How to show alert dialog in Android?

This example demonstrates how do I show alert dialog in android.

Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.

Step 2 − Add the following code to res/layout/activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Press The Back Button."
      android:textStyle="bold"
      android:textSize="24sp"
      android:layout_centerInParent="true" />
</RelativeLayout>

Step 3 − Add the following code to src/MainActivity.java

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
public class MainActivity extends AppCompatActivity {
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
   }
   @Override
   public void onBackPressed() {
      AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
      builder.setMessage("Do you want to exit?");
      builder.setTitle("Alert!");
      builder.setCancelable(false);
      builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
         @Override
         public void onClick(DialogInterface dialog, int which) {
            finish();
         }
      });
      builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
         @Override
         public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
         }
      });
      AlertDialog alertDialog = builder.create();
      alertDialog.show();
   }
}

Step 4 - Add the following code to androidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="app.com.sample">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>

Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen –

Click here to download the project code.