Develop Reference

Why does getPackageManager return null

So I am trying to start another app my inside my AccessibilityService but I keep getting the following java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference even though I use the same method in my MainActivity.class and it works fine. And I know the package name is correct, cause like I said I call it in MainActivity.class
This is the class where I call the function.
public class myAdapter extends Application
{
private void turnOn(String text)
{
Intent b = getPackageManager().getLaunchIntentForPackage(text);
startActivity(b);
}
}
I have tried the various ways to call this same function.
Intent b = getApplicationContext().getgetPackageManager().getLaunchIntentForPackage(appName);
getApplicationContext().startActivity(b);
Intent b = getApplicationContext().getgetPackageManager().getLaunchIntentForPackage(appName);
getApplicationContext().startActivity(b);
My Logcat:
Process: com.tech.myApp, PID: 17195
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
at com.tech.myApp.turnOn(myAdapter.java:198)

You say that you're trying to start an Activity from an AccessibilityService, yet your code very clearly shows that your class is extending Application. This is confusing and also likely one of the core issues you're having regarding these NullPointerExceptions. I am going to state my answer assuming that you are indeed attempting this from an AccessibilityService class, as mentioned in the description of your issue, even though this very much conflicts with the code you have posted. If this is indeed the case, you should update this question to clarify. If this answer does not help you, your question needs considerable TLC.
Once we realize that we're within an AccessibilityService, we see that there is a MUCH simpler way to accomplish starting an activity. Starting another Activity within an AccessibilityService is as simple as the following code snippet:
public class A11yService extends AccessibilityService {
void startMainActivityFromService() {
final Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(intent);
}
}
Easy as that!

Okay so what I had to do was on my MainActivity.class was create a static context. Which then I could pull from any other class even the ones that run in the background.
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private static Context context;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MainActivity.context = getApplicationContext();
// CALL MY FUCTION - THIS COULD BE ANYWHERE NOW
startApp(app name you want to start);
}
}
Then in my other classes or activities or services I use this:
public static void startApp(String appName)
{
// MAKE SURE TO GET THE CONTEXT
Context context = MainActivity.getAppContext();
Intent b = context.getPackageManager().getLaunchIntentForPackage(appName);
context.startActivity(b);
}

Finish Activity() from a separate myJavaClass.java

I have tried almost all the solutions from SO but no success :(.
I have a simple myJavaClass.java with a couple of functions.
One of the functions in myJavaClass : startActivity() starts MyCustomActivity
public startActivity(Context context)
{
Intent intent = new Intent(context, MyCustomActivity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK |Intent.FLAG_ACTIVITY_SINGLE_TOP);
context.startActivity(intent);
}
This launches MyCustomActivity() as expected.
Now I have another function in myJavaClass.java to close/finish MyCustomActivity but it is not able to do so!
I have tried
Making MyCustomActivity SingleTop in manifest and creating the activity via an intent as above
Passing an activity instance to "this" in onCreate() of MyCustomActivity and calling MyCustomActivity.activity.finish() from myJava.class but that doesnt work as well
Please help me. I have been stuck here for hours now. I know the solution is very simple and conceptual but I am a newbie. Just building Java/Android concepts!
EDIT
MyCustomActivity
public Activity activity;
OnCreate()
{
...
this = activity;
}
MyJavaClass
public closeActivity(Context context)
{
Activity customActivity = MyCustomActivity.activity;
customActivity.finish();
}

I think that what you are trying to do is fundamentally bad. For a start, outside of the Activity code, there are no guarantees that the activity still exists - the memory manager may have cleaned it up, the user may have pressed Back etc. Think of Activities as independent entities - you can start them, and you can optionally get a result back when they finish what they're doing, but that's it.
Think about whether you really have to programmatically close the activity from outside it - I'd say this is an unusual design, but there are circumstances where it may be appropriate.
If so, what I think you want is a publish/subscribe system whereby MyCustomActivity can register a listener with MyJavaClass, and then receive a callback whereupon it can 'finish' itself.
public Activity activity implements FinishListener
{
public void onCreate(...)
{
//where does MyJavaClass come from? see in a minute
MyJavaClass myjava = getMyJavaclass();
myJava.addFinishListener( this );
}
public void onFinishCallback()
{
this.finish();
}
}
and
public class MyJavaClass
{
private List<FinishListener> finishListeners = ...;
public void addFinishListener( FinishListener fl )
{
this.finishListeners.add(fl);
}
public closeActivity(Context context)
{
for ( FinishListener fl : finishListeners )
{
fl.onFinishCallback();
}
}
}
and
public interface FinishListener
{
void onFinishCallback();
}
Now the only remaining issue is how to get MyJavaClass from the Activity. That's up to you - you may already know how, you may be able to put it in your Application implementation, it could be a singleton (bad), the listeners could be static (bad) or various other options.
Oh, and don't forget to remove the listener again in the Activity's onDestroy() method!

Just try this....
public closeActivity(Activity _activity)
{
_activity.finish();
}

you can't finish activity from other class until you have the reference of instance of Activity in that class, give the reference in that class and call finish() method to stop the activity.
activity.finish();

Starting an Android Activity from an Listener defined in an external class file

I have an Activity MyActivity with a Button MyButton.
I want to attach a MySpecialOnClickListener to MyButton.
I write MySpecialOnClickListener in an external class file.
public class MySpecialOnClickListener extends ButtonHandler implements OnClickListener {
public OnClickListenerWithSpeech (Context context)
{ super.context = context; }
#Override
public void onClick(View view) { handleClick(view); }
}
and ButtonHandler looks like this
public abstract class ButtonHandler {
protected Context context;
protected void handleClick (View view){
if (view.getid()==R.id.button_B) {
context.startActivity (new Intent(context, ActivityC.class));
}
}
}
I basically want to store all logic for Buttons in the ButtonHandler.
SO...as I said, I have the MySpecialOnClickListener defined in an external class file.
When I click MyButton I get the following fatal error.
Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?
So I can't start an activity normally from within a non-Activity. Fair enough.
However, if I change MySpecialOnClickListener to be an inner class in 'MyActivity' it works fine. Remember 'ButtonHandler' is still an external class file. So it (where ActivityC is ultimately started from) doesn't change.
My question (finally) is: can someone explain the logic of why one is allowed and the other isn't. I presume its a scoping thing or something but I'm a bit confused. It seems the code to start the process of starting an activity has to literally be inside another Activity.
EDIT - PROBLEM SOLVED
See below. The location of the class is irrelevant. I just didn't pass in the context properly.

because the ButtonHandler 'context' field isn´t associated with any activity context. So, when you attach the MySpecialOnClickListener instance to a button you create it passing the context parametener, isn´t???
something like this:
MySpecialOnClickListener listener = new MySpecialOnClickListener(MyActivity.this);
aButton.setOnClickListener( listener );
in this way you´re constructing the Button with the correct context...

It's likely that you are not passing the Activity context to MySpecialOnClickListener. Could you show me the difference in the way you invoke the inner-class approach?

Apologies to those of you who tried to answer. It was my fault (and I didn't include the following info initially for people)
When I was passing in the context to the 'MySpecialOnClickListener' I would do:
view.setOnClickListener(new MySpecialOnClickListener(getApplicationContext()));
when I should have done:
view.setOnClickListener(new MySpecialOnClickListener(this));
So getApplicationContext() doesn't seem to get the "correct" context for the app.
Which leads me to my next question as to what getApplicationContext() actually returns :)

New Activity nullpointerexception

I have a beginners problem. Here is my situation:
I want to start a new activity from the main activity. The code to launch the new activity is found in a separate class file. I seem to be passing the wrong arguments and I am ending up in a nullpointerexception when trying to launch the new activity. The new activity launches fine when I place the code in the main activity class file, therefore the second activity and the manifest are fine. Here is a sample of my code:
In my main activity class where I instanciate the second class (THIS IS MY MAIN ACTIVITY. I OMITTED THE REST BECAUSE I DO NOT THINK IT IS RELATED TO THE PROBLEM):
Tester mytest = new Tester();
mytest.test(this);
In my second class file (THIS IS NOT AN ACTIVITY; IT IS A CLASS THAT IS INSTANTIATED IN THE ACTIVITY):
public class Tester extends Activity {
Intent myIntent;
public void test (Context context) {
myIntent = new Intent (Intent.ACTION_VIEW);
myIntent.setClass(context, newActivity.class);
thebutton.setOnClickListener(
new OnClickListener() {
public void onClick(View v) {
startActivity(myIntent);
}
}
):}
When I perform the click I receive a nullpointerexception at startactivity. Can anyone enlighten me on this please?I am sure that I am wrongly using the context.

Activities are started with Intents. Please read the Android Application Fundamentals first and try the Hello World app :)
I understood that you will use your separate Tester class at all cost ;) so I'm trying to adapt and help you out there.
First of all, don't let your class inherit from Activity. This won't help you, cause this calls will probably not have any valid context. Activity somehow implements the template pattern, providing you key method like onCreate(...), onPause(...) etc and is instantiated by the Android OS.
If you still want to use the class, you have to pass in the context. Probably you're aiming for some MVC/MVP pattern structure, anyway.
public class Tester {
private Context context;
public Tester(Context context){
this.context = context;
}
public void test () {
final Intent myIntent = new Intent(context, NewActivity.class);
//guess this comes from somewhere, hope through a findViewById method
thebutton.setOnClickListener(
new OnClickListener() {
public void onClick(View v) {
context.startActivity(myIntent);
}
}
)};
}
}
This would be a proposed solution from my side. A problem I still see here is on how you retrieve the button in that test() method. In order to have that work properly you have to retrieve it from some View class (with view.findViewByid(R.id.myButton)) or to create it dynamically and associate it with the view during the onCreate(...) of your Activity (probably using an Inflater).

How to move from one class to other class in android

I want to know how to move from one class to other class in android.
I have one main class i.e Addition.java(Main Activity) and i created other subactivity(Form.java).
I want to how to make my move from one class(ie.Activity)to other.
I tried this,but not working out,just trying to figure out
Intent intent = new Intent();
intent.setClass(this.getParent(),Form.class);
startActivity(intent);
here Form.class is the subactivity, this.getParent I hope it represents main activity. And I created one activity in manifest.xml file and named it as .Form
Am i working right?

The below code works perfectly.
Try it:
#Override
public void onClick(View v)
{
Intent intent = new Intent();
intent.setClass(v.getContext(),Form.class);
startActivity(intent);
}

make sure that activity is declared in Android.manifest.