Issue #18494: Add GoogleNonConstantFieldNameCheck for Google Java Style compliance

[vivek-0509]

Adds GoogleNonConstantFieldNameCheck to enforce non-constant field naming per Google Java Style Guide §5.2.5.

Part of #17842
Detailed Issue at #18494

Rules Enforced

• lowerCamelCase format required
• Single-character names rejected (f)
• Hungarian-style prefixes rejected (mValue, sCount)
• Underscores only between adjacent digits (guava33_4_5 )
• No leading/trailing/consecutive underscores

Scope

• This check validates instance fields, while excluding constants (static final), static non-final fields and local variables.

Diff Regression config: https://gist.githubusercontent.com/vivek-0509/36cbe7f3789730951f15e7b6d551c6f3/raw/7ac2050528756f02e9952ae546a9c1e8cfed29c0/base_config.xml

Diff Regression patch config: https://gist.githubusercontent.com/vivek-0509/a979757b15933ce44f9b0f676a038195/raw/77433e7a8e91c9d621be184f4efcb829cb12d868/patch_config.xml

Contirbution repo PR: checkstyle/contribution#1021

[vivek-0509]

GitHub, generate report

[vivek-0509]

Github, generate site

[github-actions]

Report generation failed on phase "make_report",
step "Generate report".
Link: https://github.com/checkstyle/checkstyle/actions/runs/21036868447

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/2508be9_20260115160126/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/2508be9_20260115160126/checks.html#Standard_Checks

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/2508be9_20260115160126/checks/naming/googlemembername.html#

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/2508be9_20260115160126/checks/naming/index.html#Naming_Conventions_Checks

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0621ade_2026170816/reports/diff/index.html

[vivek-0509]

Hey @romani, I have reviewed the diff report all are genuine violations.
https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0621ade_2026170816/reports/diff/index.html

[romani]

Diff report is very huge hard to get actual diff.
Please generate report in comparison to old module that covered this to catch actual diff. Messages likely should be overriden to be same to avoid diff in wording to be a reason of diff.

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/649199d_2026065114/reports/diff/index.html

[vivek-0509]

Hi @romani , I have generated a comparison diff report. I have overridden the messages to be identical. Here is the report https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/649199d_2026065114/reports/diff/index.html. Please let me know if I have understood your request correctly or if any changes are needed.

[romani]

diff is still wild.
please give same id to both Checks to try avoid diff detection that is purely due to name of Check.

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/649199d_2026150917/reports/diff/index.html

[vivek-0509]

@romani I updated the configurations to use the same id for both checks, but they still appear as Red/Green pairs. It seems the report tool uses the full Check class name for the rule name, so MemberName and GoogleMemberName are treated as different rules despite the shared ID. Am I missing something?

[vivek-0509]

@romani
I was investigating the report generation tool and i find that currently, the diff report compares violations using the full source string in
https://github.com/checkstyle/contribution/blob/4ac81f9f9928d695869b93c568fced0d98a1e781/patch-diff-report-tool/src/main/java/com/github/checkstyle/data/CheckstyleRecord.java#L249-L252

When an explicit id is set in the config, the ID is appended to the class name in the format ClassName#id (e.g., MemberNameCheck#GoogleMemberNameDiff). This means when two different checks share the same id, their source strings still differ because of the class name prefix (e.g., MemberNameCheck#GoogleMemberNameDiff vs GoogleMemberNameCheck#GoogleMemberNameDiff). As a result, violations with identical line, column, severity, and message are incorrectly reported as "removed" from base and "added" in patch.

the fix can be that, If source contains # (indicating an explicit module ID was set), compare only the portion after # instead of the full string. This would allow proper matching of violations from different checks that share the same ID, and the diff report would show only true behavioral differences.

[romani]

tools is right, we tried to hack it, but tool is right in it behavior to show diff.

[romani]

so, we can look at diff and see why we have more violations that it used to be.

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/649199d_2026150917/reports/diff/elasticsearch/index.html#A1
is good example, previosly we did not do violation, now we do.
https://google.github.io/styleguide/javaguide.html#s5.2.4-constant-names

google state that type immutability if critical point in decision. Checkstyle is not type aware, we can not make decision if it is immutable or not. So better to not give violation.
False positive will make users be frustrated, as it will block them, but false-negative (absence of violation) will not make users angry.

[vivek-0509]

Understood, will change the scope to skip all static fields and update the issue accordingly.

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/f342244_2026110001/reports/diff/index.html

[vivek-0509]

@romani updated the scope for the check to skip static member names
here is the report the false negatives are now getting correcty flagged below are some examples from the report
https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/f342244_2026110001/reports/diff/openjdk25/index.html#A15156

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/f342244_2026110001/reports/diff/openjdk25/index.html#A33718

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/f342244_2026110001/reports/diff/nbia-dcm4che-tools/index.html#A9

[romani]

items:

[romani]

we need to mention that static fields are skipped, as Chckstyle not able to make sure that used type is immutable.

[vivek-0509]

done

[romani]

Checks that member names conform to the

Google Java Style Guide for non-constant field naming.

hmm, why we named it GoogleMemberNameCheck ?

as it should be GoogleNonConstantFieldNameCheck.
and it will make it clear why we have limitation on static.

[vivek-0509]

done

[vivek-0509]

@romani all the changes done pr is ready for review

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/4024f27_2026132653/reports/diff/index.html

[vivek-0509]

GitHub, generate website

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/4024f27_20260118142756/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/4024f27_20260118142756/checks.html#Standard_Checks

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/4024f27_20260118142756/checks/naming/googlenonconstantfieldname.html#

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/4024f27_20260118142756/checks/naming/index.html#Naming_Conventions_Checks

[vivek-0509]

GitHub, generate website

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/ba427e9_20260118164654/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/ba427e9_20260118164654/checks.html#Standard_Checks

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/ba427e9_20260118164654/checks/naming/googlenonconstantfieldname.html#

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/ba427e9_20260118164654/checks/naming/index.html#Naming_Conventions_Checks

[vivek-0509]

@romani
I am considering improving the violation message for this check also to better guide users when special characters are used in field names.

Current message:
Non-constant field name ''{0}'' must start with a lowercase letter, min 2 chars, avoid single lowercase letter followed by uppercase.

Updated message:
Non-constant field name ''{0}'' must start with a lowercase letter, min 2 chars, avoid single lowercase letter followed by uppercase, and contain only letters, digits, or underscores.

For cases like $foo or foo$bar, the current message does not clearly explain why the name is invalid. The Google Style Guide 5.1 explicitly states:

"Identifiers use only ASCII letters and digits, and, in a small number of cases noted below, underscores. Thus each valid identifier name is matched by the regular expression \w+."

Does this violation message looks good to you??

[vivek-0509]

@romani I have updated the violation messages since this is the same case as we have done for the GoogleMethodNameCheck as discussed with @Zopsss in that PR, please let me know if any other changes are needed.

[vivek-0509]

@Zopsss Do you want me to keep the full violation message for this check's Example1 also? I have not updated it since I have covered both the violation messages in the examples.

[vivek-0509]

@romani Can you please review this PR when you have time

[Zopsss]

Do you want me to keep the full violation message for this check's Example1 also? I have not updated it since I have covered both the violation messages in the examples.

@vivek-0509 sorry for the delay, yes please do that.

[vivek-0509]

GitHub, generate website

[vivek-0509]

@Zopsss Done, please review

[vivek-0509]

GitHub, generate report

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0bb782c_20260202113633/index.html

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0bb782c_20260202113633/checks.html#Standard_Checks

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0bb782c_20260202113633/checks/naming/googlenonconstantfieldname.html#

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0bb782c_20260202113633/checks/naming/index.html#Naming_Conventions_Checks

[vivek-0509]

CI Failure is unrelated

[github-actions]

https://checkstyle-diff-reports.s3.us-east-2.amazonaws.com/0bb782c_2026121113/reports/diff/index.html

[vivek-0509]

@Zopsss ping