We need to use the AlertDialog package to create the Alert in Android

the following program demonstartes how we can create a alert dialog box

import android.content.DialogInterface; 
import android.support.v7.app.AlertDialog; 
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); 
 } 
 // Declare the onBackPressed method 
 // when the back button is pressed 
 // this method will call 
 @Override
 public void onBackPressed() 
 { 
 // Create the object of 
 // AlertDialog Builder class 
 AlertDialog.Builder builder 
 = new AlertDialog 
 .Builder(MainActivity.this); 
 // Set the message show for the Alert time 
        builder.setMessage("Do you want to exit ?"); 
 // Set Alert Title 
        builder.setTitle("Alert !"); 
 // Set Cancelable false 
 // for when the user clicks on the outside 
 // the Dialog Box then it will remain show 
        builder.setCancelable(false); 
 // Set the positive button with yes name 
 // OnClickListener method is use of 
 // DialogInterface interface. 
        builder 
 .setPositiveButton( 
 "Yes", 
 new DialogInterface 
 .OnClickListener() { 
 @Override
 public void onClick(DialogInterface dialog, 
 int which) 
 { 
 // When the user click yes button 
 // then app will close 
                            finish(); 
 } 
 }); 
 // Set the Negative button with No name 
 // OnClickListener method is use 
 // of DialogInterface interface. 
        builder 
 .setNegativeButton( 
 "No", 
 new DialogInterface 
 .OnClickListener() { 
 @Override
 public void onClick(DialogInterface dialog, 
 int which) 
 { 
 // If user click no 
 // then dialog box is canceled. 
                            dialog.cancel(); 
 } 
 }); 
 // Create the Alert dialog 
 AlertDialog alertDialog = builder.create(); 
 // Show the Alert Dialog box 
        alertDialog.show(); 
 } 
}

the above program will generte the an alert when we try to exit the app