Develop Reference

How to pass context to a module classes without using a static context?

I have created a library, and added as a module in my app. My library does not have any activity class, all classes are core java classes, so i don't have access to context.
This is what i did;
My library class is singleton
I have created a static variable like this; static Context myGlobalContext
I created a method where i set this context;
public void init(Context context){
myGlobalContext=context;
}
I set this context from the first activity;
MyClass.sharedInstance().init(MyApplication.getAppContext());
then i use this myGlobalContext in all of the library classes.
This thing works fine when i debug this, the value of myGlobalContext is never null, but i am getting some crashes from crashlytics, where context is null.
java.lang.NullPointerException: Attempt to invoke virtual method
'android.content.SharedPreferences
android.content.Context.getSharedPreferences(java.lang.String, int)'
on a null object reference
When i run lint, it also warns about the static global context
This is my application class, here i create context;
public class MyApplication extends Application {
private static Context context;
public void onCreate() {
super.onCreate();
MyApplication.context = getApplicationContext();
}
public static Context getAppContext() {
return MyApplication.context;
}

Replace
MyApplication.context = getApplicationContext();
With
MyApplication.context = this;
And you need to add the application name to the manifest for this Context to ever be set
However, if you have a core Java library, you should not be using an Application because you have no life cycle methods to handle. Just define methods that use Context objects, import your library externally, and use Application Context sparingly if you're actually in an Activity, Service, or other Context subclass

according to your code you should have at least one instance of MyApplication or just set some default data to MyApplication.context, then your static getAppContext will return some value

Get Android application from application context

I have access to application context but not to application. I want to get the application (so I can get all running activities), but couldn't find a way to do so. Is there an existing API to get application from application context or I will have to override getApplicationContext for that?

No, there's no such API out of the box. However, you can either get application context and cast it to Application object, or Extend Application class, and make it a singleton so you can grab an instance of it from everywhere.
public class MyApplication extends Application {
private static MyApplication singleton;
// Returns the application instance
public static MyApplication getInstance() {
return singleton;
}
public final void onCreate() {
super.onCreate();
singleton = this;
}
}

Difference between my application object and application context object

I am looking for information regarding writing my own application class. In many tutorials on the net I have seen the following code:
class myapp extends Application
{
private static myapp mm;
private Context context;
public Context getContext()
{
return getApplicationContext();
}
public myapp getmyapp()
{
if(mm == null)
mm = new myapp();
return mm;
}
}
What is the difference in getting object of myapp and getApplicationContext and where to use object of myapp and where to use context object. I just want to clear the concept of usage of these objects.

that code is completely wrong:
public myapp getmyapp()
{
if(mm==null)
mm=new myapp();
return mm;
}
only the Android framework can instantiate the Application object. I mean, you can call new but the object won't be "connected" to the underlying framework
To have a static reference of the application object you should do as follows:
class MyApp extends Application{
// I fixed the names to follow some Java convention
private static MyApp instance;
#Override
public void onCreate() {
super.onCreate();
instance = this;
}
public static MyApp getMyApp(){
return instance;
}
Regarding the context, the code is not wrong, but simply doesn't make any sense. That's because the Application object already is the application context. So there's not need to ask for it.
Context is what gives Android apps access to resources, file system specific folders, permissions, etc (what I said about the Android framework creates it). The Application is one class that extends Context, other examples are Activity and Service classes.
I hope it's a bit clearer.

Error while trying to execute Sugar database CRUD operation

I (having mediocre developing skills) actually try to use Sugar as a database wrapper for my android project.
Therefore, I was following along the "Getting-Started-Guide" (http://satyan.github.io/sugar/getting-started.html) to get ready as soon as possible.
I created a class for my entities, called DataSet.java :
import com.orm.SugarRecord;
public class DataSet extends SugarRecord{
int someData;
double evenMoreData;
public DataSet(Context ctx){
super(ctx);
}
public DataSet(Context ctx,
int someData,
long evenMoreData) {
super(ctx);
this.someData = someData;
this.evenMoreData = evenMoreData;
}
}
I call the class in the following way:
someGreatClass something;
someMoreGreatCode somemore;
DataSet dataSet = new DataSet(
ctx, // Here Eclipse throws the error
something.method(),
somemore.anothermethod());
DataSet.save();
When I try to build this and to push it onto my device, Eclipse refuses to compile and throws this error:
ctx cannot be resolved to a variable
Considering the fact that I'm relatively new to Android development, the error may be obvious and I hope to get a tip how to solve this.
P.S.: Furthermore, I don't fully get the developer's statement in the getting-started-Note:
Please retain one constructor with Context argument. (This constraint will be removed in subsequent release.)
Thank you very much!
// Edit: Did edit the class name from LocationDataSet to Data set for clarification

First of all, the getting-started-note tells you that you need a constructor with only a context parameter, you did this here so that's ok
public DataSet(Context ctx){
super(ctx);
}
about
ctx cannot be resolved to a variable
I think you don't have a variable called ctx, I don't know if you're familiar with android context? (basically a context is a service or an activity), if you're using this code in an activity or a service, just use the 'this' keyword and not the ctx variable
The code you provide doesn't really show what you're doing, but you showed us the code from 'DataSet', but the error happens with a LocationDataSet? And you're calling save on DataSet?
The save method must be called on an object, not a class.
Also don't forget that sugar needs the special application class in the manifest
UPDATE with example:
Your dataset class (the sugarrecord) should look like this, that's ok in your code as far as I can see
public class DataSet extends SugarRecord<DataSet>{
private String someData;
public DataSet(Context c){
super(c);
}
public DataSet(Context c, String someData){
super(c);
this.someData = someData;
}
}
An activity that uses the record should look like this
public class SomeActivity extends Activity {
public void someMethodThatUsesDataSet(){
// Create a dataset object with some data you want the save and a context
// The context we use here is 'this', this is the current instance of SomeActivity,
// you absolutely need this, I think this is what you're doing wrong,
// you can't use ctx here because that's not a known variable at this point
DataSet example = new DataSet(this, "data you want to save");
// Tell Sugar to save this record in the database
example.save();
}
}

How to obtain AssetManager without reference to Context?

I have a class that needs to obtain a reference to it's application's AssetManager. This class does not extend any sort of android UI class, so it doesn't have a getContext() method, or anything similar. Is there some sort of static Context.getCurrentApplicationContext() type of method?
To clarify: my class is intended to be used like a library, for other applications. It has no associated AndroidManifest.xml or control over the context which is calling it.

Create a subclass of Application, for instance public class App extends Application {
Set the android:name attribute of your <application> tag in the AndroidManifest.xml to point to your new class, e.g. android:name=".App"
In the onCreate() method of your app instance, save your context (e.g. this) to a static field named app and create a static method that returns this field, e.g. getApp():
This is how it should look:
public class App extends Application{
private static Context mContext;
#Override
public void onCreate() {
super.onCreate();
mContext = this;
}
public static Context getContext(){
return mContext;
}
}
Now you can use: App.getContext() whenever you want to get a context, and then getAssetManager() (or App.getContext().getAssetManager()).

I am not sure of the best answer to the OP question. However, I do know that you have to be very careful when using a static context as suggested in Android developer resources:
In the onCreate() method of your app instance, save your context (e.g. this) to a static field named app and create a static method that returns this field, e.g. getApp():
Using static contexts can leak to leaked memory issues, especially if the static context is used for references to views.