Convert all Android apps (Java) to Kotlin

Overview

I wanted to convert my company's app to Java-> Kotlin, so I first tried converting everything to Kotlin with my Android app (all Java code).

About the measures taken at that time. It didn't take much effort, probably because my app was small.

target

• Those who know Kotlin somehow and want to rewrite android application written in java to kotlin.
• Excludes test projects

My Android, Kotlin experience

• Both are about half a year.
• The Android app was privately created and released around August 10th, 2015.
• Kotlin learned around 2016/8, and wrote and released a webview + push notification app in kotlin around 2016/8 and 2017/1.

App converted to Kotlin

A peripheral search app called PLACE SEARCH.

https://play.google.com/store/apps/details?id=com.hikarusato.placesearch

Corresponding man-hours

Day 2

• Rewrite to Kotlin code until most of the functions work

manner

1. If Kotlin is not included in Android Studio, install the plugin. There are ways to do it in various articles.
   http://blog.ch3cooh.jp/entry/20160217/1455676200
2. Edit build.gladle.

build.gradle (for the project)

buildscript {
　　　　　　　　
    ext.kotlin_version = '1.0.6'//add to
    repositories {
        jcenter()
    }
    dependencies {
        
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"//add to
    }
}

bash:build.gradle（Module:app)

apply plugin: 'kotlin-kapt'//Added if you want to make annotations available in kotlin.
apply plugin: 'kotlin-android'//add to.

・
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・

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"//add to.
    compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"//Added when using reflection (property name or class name acquisition)
    testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"//Added when using a test project
    testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"//Added when using a test project
}

Reference: https://kotlinlang.org/docs/reference/using-gradle.html

3. In the Android project navigation, select the app-> java folder and select the main menu [Code]-> [Convert Java File to Kotlin File].

This will convert all java code into Kotlin code.

Crushed compile error

-NullableType object. Add !! or? Where it is in the method format.

• In Android studio 2.3, it is ~ !!. ~ At the time of [Convert Java File to Kotlin File].

Example

        //Java in android studio->After converting to Kotlin

        mWaitDialog = ProgressDialog(this, AlertDialog.THEME_DEVICE_DEFAULT_LIGHT)
        //Here, mWaitDialog!!.setMessage or 　mWaitDialog?.I get an error when I change to setMessage
        mWaitDialog.setMessage(resources.getString(R.string.now_updating))

• Mostly this error (more than 100).
• Appropriate, object !!. Change to method or object ?. method.
  Due to the large number, I changed it to almost object !!. Method format from the middle (thinking that null check should be done in java code).

-Unable to recognize private Enum in the same class in the internal class.

Kotlin code after automatic conversion

class A {
   private enum class TYPE {
        TYPE_1,
        TYPE_2
    }

   internal class B {
        var state = TYPE.TYPE_1//Error not recognizing enum TYPE here
   }
}

Revised

class A {
    //Remove private
    enum class TYPE {
        TYPE_1,
        TYPE_2
    }

   internal class B {
        var state = TYPE.TYPE_1
   }
}

· Java Enum.valueOf (Enum name.class, "Enum enum") is not converted correctly

Kotlin code after automatic conversion

    enum class TYPE {
        TYPE_1,
        TYPE_2
    }

・
・
・

    Enum.valueOf<TYPE>(TYPE::class.java!!, "Enum enumeration")//Enum.valueOf<TYPE>Error in

Revised

    enum class TYPE {
        TYPE_1,
        TYPE_2
    }

・
・
・

    TYPE.valueOf("Enum enumeration")//Enum name.Change to valueOf format

Crushed run-time error

・ Null, parameter favicon at ~ .onPageStarted

Before correction

override fun onPageStarted(view: WebView, url: String, favicon: Bitmap) {

Revised

//The favicon can be null, so?Attach.
override fun onPageStarted(view: WebView, url: String, favicon: Bitmap?) {

· Java.lang.IllegalStateException: ViewUtils.createNaviTran… transit, enter, nextAnim) must not be null

Before correction

 override fun onCreateAnimation(transit: Int, enter: Boolean, nextAnim: Int): Animation

Revised

//Animation can be null, so?Attach.
 override fun onCreateAnimation(transit: Int, enter: Boolean, nextAnim: Int): Animation?

Advantages / disadvantages of kotlin conversion (impression)

merit

• Reduces the amount of code
• Learning cost is low.
• I couldn't get used to Objective-C-> Swift for about a week, but I got used to java-> kotlin in about 3 days.

• Extensive standard functions. The list operation system is especially convenient (filter, map, find, etc.).
• Android Studio warns you of code that you can write efficiently in kotlin
• Since it can be used interchangeably (because java code can be used from Kotlin and vice versa), existing libraries can also be used.

Demerit

• The trouble of converting java-> kotlin of existing project.
• apk size increases a little.
• According to some blogs, it will increase by 50KB to 800KB.
• In my case, I updated the sdk version and library, so it's not helpful, but it increased by 2.2MB (10MB-> 12.2MB).
• I feel that the build is a little slower than when it was java.

Summary

• Java-> Kotlin doesn't seem to be that hard.
• Impression that the major version upgrade of Swift is more difficult.
• The Kotlin conversion accuracy of Android Studio should improve with repeated version upgrades.