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App crahing when Button declared outside onCreate method [duplicate]

This question already has answers here:
Why does my Android app crash with a NullPointerException when initializing a variable with findViewById(R.id.******) at the beginning of the class?
(9 answers)
Closed 1 year ago.
The following code crashes the app
public class SharedPreferencesActivity extends AppCompatActivity {
Button b1=(Button)findViewById(R.id.buttonSave); //**ERROR**
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_shared_preferences);
}
Whereas the following code works fine
public class SharedPreferencesActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_shared_preferences);
Button b1=(Button)findViewById(R.id.buttonSave);//**REPLACED BUTTON DECLARATION**
}
Why is this happening?

If you want a reference to your button globally then you should declare it outside then initialize it in onCreate
Button b1; //declare the button variable
public class SharedPreferencesActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_shared_preferences);
b1 = (Button) findViewById(R.id.buttonSave); //initialize on creation
}
You should read more into the basics of Java when learning Android

Why must be objects initialized in onCreate?

I have the following code:
public class MainActivity extends Activity {
TextView number=(TextView)findViewById(R.id.number2);
TextView number2=(TextView)findViewById(R.id.number2);
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
number.setText("Text");
number.setText("Text");
}
followed by more code, but when I run it it crashes.
After doing that i tried to initialize TextViews in onCreate()
public class MainActivity extends Activity {
TextView number;
TextView number2;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
number=(TextView)findViewById(R.id.number);
number2=(TextView)findViewById(R.id.number2);
number.setText("Text");
number.setText("Text");
}
and it worked. Why must objects be initialized in onCreate?

Your activity won't have a Window until onCreate(). Attempting to find any views before the window is initialized will lead to NPE.
Additionally, attempting to find views before setContentView() will return nulls and so the return values are not good for anything.

What could possibly be causing this NPE in onCreate?

I am getting an NPE in onCreate of the following file (MySubActivity):
public class MySubActivity extends MySuperActivity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
myTextView.setText(getResources().getString(R.string.myString));
}
}
MySuperActivity:
public class MySuperActivity extends Activity {
protected TextView myTextView;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.my_layout);
myTextView = (TextView)findViewById(R.id.myTextViewid);
}
}
The strange thing is that I have never seen this crash while testing the app. The page works fine when I test it. However I got a crash report from Google notifying me of the crash. I cannot reproduce it, and I have no idea under what scenario this crash could happen. Seeing as how it works for me, the resource ids and string names etc. must be correct.
The only thing that came across my mind was that maybe the user had their phone set to a different language, so it couldn't properly pull the resources. However, there are default resources for all of them, and I tested changing the language of my emulator and it didn't crash. Any ideas?

Set your view in another method, like this:
public class MySubActivity extends MySuperActivity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
private void setView(){
myTextView.setText(getResources().getString(R.string.myString));
}
}
Edit: Don't call setView() in MySuperActivity
public class MySuperActivity extends Activity {
protected TextView myTextView;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.my_layout);
myTextView = (TextView)findViewById(R.id.myTextViewid);
// Or, you can do this in a method called getView() if you like
}
}

How to set the titlebar in every activity programmatically?

Is it possible to set the titlebar in every activity programmatiaclly from one particular activity?
I can set the titlebar of a particular activity programmatically from within that activity, but subsequent activities have the string set from the manifest thereafter.
To set a title I use the following from within the activity. I would like every activity set like this in the application.
setTitle(carerName + " is now logged in");

you can write base activity and call a method of super .
public class DerivedActivity extends BaseActivity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...}
}
public class DerivedActivity2 extends BaseActivity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...}
}
public class BaseActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setTitle(carerName + " is now logged in");
}
}

Want to Run a function without click any button when application start

I have a function named 'func()'. I want to run this function when application start without clicking any button. just when application load I want to show a massage.that massage in that function. I just want to run that function when app start what will be the code.
public class TextViewActivity extends Activity {
public static EditText etxt;
public final void func(){
etxt.setText("Massage");
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
etxt= (EditText) findViewById(R.id.etxt2);
}
}

Just put a call to the function on the onCreate
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
etxt= (EditText) findViewById(R.id.etxt2);
func(); //A call to the function.
}
Hope that helps.

I don't recommend subclassing the Application in order to do this. When the application starts it will go to the main activity. So I would say just keep a SharedPreference boolean value if it has been set. If not show the message.
So keep state of the application here: http://developer.android.com/reference/android/content/SharedPreferences.html, just set a boolean. Remember when you app gets called, the activity onCreate always gets called of the main activity, so its just a matter of not calling it again.

try using below code.. you need to call ur function after you initialize edittext etxt. so it can not cause you NPE
public final void func(){
etxt.setText("Message");
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
etxt= (EditText) findViewById(R.id.etxt2);
func();// here your function call.
}

Try this:
public class TextViewActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
findViewById(R.id.etxt2).setText("SMTH");
}
}
In case you really need a function to be called, you can use this:
public class TextViewActivity extends Activity {
public final void func() {
findViewById(R.id.etxt2).setText("SMTH");
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
func();
}
}