Develop Reference

How to initialize context where we cannot get context from getApplicationContext()?

I want to access a class extending SQLiteOpenHelper to get the context of database from a java class. I need to pass application context to get that but don`t have access to getApplicationContext().
How can I get Application Context in java class that is not activity?

I suggest you create a constructor that has a parameter of the Context type.
public class MySQLiteOpenHelper extends SQLiteOpenHelper {
// Variables
private Context ctx;
public MySQLiteOpenHelper(Context ctx) {
this.ctx = ctx;
}
//More code
}
Now, in your activities, you can do this:
MySQLiteOpenHelper helper = new MySQLiteOpenHelper(this);
And in your fragments, you can do this:
MySQLiteOpenHelper helper = new MySQLiteOpenHelper(getActivity().getApplicationContext()); //getActivity() would work too, Activity (indirectly) extends Context.

You can create custom Application class and write method getContext().
Someting like this:
public class MyApplication extends Application {
private static MyApplication mCurrentInstance;
#Override
public void onCreate() {
super.onCreate();
mCurrentInstance = this;
}
public static MyApplication instance() {
return mCurrentInstance;
}
public static Context context() {
return mCurrentInstance.getApplicationContext();
}
}
And add this class into manifest:
<application
android:name=".MyApplication">

Class throwing Null Pointer Exception

Why does the following method throws an NPE,
public ActivityOne extends Activity{
DataManager dtMan = new DataManager(this)
public onCreate (){
...some source code here...
dtMan.check();
}
}
public class DataManager(){
private Context myContext;
public DataManager (Context context){
myContext = context;
}
Helper helper = new Helper(this);
public boolean check(){
helper.open();
...some source code here...
}
}
When I view the logcat; I get a java.null.pointer exception, so I did something like
public class DataManager(){
private Context myContext;
public DataManager (Context context){
myContext = context;
}
public boolean check(Context context){
**Helper helper = new Helper(context);**
helper.open();
...some source code here...
}
}
And it worked, so what is the difference between the two DataManager in Java/Android programming perspective, thus this approach if I understand correctly must be replicated to as follow:
public class DataManager(){
private Context myContext;
public DataManager (Context context){
myContext = context;
}
public boolean check(Context context){
**Helper helper = new Helper(context);**
helper.open();
...some source code here...
}
public boolean check2(Context context){
**Helper helper = new Helper(context);**
helper.open();
...some source code here...
}
public boolean check3(Context context){
**Helper helper = new Helper(context);**
helper.open();
...some source code here...
}
}
Meaning I just can't declare the Helper Class once and use it anywhere the calling class, did I forgot some fundamentals? Please clarify.
Will the context also lead to memory leaks?, if so, how will I fix it?
#EDIT:
Well I forgot to include the Helper Class
public class Helper{
private Context myContext;
public Helper(Context context){
myContext = context;
}
public void open(){
//do stuff here
}
}

Your code does not work because the DataManager is initialized at field scope in the Activity.
Classes which need a Context should always be initialized in one of the Activity's life cycle methods:
onCreate, onStart, onDestroy, etc.
Like this:
private DataManager dataManager;
public void on create(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
dataManager = new DataManager(this):
}
You also should initialize the Helper class in the DataManager constructor:
public class DataManager(){
private Context context;
private Helper helper;
public DataManager (Context context){
this.context = context;
this.helper = new Helper(this.context);
}
public boolean check(Context context){
helper.open();
//...some source code here...
}
}
To prevent memory leaks you just need to make sure the DataManager class is not a static instance with an Activity Context.
If you don't need an Activity Context but you're also fine with an Application Context you should use it:
public DataManager(Context context){
this.context = context.getApplicationContext();
//...
}

Try put
dtMan = new DataManager(this)
in onCreate method in Activity?

Starting an Android Activity from a static method

I want to start an activity from a static java method on an android device.
I do not have any context or anything passed as parameter to the static function.
For starting the activity I must call "startActivity" with the current running method as "this" pointer. So is there a way to get the current running activity?

You can access only static variables/objects inside static method.
So You need to Implement this way
public class MainActivity extends Activity {
private static Context mContext;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mContext = this;
}
public static void goToLoginActivity() {
Intent login = new Intent(mContext, LoginActivity.class);
mContext.startActivity(login);
}
}
NOTE : But this is not the proper way to do so, this may cause window leak issue.
Better approach is pass activity/context object as parameter like this.
public static void goToLoginActivity(Context mContext) {
Intent login = new Intent(mContext, LoginActivity.class);
mContext.startActivity(login);
}

Create a Class in your app extending class Application, define a static context and initialise this with your application context. You can expose a static method from this class for accessing defined static reference. Thats it.
class MyApp extends Application{
private static Context mContext;
public void onCreate(){
mContext = this.getApplicationContext();
}
public static Context getAppContext(){
return mContext;
}
}
Now you can use this static method for accessing context anywhere in your app.

accessing a calling activity's method from within an object it created

MyI don't understand why I get a compile error for this:
public class Main_screen extends ListActivity {
List<Object> completeList;
private My_ArrayAdapter adapter;
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
completeList = getCompleteList();
adapter = new My_ArrayAdapter(this, completeList);
setListAdapter(adapter);
}
public void doSth() {
...
}
}
and in My_ArryAdapter:
public class My_ArrayAdapter extends ArrayAdapter<Object> {
private final List<Object> list;
private final Activity context;
public My_ArrayAdapter(Activity context, List<Object> list) {
this.context = context;
this.list = list;
}
public void tryIt() {
context.doSth(); // <-------- THIS DOES NOT WORK, this method can not be called
}
}
Please explain, is there something fundamental I have not understood. I am just passing the context into the ArrayAdapter instance I create. And from within this instance I would like to acccess the caller's method.
Why shoudl this not be possible?
Many thanks!

try this:
public void tryIt() {
((Main_screen)context).doSth();
}
context is Activity and it hasn't doSth(), but Main_screen has, so you should cast to this class

Actually you are making Activity context object and passing a child of Activity (i.e Main_Screen), Its called upward cast (Implicit Casting).
So the Activity (as parent) has no method of doSth(). So you need downward Casting (Explicit casting) to make it a Main_Screen.
Two ways to do this.
make an Object of Main_Screen context instead of Activity context
or
cast it as Main_Screen in tryIt() method to avail Main_Screen methods like this way:
if(context.isInstance(Main_Screen.class))
{
((Main_Screen)context).doSth()
}
you can also use try catch to minimize the chances of ClassCastException

You can use the below code. Obviously class context don't contain an object doSth(). doSth() is declared in class Main_screen.
public class My_ArrayAdapter extends ArrayAdapter<Object> {
private final List<Object> list;
private final Activity context;
public My_ArrayAdapter(Activity context, List<Object> list) {
this.context = context;
this.list = list;
}
public void tryIt() {
Main_screen.doSth();
}
}

How I did it
StaticCommonDataClass -> maintains static data here I will keep the instance of Activity one in it.
ActivityOneClass -> Contains the method that I have to access in ActivityTwo actually.
ActivityTwoClass => Will access the ActivityOne Method.
What I hate is to pass two many parameters from one function to other function or one class to other class,
that too when it has to be done for similar values again and again.
Here i will store refrence of ActivityOneClass in static Variable.
public class CommonStaticData {
private static Activity _activity;
private static Context _context;
public static void setactivity(Activity activity) {
_activity = activity;
}
public static Activity getactivity() {
return _activity;
}
public static void setcontext(Context context) {
_context = context;
}
public static Context getcontext() {
return _context;
}
}
public class ActivityOneClass extends Activity {
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity1);
CommonStaticData.setactivity(ActivityOneClass.this); //will keep the instance alive for this activity
}
Public void activityOneMethod()
{
//Set of statements
}
}
public class ActivityTwoClass extends Activity {
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity2);
((ActivityOneClass) CommonStaticData.getactivity()).activityOneMethod();
//we need to typecast the instance stored in CommonStaticData.getactivity() to "ActivityOneClass" thats is the
//activity containing the method so as to access the method otherwise it will not come in the intellisense window and will generate Compiler Error
}
}

Create notification from non-activity class

Is it possible to create notification from non-activity class? If so, how?

pass the context in to the class and then create it normally

class A extends Activity{
//required stuff go here
new B().createDialog(A.this);
}
other class
class B{
public void createDialog(Context context){
//create your dialog or notification here
}
}

As subspider said above, pass the context into the class and you'll be fine:
public class DoSomethingClass {
//Here's a context
private Context _CONTEXT;
//Construct that sets the context
public DoSomethingClass(Context c) {
this._CONTEXT = c;
}
public void createNotification() {
/*
Create the notification as usual, just make sure you alter the following lines:
Intent notificationIntent = new Intent(this, MyClass.class);
Context context = this.getApplicationContext();
^Make sure you alter this into this._CONTEXT above
*/
}
}