Omit java.util.Map field when empty?

[Chris-753]

Hello there,

when i got a class like this:

public class Foo {
   private String name = "bla";
   private Map<String, String> events = new HashMap<String, String>();
   ...
}

And the events-Map has contents, im ok with output like this:
<Foo>
  <name>bla</name>
  <events>
    <entry>
      <string>foo</string>
      <string>bar</string>
    </entry>
  </events>
</Foo>

But whats the best way to get output like this, if the events Map is empty??

<Foo>
  <name>bla</bla>
</Foo>

currently im getting:

<Foo>
  <name>bla</bla>
  <events />
</Foo>

I realize that an empty map should be different from null, but since
im using a lot of empty Maps i'd like to avoid these tags in the
output.

Using a custom Map Converter doesnt solve this, since the <events> tag
comes from the fieldname and not from the Map deserialization.

Another Problem: the Map is a field in a common baseclass.

Do i have to write a custom MarshallingStrategy that uses the original
one, but omits fieldnames for empty Maps?
If so, what is the easiest way?

Would it work to initialize this field with null by default, and only
create the map, when adding an item?

Thanks in advance.

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[Jörg Schaible-2]

Hi Chris,

Chris wrote:

> Hello there,
>
> when i got a class like this:
>
> public class Foo {
>    private String name = "bla";
>    private Map<String, String> events = new HashMap<String, String>();
>    ...
> }
>
> And the events-Map has contents, im ok with output like this:
> <Foo>
>   <name>bla</name>
>   <events>
>     <entry>
>       <string>foo</string>
>       <string>bar</string>
>     </entry>
>   </events>
> </Foo>
>
> But whats the best way to get output like this, if the events Map is
> empty??
>
> <Foo>
>   <name>bla</bla>
> </Foo>
>
> currently im getting:
>
> <Foo>
>   <name>bla</bla>
>   <events />
> </Foo>
>
> I realize that an empty map should be different from null,

That's the reason.

> but since
> im using a lot of empty Maps i'd like to avoid these tags in the
> output.
>
> Using a custom Map Converter doesnt solve this, since the <events> tag
> comes from the fieldname and not from the Map deserialization.

Yep.

> Another Problem: the Map is a field in a common baseclass.
>
> Do i have to write a custom MarshallingStrategy that uses the original
> one, but omits fieldnames for empty Maps?

No, the tags are written by the converter used for Foo (presumably the
ReflectionConverter).

> If so, what is the easiest way?
>
> Would it work to initialize this field with null by default, and only
> create the map, when adding an item?

Yes. Also keep in mind, that the deserialization of XML without the empty
events tag will also set this field to null - you have to handle it then
anyway.

As alternative you may write writeReplace/readResolve methods if your
objects are cloneable:

private Object writeReplace() {
    Foo written;
    if (!events.isEmpty()) {
        written = this;
    } else {
        written = clone();
        clone.events = null;
    }
    return written;
}

private Object readResolve() {
    if (events == null) {
        events = new HashMap<String, String>();
    }
    return this;
}

- Jörg


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