In the past ten years, Java has been the most used language in android development, but over time, Java has gradually exposed many problems. For example, features that do not support lambda and try-with-resources. Also, have a terrible NullPointerException (NPE) problem. In this context, Kotlin has appeared. Do not worry about NPE, powerful IDE, and more short lines of code. Kotlin has been Google's preferred language for Android app development since 7 May 2019. It is foreseeable that more and more Android applications will be developed using Kotlin instead of Java in the future. In this project, I will explore the advantages of Kotlin compared to Java by rewriting some code in COMP1601 and COMP2601. Moreover, finally, determine whether it is essential to use Kotlin instead of Java to develop Android. Similar to Kotlin, swiftUI was born to solve some of the UIKit problems that have accompanied us for decades [4]. SwiftUI, based on a declarative interface development approach, will undoubtedly bring about reforms in IOS development [3]. This project will also explore some new features of swiftUI through some examples.