Develop Reference

Why do I get a NullPointerException here?

I have my code defined the way below. There are two crucial activities. Activity (1) shows some images in a ViewFlipper. It uses methods to load desired image directly. The onOptionsItemSelected() method fetches data from a menu defined within linked XML layout R.layout.browse. The other method, displaySelectedFlag(), gets a tag parameter passed from a different activity, let's call it activity (2).
Activity (1):
public class BrowserActivity extends AppCompatActivity implements SimpleGestureListener, View.OnClickListener {
public ViewFlipper vFlipper;
(...)
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.browse);
vFlipper = (ViewFlipper) findViewById(R.id.viewFlipperBrowser);
(...)
} // onCreate() ends here
// this method below works fine:
#Override
public boolean onOptionsItemSelected(MenuItem item) {
vFlipper.setDisplayedChild(item.getOrder());
return true;
}
// and this one doesn't:
public void displaySelectedFlag(int orderTag) {
vFlipper.setDisplayedChild(orderTag); // crashes here
}
}
Activity (2):
public class ListActivity extends Activity implements View.OnClickListener {
private BrowserActivity browserActivity = new BrowserActivity();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list);
ImageButton imageA = (ImageButton) findViewById(R.id.img_a);
imageA.setOnClickListener(this);
ImageButton imageB = (ImageButton) findViewById(R.id.img_b);
imageB.setOnClickListener(this);
}
public void displayImageInfo(View view) {
String tagValue = (String) view.getTag();
int tagId = Integer.parseInt(tagValue);
Intent intent = new Intent(this, BrowserActivity.class);
startActivity(intent);
browserActivity.displaySelectedImage(imageId);
}
#Override
public void onClick(View view) {
displayImageInfo(view);
}
}
As I checked, the method onClick() called in activity (2) fetches an ID of an ImageButton and passes it to activity (1). Unfortunately, I get a NullPointerException when calling the ViewFlipper (the line is marked in the code above, activity (1)).
Any idea why it happens?

You cannot reference one Activity from another activity. You must let the Android OS create the Activity object via the call to "startActivity". Allocating a local variable as an instance of an Activity doesn't actually mean anything (like your instantiation of the BrowserActivity). Apoorv's comment links to a decent article on the subject.
If you want to pass data from one Activity to another, you need to pass extras within the Intent's bundle. This post goes into detail: https://stackoverflow.com/a/819427/504252

Android Activity not Retaining the Variables after being called by another Activity

I am developing a Quizz App in which an activity shows question and options from SQLite and on selecting option, another activity is showing result for 2000 ms(it has a timer)and then it calls First Activity via an Intent.
So, Most of the interaction is between 2 activities. But each time my MainActivity is called, it re-initializes all the variables again and again.
I am opening my database connection in onCreate() and also keeping a counter (that can count how many questions have been asked yet) whose value is not retained after the intent from Second Activity. I am worried on how to solve this.
I am a bit confused about the life cycle that is followed. Whether the call to First Activity from Second one starting with onCreate() or it's also initializing the instance variables again.
This is onCreate() method I wrote:
public class MainActivity extends Activity {
protected static final int SCORE_INCREMENT = 5;
TextView question;
Button score, opt1, opt2, opt3;
MyDatabaseManager dbManager;
QuizManager quizManager;
private int quiz_counter =1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
dbManager = new MyDatabaseManager(getApplicationContext());
dbManager.open();
quizManager = new QuizManager(MainActivity.this, dbManager);
Toast.makeText(MainActivity.this, "Asking The First Question", 0).show();
askQuestion();
}
}
Is there any difference between the above written code and the one I am writing now... if the activity is called again via an Intent
public class MainActivity extends Activity {
protected static final int SCORE_INCREMENT = 5;
TextView question;
Button score, opt1, opt2, opt3;
MyDatabaseManager dbManager = new MyDatabaseManager(getApplicationContext());
QuizManager quizManager = new QuizManager(this, dbManager);
private int quiz_counter =1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toast.makeText(MainActivity.this, "Asking The First Question", 0).show();
askQuestion();
}
}
This might be a silly question. But it's a bit confusing for me. Suggestions are welcome.

If you have variables that you want to maintain between changing activities, then you should either
Store them in SharedPreferences
or
Pass them between the Activites in the Intents (see Starting another activity)

Clearing TextView in Android

I'm facing this problem with TextView. It is not erasing the previous instances of data.
When I'm running my application in emulator it displays output data in TextView. That's fine. But when I'm clicking back button in my emulator and re opening the application it does not clear the previous data. Instead it appends the data to already existing data.
Any help is appreciated.
My code is as below:
public class MainActivity extends ActionBarActivity
{
private TextView mTextView;
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView) findViewById(R.id.text_test);
new Thread(new TestLocalHost()).start();
}
private class TestLocalHost implements Runnable
{
#Override
public void run()
{
final String s = JSONParser.doGet("http://192.168.0.107:15071/GetResult.ashx?op=getInfo",null);
runOnUiThread(new Runnable()
{
#Override
public void run()
{
mTextView.setText(s);
}
});
}
}
}

It's happening because your app still lives in the emulator memory, you need to force kill it if you want your app to run again from scratch.
You should click on the "running processes" button and swipe left/right the app process.
In the scenario that you described the activity wasn't destroyed yet, and when you reopen it only its onResume() method is being called.

Android application closes on click

I do my android app , but I have a bug I don't know how to fix it.
My code is below :
This is my Main Activity :
public class MainActivity extends Activity {
private ImageView imgHot;
public final static String EXTRA_MESSAGE="com.cuonglm.KhoHinh.MESSAGE";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
imgHot=(ImageView)findViewById(R.id.imageViewHot);
imgHot.setOnClickListener(toContentHot);
}
View.OnClickListener toContentHot=new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
Intent content=new Intent(MainActivity.this,ContentActivity.class);
String signal="1";
content.putExtra(EXTRA_MESSAGE,signal);
startActivity(content);
}
};
And this is my second Activity :
public class ContentActivity extends Activity {
private TextView viewMessage;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
setContentView(R.layout.activity_content);
}
I want to click on the image on the Main Activity , string "1" or number "1" will send to the Second Activity via Intent and view on the TextView.
But my app will be close "Unfortunately..."
Thanks

Change to
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
In your ContentActivity
findViewById looks for a view with the id for the current infalted layout. SO you need to set the content of your layout to the activity first and then initialize your views.
You are probably gettting NullPointerException coz your initialization fails.

You need to call setContentView() in your second Activity before trying to access any of the Views in that layout. Change it to
public class ContentActivity extends Activity {
private TextView viewMessage;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
}
If this doesn't fix your problem then please post your logcat so we can see the error. Also always post logcat in the future when your app crashes. They aren't always this easy to see.
Also, I'm not sure you understand how putExtra() works. It is a key, value pair so when you put EXTRA_MESSAGE as the key then that is what you would use to retrieve the value added in the second param. So the way you are doing it may work if the Activity gets destroyed but it looks really strange to me and probably not realy safe or efficient. I would change it to something like
content.putExtra("message",signal);
in your first Activity then get it with
String messageReceive = content.getStringExtra("message");
in your second Activity

You need to set the layout before trying to reference the Views associated with it.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
setContentView(R.layout.activity_content);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
}

Is it possible to access an object of an activity fom other place?

This maybe a stupid idea, but does anyone know is it possible to access one activity's object form other places?
To be specific, lets say if you have an activity A (with a textView t) and you create a normal java class B.
At onCreate, you start to run B for some calculation like below,
public class MyActivity extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
t = (TextView)findViewById(R.id.outputtext);
Somejava B = new Somejava();
B.run();
}
}
Is there a way for B to update the textView?
I know the simple way (maybe the correct way) is to return the result from B class and use t.setText(result) in MyActivity, but I'm just want to know is it possible to update the textview in B?

Use Intent or public static variables

can simply pass activity refernce to b in constructor and create the method in your acitivty to update textview. if you using another thread not forgot to use handler or other ways to update UI thread.

Yes, it is possible if the Activity's field is public and post the UI changes in a public Handler created on the first Activity but in facts, it's really ugly to do that...
You can use startActivityForResult(...) to notify an other activity how the process has passed with some serialiezable data in the Bundle extras of the Intent and catch the result in the overrided method onActivityResult(...)...

For a "normal java class" B I would work with interfaces
public interface SomejavaListener{
void onSomejavaFinish(Object result);
}
public class MyActivity implements SomejaveFinish extends Activity {
TextView t;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
t = (TextView)findViewById(R.id.outputtext);
Somejava B = new Somejava();
B.run(MyActivity.this); //notice the extra argument!
}
public void onSomejavaFinish(Object result){
t.setText("updated! ^,^");
}
}
public class Somejava {
//...
public void run(SomejavaListener callback){
//working working
callback.onSomejavaFinish( new Object() );
}
}
However in respect to the android environment the question is sitting in I got the feeling maybe an AsyncTask would be the right thing for you. It has an doInBackground method to do work and not spoiling your UI Thread (resulting in ANR Errors.)
Another advantage is the onPreExecute and onPostExecute methods are running in the UI Thread itself again, so it just takes a blink to update your TextView
public class MyActivity extends Activity {
TextView t;
private class MyAsyncTask extends AsyncTask<Void, Void, Void> {
protected Void doInBackground(Void... void) {
//do your stuff
return null
}
protected void onPostExecute(Void void) {
MyActivity.this.t.setText("updated ^^v");
}
}
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
t = (TextView) findViewById(R.id.outputtext);
MyAsyncTask myAsyncTask = new MyAsyncTask();
myAsyncTask.execute();
}
}