How to create alerts in android

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