What is @Builder.Default used for?

Normally, a field's default field initializer (like private int retries = 3;) is silently ignored when using @Builder, because the builder constructs the object through its own constructor logic rather than the field's normal initialization path — without @Builder.Default, an unset field in the builder would end up as its type's zero value (0, null, etc.), not the initializer you wrote.

@Builder
public class RetryConfig {
    @Builder.Default
    private int maxRetries = 3;   //  without this annotation, unset -> 0, not 3
}

RetryConfig cfg = RetryConfig.builder().build();
//  cfg.getMaxRetries() == 3, thanks to @Builder.Default

This is a common gotcha for developers new to Lombok's builder: field initializers you'd expect to "just work" are silently dropped unless explicitly marked with @Builder.Default, so any field with a meaningful default value needs this annotation to actually preserve that default when built via the builder.

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