add "fallbackStringForNullValue" attribute

[mariok]

This change allows to define fallback-values for fields, which will be used by the formatter, if the field value is null. This is important to keep the originally defined positions of the fields when formatting an object back into a fixed length string.

With this change an object, which has been created by parsing a fixed length string, can be formatted back into the exact same fixed length string again, even if some fields are empty or ignored in the original fixed length string.

[g0ddest]

Hey @mariok, thanks for the PR! The idea makes sense - being able to round-trip parse/format with null fields is definitely useful.

A few things though:
Can you add some tests? At minimum:

Formatting with null field using the fallback
Round-trip: parse → format → parse (should get the same string back)
Make sure it doesn't break existing code without this attribute

What happens if the fallback length doesn't match the field length?

@FixedField(offset = 1, length = 10, fallbackStringForNullValue = "N/A") // ??

Should probably validate or auto-pad it.
Docs would be nice - just a quick example in the README showing when/why to use this.
Also curious how it works with padding and align - does the fallback get padded/aligned or is it used as-is?

[mariok]

Hi @g0ddest,

thanks for your feedback. I have added the padding/align feature, so that the given fallback string will be automatically aligned and padded, if it is shorter than the field length. In case it is longer than the field length, an exception will be thrown with the necessary hints how to fix it.

Also, some test cases and documentation has been added.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[g0ddest]

Now it is much better, thank you! Could you also check the ./gradlew checkstyleMain output? Looks there are some issues there with long lines.

[mariok]

Sure, the checkstyle issues are fixed now.

[g0ddest]

Thank you! Will update in maven central shortly

[g0ddest]

@mariok , the patch is accessible now on maven central with 0.14 https://central.sonatype.com/artifact/name.velikodniy.vitaliy/fixedlength/0.14

[mariok]

Nice, thanks!