Tuesday, 2 December 2014

NFC Mifare UID (Card Serial Number) reverse issue

If you are using NFC in your application, you should be careful while using UID of NFC card.

When any card is manufactured, it will be assigned a card serial number and it must be there if there is no any data.

When you try to fetch that UID using Android SDK NFC tag technology, it will return you UID in reverse order.

Generally when card ia manufactured, bytes are stored in LSB (Least Significant Byte) order while when Android read card serial number, it will read in MSB(Most Significant Byte) fashion.

For example. Any card has UID "12345678" then in MSB fashion, it will be like "78563412". 
So you need to convert this order from MSB to LSB.

Following short of code will help you to do this.

 String cardID = getHax(tag.getId());  
 String cardIdArr[] = cardId.split(" ");  
 String reverseCardId = "";  
 for (int i=0;i < cardIdArr.length;i++)  
   reverseCardId += cardIdArr[i];  
 Log.d("reverse"," card id = " + reverseCardId );  

Thursday, 27 November 2014

Android Fragment Back Stack handling


Fragment Back Stack manager while displaying fragments on single activity and need to maintain on back press


We know that there is activity stack in Android. We don't need to maintain the stack while opening or closing activity. It will automatically handle the stack and show you the top of activity when you pressed back button.
But in fragment, its neccessary to handle them. Because Android is not going to handle them. We need to create a stack of fragment and manage them while pressing back button.
So, I have created one demo to represent how to handle the fragment in Back Stack.


In the sample application, you will find one object named fragmentStack. Its a Stack which will push and pop the fragment as per requirement.
Whenever you are displaying any new fragment, just push that fragment into stack using following code.

 //here this fragment is our first fragment  
 homeListFragment = new HomeListFragment();  


And when you are displaying any other fragment over this fragment, use following code.
We will create a new object of second fragment and add it to stack.

 //here this fragment is second fragment  
 resultListFragment = new ResultListFragment();  
 //hide the last fragment  
 //push the new fragment into stack  

Second Fragment.png

When backPressed event fires, we will check whether stack size is 2 or not. If it is, then we will pop last fragment and display the previous fragment by following code.

 if (fragmentStack.size() == 2) {  
   FragmentTransaction ft = fragmentManager.beginTransaction();  
 } else {  
   //if size is `1` it means first fragment is visible and we can exit from application  

Click here to download full source code

Sunday, 14 September 2014

Exit from Activity with twice "Back" button click

Description: When user presses two times "back" button, he is allow to exit from activity

Complexity Level: Beginner

Keywords: back, back button, android, android studio, alert, onbackpress, twice

      When we are doing some important operation or work on activity and it may possible that by mistake we pressed "back" button. This will simply stop all your important work and you are redirect to previous activity.

     To overcome this situation, we can provide a confirmation to the user that you need to press once again to exit from activity. This will give user a great experience in application.

To add this functionality in you application, use following code snippet.

 public void onBackPressed() {  
   if (doubleBackToExitPressedOnce) {  
   this.doubleBackToExitPressedOnce = true;  
   Toast.makeText(this, "Press once again to exit", Toast.LENGTH_SHORT).show();  
   new Handler().postDelayed(new Runnable() {  
     public void run() {  
   }, 2000);  


doubleBackToExitPressedOnce is core of this snippet. When user presses "back" button, value for `doubleBackToExitPressedOnce` is "false" and he will get a message for "Press once again to exit".

If user presses "back" button one more time, he will redirect to previous activity as "doubleBackToExitPressedOnce" is now "true".

Monday, 1 July 2013

Display Alert on Back button Pressed in Android Studio

Description: When user wants to exit from application, he is prompted to exit from application.
Complexity Level: Beginner
Keywords: back, back button, android, android studio, alert, dialog, onbackpress

                Alert means to get user’s input when needed. Most of us played games and when we try to exit it by pressing “Back” button in Android phone, it ask for “Do you really want to exit?”.             So, in this article we are going to learn how to prevent user to exit from application without giving response.

Step 1:
Create a new Project with following parameters.

Step 2:
Open your Main Activity file, in my case it is “BackPressActivity” and paste following code inside.
package com.example.alertonbackpressdemo;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class BackPressActivity extends Activity {
    protected void onCreate(Bundle savedInstanceState) {

    public void onBackPressed() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setMessage("Do you want to Exit?");
        builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
                //if user pressed "yes", then he is allowed to exit from application
        builder.setNegativeButton("No",new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
                //if user select "No", just cancel this dialog and continue with app
        AlertDialog alert=builder.create();

Let’s understand Alert Dialog and its different methods.
In code there is written “builder.setCancelable(false);” which means that user can’t cancel this dialog by pressing back again or touch outside the alert dialog box and dismiss it. So user has to press one of the options you have provided to him.
Next is “builder.setMessage("Do you want to Exit?");”. Here you can write your own message which will be displayed to user in alert.
builder.setPositiveButton” will set a positive button in left side. Parameter will accept the name of that button as you can see in code is “Yes”. Same thing is set for “builder.setNegativeButton”. When you set these buttons, you need to pass a listener which will be fired when user click one of the button.
Step 3:
Run your application.

                In this article, we learned “How to show Alert Dialog” and “Show dialog at BACK button pressed”.