Convert to Kotlin

app/build.gradle

@@ -2,6 +2,8 @@ plugins {
     id 'com.android.application'
 }
 
+apply plugin: 'kotlin-android'
+
 android {
     compileSdkVersion 30
 
@@ -35,4 +37,9 @@ dependencies {
     testImplementation 'junit:junit:4.+'
     androidTestImplementation 'androidx.test.ext:junit:1.1.2'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
+    implementation "androidx.core:core-ktx:+"
+    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+}
+repositories {
+    mavenCentral()
 }

app/src/main/java/com/example/kotlinhomework/MainActivity.kt

@@ -0,0 +1,52 @@
+package com.example.kotlinhomework
+
+import android.content.Intent
+import android.os.Bundle
+import android.widget.Button
+import android.widget.TextView
+import androidx.appcompat.app.AppCompatActivity
+
+class MainActivity : AppCompatActivity() {
+    private var greetings: String? = null
+    private var name: String? = null
+    private var textView: TextView? = null
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_main)
+        greetings = getString(R.string.hello)
+        name = savedInstanceState?.getString(NAME_KEY) ?: getString(R.string.anon)
+        textView = findViewById(R.id.textViewHello)
+        val button = findViewById<Button>(R.id.buttonNameYourSelf)
+        button.setOnClickListener {
+            val intent = Intent(this@MainActivity, com.example.kotlinhomework.SecondActivity::class.java)
+            startActivityForResult(intent, com.example.kotlinhomework.SecondActivity.GET_NAME_REQUEST_CODE)
+        }
+    }
+
+    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
+        super.onActivityResult(requestCode, resultCode, data)
+        if (requestCode == com.example.kotlinhomework.SecondActivity.GET_NAME_REQUEST_CODE && resultCode == RESULT_OK)
+            name = data?.getStringExtra(com.example.kotlinhomework.SecondActivity.NAME_KEY) ?: name
+    }
+
+    override fun onResume() {
+        super.onResume()
+        textView!!.text = String.format(
+                "%s, %s!",
+                greetings,
+                name
+        )
+    }
+
+    override fun onSaveInstanceState(outState: Bundle) {
+        super.onSaveInstanceState(outState)
+        outState.putString(
+                NAME_KEY,
+                name,
+        )
+    }
+
+    companion object {
+        private const val NAME_KEY = "com.example.kotlinhomework.MainActivity.NAME_KEY"
+    }
+}

app/src/main/java/com/example/kotlinhomework/SecondActivity.kt

@@ -0,0 +1,47 @@
+package com.example.kotlinhomework
+
+import android.content.Intent
+import android.os.Bundle
+import android.text.Editable
+import android.text.TextUtils
+import android.text.TextWatcher
+import android.view.View
+import android.widget.Button
+import android.widget.EditText
+import androidx.appcompat.app.AppCompatActivity
+
+class SecondActivity : AppCompatActivity() {
+    private var editText: EditText? = null
+    private var button: Button? = null
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_second)
+        editText = findViewById(R.id.editTextTextPersonName)
+        button = findViewById(R.id.buttonName)
+        button?.isEnabled = false
+        editText?.addTextChangedListener(object : TextWatcher {
+            override fun beforeTextChanged(charSequence: CharSequence, i: Int, i1: Int, i2: Int) {
+                // Nothing to do
+            }
+
+            override fun onTextChanged(charSequence: CharSequence, i: Int, i1: Int, i2: Int) {
+                // Nothing to do
+            }
+
+            override fun afterTextChanged(editable: Editable) {
+                button?.isEnabled = !TextUtils.isEmpty(editable)
+            }
+        })
+        button?.setOnClickListener(View.OnClickListener {
+            val intent = Intent()
+            intent.putExtra(NAME_KEY, editText?.text.toString())
+            setResult(RESULT_OK, intent)
+            finish()
+        })
+    }
+
+    companion object {
+        const val NAME_KEY = "com.example.kotlinhomework.SecondActivity.NAME_KEY"
+        const val GET_NAME_REQUEST_CODE = 1234
+    }
+}

build.gradle

@@ -1,11 +1,13 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 buildscript {
+    ext.kotlin_version = '1.4.21'
     repositories {
         google()
         jcenter()
     }
     dependencies {
         classpath "com.android.tools.build:gradle:7.0.0-alpha02"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files