property from the element if it really exists, and to get nothing or a default behaviour/property otherwise. So, a typical use of Options consists in

• applying a function A->B on the Option[A] in order to get an Option[B];
• switching your code on the basis of the real content of the Option (for instance, by using Pattern matching if the language supports it and if Option has two subtypes, or by using method overload);
• or filtering the Options that represent (non-)existing elements.

As an alternative, you could use the Null Object Pattern, in which you have an abstract class MyClass and two concrete subclasses: MyRealClass and MyNullClass. You manipulate instances of MyClass, but generate instances of MyRealClass (if the element is really existing) or MyNullClass (if the element doesn't exist). MyNullClass contains the default behaviours/properties. If the null objects are stateless (which is typically the case), one could cache them, or even make them singletons.

This pattern is described in [Fowler].

[Fowler] Martin Fowler, Kent Beck, Refactoring: Improving the Design of Existing Code.

You can't do it directly, you should provide your own way to check this. Eg.

class MyClass {
  Object attr1, attr2, attr3;

  public boolean isValid() {
    return attr1 != null && attr2 != null && attr3 != null;
  }
}

Or make all fields final and initialize them in constructors so that you can be sure that everything is initialized.

I was also required to produce such a structure for legacy client compatibility, here is my solution (depends on Spring Boot since uses @JsonComponent annotation)

Create "special object" that will be treated as empty

public class EmptyObject {
}

Create property in your model

@JsonProperty("data")
private EmptyObject data = new EmptyObject();

public EmptyObject getData() {
    return data;
}

Create serializer that will process empty object above

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import com.sevensenders.datahub.api.service.response.model.EmptyObject;
import org.springframework.boot.jackson.JsonComponent;

import java.io.IOException;

@JsonComponent
public class EmptyObjectSerializer extends StdSerializer<EmptyObject> {

    public EmptyObjectSerializer() {
        this(null);
    }

    public EmptyObjectSerializer(Class<EmptyObject> t) {
        super(t);
    }

    @Override
    public void serialize(EmptyObject value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        // to maintain AF compatible format it is required to write {} instead of null
        gen.writeStartObject();
        gen.writeEndObject();
    }
}

Output:

{
  ...
  "data": {}
}

There is a way to initialize the destination object in Dozer.

1. Define a factory class for the destination Object.
2. Specify the create-method for the destination object as the method in the above factory class.

Take your example, I presume you want to map Person to PersonVO and Address to AddressVO.

Create a factory class for PersonVO

public abstract class PersonVOFactory {
    private PersonVOFactory() {
    }
    public static PersonVO createPersonVO() {
        PersonVO personVO = new PersonVO();
        personVO.setAddressVO(new AddressVO());
        return personVO;
    }
}

Specify the above create method for Person to PersonVO mapping.

<mapping map-id="person-mapping">
    <class-a>Person</class-a>
    <class-b create-method="PersonVOFactory.createPersonVO">PersonVO</class-b>
    <field>
        <a>address.addressField1</a>
        <b>addressVO.addressField1</b>
    </field>
    <field>
        <a>address.addressField2</a>
        <b>addressVO.addressField2</b>
    </field>
</mapping>

Now the statement, personvo.getAddressVO.getAddressField1() will not throw an NPE.

An "empty object" is pretty ambiguous in java terms. I could interpret that as this:

Object empty = new Object();

which is about the emptiest object you can create.

However in your example,

Name myName = new Name();

That's going to create an object based on whatever code you've put in your default constructor. (Which, i guess if you're setting everything to a default value, is pretty empty)

Can't you just replace {} with NULL before passing it to the GSON?

You may try the COALESCE function to return a new Country if the field is null : SELECT COALESCE(Co, new Country()) .... But you may have to bind your main Entity to the Country entity instead of using the country_Id.

[EDIT] It can be used with a CriteriaBuilder also : cb.coalesce(root.get("Co"), new Country()) /!\ will use the same default Country instance for all result lines

If you return an empty collection (but not necessarily Collections.emptyList()), you avoid surprising downstream consumers of this method with an unintentional NPE.

This is preferable to returning null because:

• The consumer doesn't have to guard against it
• The consumer can operate on the collection irrespective of how many elements are in it

I say not necessarily Collections.emptyList() since, as you point out, you're trading one runtime exception for another in that adding to this list will be unsupported and once again surprise the consumer.

The most ideal solution to this: eager initialization of the field.

private List<String> bone = new ArrayList<>();

The next solution to this: make it return an Optional and do something in case it doesn't exist. Instead of throwing you could also provide the empty collection here if you so desired.

Dog dog = new Dog();
dog.get().orElseThrow(new IllegalStateException("Dog has no bones??"));

Returning null is usually the best idea if you intend to indicate that no data is available.

An empty object implies data has been returned, whereas returning null clearly indicates that nothing has been returned.

Additionally, returning a null will result in a null exception if you attempt to access members in the object, which can be useful for highlighting buggy code - attempting to access a member of nothing makes no sense. Accessing members of an empty object will not fail meaning bugs can go undiscovered.

MattPutnam's answer is right on point, and I second it. I'd add this: the concept of "null object," when you analyze it, seems to boil down to the mathematical concept of a monoid. You can think of it this way: a monoid is a type that has both of these things:

1. An "append," "sum" or similar operation, which needs to be associative: a.op(b).op(c) is the same as a.op(b.op(c)).
2. An "empty," "zero" or "null" value, that acts as the neutral element or identity element of the operation.

The classic example of the null object pattern is to return an empty list or array instead of null. Well, lists are a monoid, with append as the operation and the empty list as the neutral element.

Now, the problem that you face in your Car example is that Car isn't really a monoid; there is no notion of "the empty car" or "the neutral car", and there isn't really a sensible operation that you could use to combine two Cars into one.

So the recommendation you're rightly getting is to use something like the Java 8 Optional. And the trick is that no matter what type T is, Optional<T> is a monoid:

1. The monoid's "combine" operation is "pick the first value if it's not empty, otherwise pick the second value":
   • x || empty = x
   • empty || x = x
2. The neutral element is Optional.empty(), because Optional.empty().orElse(anything) is the same as just anything.

So basically, Optional<T> is a wrapper that adds a null object to types like Car that don't have one. The Optional<T>.orElse(T value) method that is a slightly refactored version of the "pick first non-empty value" monoid.

Method overloading can make your implementations more efficient and cleaner:

public static boolean isEmpty(Collection obj) {
    return obj == null || obj.isEmpty();
}

public static boolean isEmpty(String string) {
    return string == null || string.trim().isEmpty();
}

public static boolean isEmpty(Object obj) {
    return obj == null || obj.toString().trim().isEmpty();
}

The Collection version is as efficient as possible.

The String version would be more efficient without the trimming. It would be best to trim your strings as soon you see them, long before they reach this call. If you can review the callers and make sure that the strings are always trimmed at their origins, then you can remove .trim() for best performance.

The Object version can be inefficient, depending on the toString implementation of the objects that will be passed to it, and because of the trimming.

I removed the comparison with null from there, because it seems pointless to me. I mean, a class whose toString method says "null" would seem very very odd.

In any case, you don't really want the Object version to be called, at all. Most importantly because it probably won't even work. Take for example an empty Map. Its toString method returns the string {}, which won't match your conditions of emptiness. (For this type you should definitely add isEmpty(Map<?, ?> map) to benefit from its isEmpty method.)

If performance is so critical, then add more overloaded implementations for all other types that you care about, for example:

public static boolean isEmpty(Something obj) {
    return obj == null || obj.isEmpty();
}

Finally, especially when something is so important, you definitely want to unit test it, for example:

@Test
public void testEmptyObject() {
    assertTrue(isEmpty((Object) null));
    assertFalse(isEmpty(new Object()));
}

@Test
public void testEmptyString() {
    assertFalse(isEmpty("hello"));
    assertTrue(isEmpty(""));
    assertTrue(isEmpty(" "));
    assertTrue(isEmpty((Object) null));
}

@Test
public void testEmptySet() {
    assertFalse(isEmpty(new HashSet<String>(Arrays.asList("hello"))));
    assertTrue(isEmpty(new HashSet<String>()));
}

@Test
public void testEmptyMap() {
    Map<String, String> map = new HashMap<String, String>();
    assertTrue(isEmpty(map));
    map.put("hello", "hi");
    assertFalse(isEmpty(map));
}