Develop Reference

Android App keeps on crashing upon new Intent

So I've got this error code: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
The line: Intent callMode = new Intent(MainActivity.this, IncallActivity.class);
What I'm trying to do is to set a new activity when I receive a call. The following function is what should "switch" to that activity.
public void callMode() {
Intent callMode = new Intent(MainActivity.this, IncallActivity.class);
startActivity(callMode);
}
And it is located inside of the MainActivity class, outside of the onCreate function.
When I try to put it inside of the onCreate function (without public void callMode() of course) it works, but that's not the goal.
I'm trying to activate this new activity from another class, which listens for calls.
public class CallReceiver extends BroadcastReceiver {
IncallActivity incallActivity = new IncallActivity();
public MainActivity mainActivity;
#Override
public void onReceive(Context context, Intent intent) {
if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(TelephonyManager.EXTRA_STATE_OFFHOOK)) {
mainActivity.callMode();
} else if (intent.getStringExtra(TelephonyManager.EXTRA_STATE).equals(TelephonyManager.EXTRA_STATE_IDLE)) {
incallActivity.endCallMode();
}
}
}
So why does this error show up?

I found a way to do it.
You need to add a public static Context context to your code, and then in onCreate, do context = getBaseContext();
From there you can launch the second activity from any class you'd like.

Android - StartActivityForResult on finish() method freezes UI

I have the following situation:
Activity A starts Activity B, by using startActivityForResult
Activity B then returns an ArrayList of Strings to Activity A by using on finish().
Here is a code example of what exactly Activity B does:
ArrayList<String> urls = new ArrayList<>();
urls.add("Some string");
Intent intent = new Intent();
intent.putStringArrayListExtra(KEY, urls);
setResult(Activity.RESULT_OK, intent);
finish();
Then Activity A receive the data in onActivityResult(...)
The issue I have is that when the user taps the done button and Activity B's code example executes, Activity B freezes for about 3 seconds (when I have about 2 strings in the ArrayList). The more strings I have in the ArrayList the longer it freezes. I have more or less determined that it is finish() that causes the UI thread to freeze.
Is there a way to call finish() without freezing Activity B? If not, why is this happening?
EDIT:
Here is the full example:
/**
* Background task
*/
private class gatherUrlsTask extends AsyncTask<ArrayList<PictureEntry>, Integer, Intent> {
#Override
protected void onPreExecute() {
progressBar.setVisibility(View.VISIBLE);
bt_done.setVisibility(View.GONE);
fab_add_picture.setVisibility(View.GONE);
}
#SafeVarargs
#Override
protected final Intent doInBackground(ArrayList<PictureEntry>... params) {
ArrayList<String> imagePaths = new ArrayList<>();
for (PictureEntry pictureEntry : params[0]) {
if (pictureEntry.isSelected()) {
imagePaths.add(pictureEntry.getPath());
}
}
if (imagePaths.size() == 0) {
Toast.makeText(getApplicationContext(), R.string.please_select_atleast_one_image, Toast.LENGTH_LONG).show();
return null;
} else {
Intent intent = new Intent();
intent.putStringArrayListExtra(SELECTED_IMAGES_KEY, imagePaths);
return intent;
}
}
#Override
protected void onPostExecute(Intent intent) {
setResult(Activity.RESULT_OK, intent);
finish();
}
}
However I can remove everything from the AsyncTask since it did not have any effect on performance.

i don't know what is cause of that, but for prevent ui getting freezed, use asyncTask and then in the onPostExcecute call finish()

How to open browser in android app?

i'm doing a project where i have to call a browser via my android app, but when i call it, the app stops.
the code can be found here: https://github.com/coppetti/android-pulsometer
but for fast view, i have a "Pulsometro" class where
public void onPreviewFrame(byte[] data, Camera cam) {
...
Browser browser = new Browser();
browser.callBrowser(beats);
return;
...
}
and a Browser class where:
public class Browser extends Activity{
public void callBrowser(int beats){
String url = "http://www.higia.info/?q="+beats;
Intent i = new Intent(Intent.ACTION_VIEW);
i.setData(Uri.parse(url));
startActivity(i);
return;
}
}
There's a way to call a browser and my app doesn't breaks?

Try this:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void onOpenWebBrowser(View v)
{
Intent webPageIntent = new Intent(Intent.ACTION_VIEW);
webPageIntent.setData(Uri.parse("https://www.google.co.in/"));
try {
startActivity(webPageIntent);
} catch (ActivityNotFoundException ex) {
}
}

Do not just randomly choose superclasses. Do not just create some subclass of Activity and expect it to work.
Move your callBrowser() method into some real Activity implementation, and get rid of Browser entirely.
Or, remove the superclass from Browser, have callBrowser() take a Context as a parameter, and call startActivity() on that Context.

Try this class. Call the callBrowser method and give it and Activity for the context parameter.
public class Browser{
public void callBrowser(Context context, int beats){
String url = "http://www.higia.info/?q="+beats;
Intent i = new Intent(Intent.ACTION_VIEW);
i.setData(Uri.parse(url));
context.startActivity(i);
}
}

Calling another activity from AsyncTask

I'm new to android and I have a problem with this code. I'm trying to get a JSON String and start another activity to display it as a ListView.
I'm not able to start the activity. It says that the The constructor Intent(RequestJsonString, Class) is undefined and The constructor Intent(RequestJsonString, Class) is undefined .
Here:
Intent intent = new Intent(RequestJsonString.this,DisplayResults.class);
and Here:
RequestJsonString.this.startActivity(intent);
I have read many posts on this on stackoverflow and tried with activity, context and this. But still I'm not getting it right. I think I should be missing something. Any help is appreciated.
public class RequestJsonString extends AsyncTask<String, Void, JSONObject> {
#Override
protected JSONObject doInBackground(String... urls) {
// Code HTTP Get Request and get JSONObject
return jsonObject;
}
protected void onPostExecute(JSONObject jsonObj){
try {
Intent intent = new Intent(RequestJsonString.this,DisplayResults.class);
intent.putExtra("JSON_Object", jsonObj.toString());
RequestJsonString.this.startActivity(intent);
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Log.v("Json_OutPut","Done");
}
}

To start the activity from AsyncTask.
Intent intent = new Intent(YourActivityName.this,DisplayResults.class);
or you can do same like below.
Declare the context instance variable and initialize it in onCreate method.
private Context context;
public void onCreate(Bundle bundle) {
............
context = this;
........
}
Start the activity like this.
Intent intent = new Intent(context,DisplayResults.class);
intent.putExtra("JSON_Object", jsonObj.toString());
startActivity(intent);

In your case you are referring to asynctask class context
Intent intent = new Intent(RequestJsonString.this,DisplayResults.class);
Use a Activity Context
Intent intent = new Intent(ActivityName.this,DisplayResults.class);
Check the link to know when to use getApplicationContext() and when to use Activity Context
When to call activity context OR application context?
Edit:
Pass the Activity context to the asynctask constructor
new RequestJsonString(ActivityName.this).execute(params..);
In your asynctask constructor
Context c;
public RequestJsonString( Context context)
{
c= context;
}
Then
Intent intent = new Intent(c,DisplayResults.class);
startActivity(intent);

NullPointerException when using getInt(String)

I'm android beginner so please be easy on me. I'm doing some "exercises" and i'm writing simple app which will tell RSSI strength of home wifi network. Getting that number is pretty easy, but updating it and showing that on screen it's a little more complicated as i thought.
First this is my onCreate Activity. In this activity i'm launching another android component - Service. Because the code will run in background (i know i could use thread or something else, but this is for "practice" sake, and i have a few ideas what to do with this app, while running service and not interacting with UI )
public class MainActivity extends Activity {
TextView wifi_check;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
referenceViews();
startService(new Intent(this, CheckingWifiService.class));
//wifi_check.setText(""+getIntent().getExtras().getInt("RSSI"));
}
private void referenceViews() {
wifi_check = (TextView) findViewById(R.id.wifiCheck_TV);
}
}
Because my code will run every second or so, i will use TimerTask for this purpose. And here is my TimerTask class, which includes run() method, and code for executing inside
public class TimerTsk extends TimerTask {
Context act;
WifiManager wifiMan;
WifiInfo info;
Bundle sendInfo;
Intent intent;
int rssi;
public TimerTsk(Context context) {
act = context;
}
#Override
public void run() {
intent = new Intent();
sendInfo = new Bundle();
wifiMan = (WifiManager) act.getSystemService(Activity.WIFI_SERVICE);
info = wifiMan.getConnectionInfo();
rssi = info.getRssi();
Log.d("WORKED", "RUNNING SUCESSFULLY");
// i want to send info to my activity
sendInfo.putInt("RSSI", rssi);
intent.putExtras(sendInfo);
}
}
From this class , i want to send result of RSSI to my activity and then update a text. But when i call this code below, on activity i always get NullPointerException.
wifi_check.setText(""+getIntent().getExtras().getInt("RSSI"));
To be honest i had hard time figuring out which part of code is throwing an exepction. And i found that more exactly, this part of code is throwing an exepction.
getInt("RSSI")
Overall i see that service is running, because in my LOGCAT i see a message that i create with Log.d in TimerTsk class.
Any ideas why is this happening?
Here is my service class:
public class CheckingWifiService extends Service{
int rssi;
Timer time;
TimerTsk ttsk;
#Override
public IBinder onBind(Intent intent) {
return null;
}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
time = new Timer();
time.schedule(new TimerTsk(getApplicationContext()), 500);
return START_STICKY;
}
}
Here is my LogCat:

I see a common mistake. Don't do this:
sendInfo.putInt("RSSI", rssi);
intent.putExtras(sendInfo); // This adds a Bundle to your existing Bundle!
You are creating an Intent, with a Bundle of extras, with a Bundle that holds rssi. Leave out this unnecessary Bundle:
intent.putExtras("RSSI", rssi);
Now in your next Activity you can use:
getIntent().getIntExtra("RSSI", 0);
However you should always check to make sure there aren't any surprise null variables:
Intent in = getIntent();
if(in != null) {
int rssi = in.getIntExtra("RSSI", -1);
if(rssi < 0)
wifi_check.setText(""+rssi);
else
wifi_check.setText("Unknown");
}

is your activity starting? I don't see any call to startActivity(). In any case as mentioned by Sam you just need to call putExtra for your intent. don't forget to call
is your activity starting? I don't see any call to startActivity(). In any case as mentioned by Sam you just need to call putExtra for your intent. don't forget to call
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
you need to put this flag when start activies from background