Default values for builder fields via @Default / @DefaultValue (#227)

[AndreasIgel]

Summary

Implements default values for builder fields (issue #227): a field annotated with @Default (or any @Default/@DefaultValue with a String value()) gets that value applied in build() when the field was never set. Works for both record constructor parameters and class setter-fields.

@SimpleBuilder
public record Product(
    String name,
    double price,
    @Default("GENERAL") String category,   // -> "GENERAL"
    @Default("true") boolean active) {}     // -> true

ProductBuilder.create().name("Laptop").price(1500.0).build();
// category == "GENERAL", active == true

Why the existing implementation didn't cover this: the builder always constructs via the max-parameter (canonical) constructor and passed null/0 for unset fields, so a smaller delegating record constructor is never selected and there was no "use this when omitted" hook. This PR adds that hook without changing constructor selection.

How it works

• Detection — framework-agnostic by simple name (Default, DefaultValue, any package, e.g. Jakarta REST @DefaultValue). Extracted in BuilderDefinitionCreator for both constructor params and the backing field element (FieldAnnotationExtractor), then formatted per field type (String -> quoted, char -> single-quoted, numeric/boolean -> as-is, complex -> raw expression).
• Model — FieldDto gains defaultValue + isRequired() (a defaulted field is never required, even with @NotNull, so no validation error when unset).
• Codegen — CoreMethodsEnhancer.build():
  • constructor args: this.field.valueOr(<default>)
  • setter fields: this.field.ifSet(result::setX).orElse(<default>)
• Runtime — TrackedValue gains valueOr(T) and a backward-compatible fluent ifSet(consumer).orElse(default) (existing ifSet(consumer); callers are unaffected):

public DefaultValueApplier<T> ifSet(Consumer<T> consumer) {
  if (isSet()) consumer.accept(value);
  return new DefaultValueApplier<>(isSet(), consumer);   // .orElse(default) no-ops if already applied
}

Docs / examples / tests

• README "Default Values" usage section.
• New example DTOs ProductWithDefaults (record) and OrderWithDefaults (class) + their committed generated builders.
• DefaultValueTest (7 cases) and TrackedValueTest additions.
• example/pom.xml: registers generated-example-builder as a compiled source root via build-helper-maven-plugin so the committed generated builders are compiled.

Full build green, 312 tests pass; the committed generated example builders regenerate byte-identical to a fresh mvn -pl example clean compile (generated-source gate passes).

[codecov]

Codecov Report

❌ Patch coverage is 92.18750% with 5 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...processor/processing/BuilderDefinitionCreator.java	88.88%	1 Missing and 2 partials ⚠️
...s/processor/analysis/FieldAnnotationExtractor.java	88.88%	1 Missing and 1 partial ⚠️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud