A common, repetitive, and time-consuming task in the construction of Android applications is creating interface field validation. Although there are some tools that address that issue, they are not intuitive and require effort for configuration, which may hinder their use. This paper introduces Convalida, an annotation-based library that generates code automatically at compile time for field validation of Android applications, thus allowing the developer to focus on the implementation of business rules. A comparative study considering other field validation tool and a manual approach is also provided and shows that our library has improved the final code.