Added kotlin dependency

app/build.gradle

@@ -1,4 +1,5 @@
 apply plugin: 'com.android.application'
+apply plugin: 'kotlin-android'
 
 android {
     compileSdkVersion 26
@@ -17,4 +18,5 @@ dependencies {
     compile 'com.android.support:appcompat-v7:26.0.0-alpha1'
     compile 'com.android.support:design:26.0.0-alpha1'
     compile 'com.android.support:support-vector-drawable:26.0.0-alpha1'
+    compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
 }

app/src/main/java/me/vadik/knigopis/MainActivity.java

@@ -1,45 +0,0 @@
-package me.vadik.knigopis;
-
-import android.os.Bundle;
-import android.support.annotation.NonNull;
-import android.support.design.widget.BottomNavigationView;
-import android.support.v7.app.AppCompatActivity;
-import android.view.MenuItem;
-import android.widget.TextView;
-
-public class MainActivity extends AppCompatActivity {
-
-    private TextView mTextMessage;
-
-    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
-            = new BottomNavigationView.OnNavigationItemSelectedListener() {
-
-        @Override
-        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
-            switch (item.getItemId()) {
-                case R.id.navigation_home:
-                    mTextMessage.setText(R.string.title_home);
-                    return true;
-                case R.id.navigation_dashboard:
-                    mTextMessage.setText(R.string.title_dashboard);
-                    return true;
-                case R.id.navigation_notifications:
-                    mTextMessage.setText(R.string.title_notifications);
-                    return true;
-            }
-            return false;
-        }
-
-    };
-
-    @Override
-    protected void onCreate(Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-        setContentView(R.layout.activity_main);
-
-        mTextMessage = (TextView) findViewById(R.id.message);
-        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
-        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
-    }
-
-}

app/src/main/java/me/vadik/knigopis/MainActivity.kt

@@ -0,0 +1,26 @@
+package me.vadik.knigopis
+
+import android.os.Bundle
+import android.support.design.widget.BottomNavigationView
+import android.support.v7.app.AppCompatActivity
+import android.view.MenuItem
+
+class MainActivity : AppCompatActivity() {
+
+  private val onNavigationItemSelectedListener = { item: MenuItem ->
+    supportActionBar!!.setTitle(when (item.itemId) {
+      R.id.navigation_home -> R.string.title_home
+      R.id.navigation_dashboard -> R.string.title_dashboard
+      R.id.navigation_notifications -> R.string.title_notifications
+      else -> throw UnsupportedOperationException()
+    })
+    true
+  }
+
+  override fun onCreate(savedInstanceState: Bundle?) {
+    super.onCreate(savedInstanceState)
+    setContentView(R.layout.activity_main)
+    val navigation = findViewById(R.id.navigation) as BottomNavigationView
+    navigation.setOnNavigationItemSelectedListener(onNavigationItemSelectedListener)
+  }
+}

build.gradle

@@ -1,15 +1,20 @@
 buildscript {
+    ext {
+        kotlin_version = '1.1.4-3'
+    }
     repositories {
         jcenter()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:2.3.3'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
 
 allprojects {
     repositories {
         jcenter()
+        mavenCentral()
     }
 }