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Static Context Warning

Android Studio now displays a warning when a class contains a static Context object. It says this causes a memory leak. However, I have noticed this is also done in android libraries. For instance, the LocalBroacastManager class has a static instance and it contains a context object.
So how bad is this as a memory leak?
I have a singleton geofencing class that runs in the background and saves a boolean to sharedPreferences indicating if the user is in the geofence or not. To save the preferences I need a context object, but since the method is an overridden method, I have no way of passing a context object in.
How can this be accomplished without a context instance variable?

Its about the worst you can have. Lets say that you have an Activity, and store that as a static Context. Unless you null it out when the activity finishes, you now have leaked a whole Activity. That means every variable that the activity holds is leaked, including the entire view hierarchy. Its basically preventing anything in that Context from freeing up.
The best thing to do is not to store a Context, but instead to pass it in as a parameter to functions that need it. If you must store a Context, do not make it static. A non-static variable won't leak it so long as nothing in the framework continues to hold a reference to the object after the activity is finished.
If you absolutely must use a static Context, make it the Application context. That one is valid over the length of your app, so it can't really be leaked.

Statics should be rarely used, especially in Android. There are ways and reasons of using statics, but in 90% of cases, it is just a misuse of them.
Keeping the context as a static variable is a big no-no. Imagine following scenario:
You go from activity A to activity B.
While in activity B you keep activity B reference as static Context somewhere.
You go back to activity A. Activity B should be destroyed, but instead it is kept, since you keep a static reference to it.
Now you go again to B from A, et voila - you have two B instances: the one you see and another one kept as static context.
You may NEVER have two instances of the same activity. It could cause a lot of issues.
Now, I'm aware that a lot of developers (even ones making libraries) every now and then make mistakes and use anti-patterns, so you should not blindly rely on practice/pattern seen in someone other's code. Hell, I've even seen a lot of garbage code in something written by Google devs.
If you really need a singleton (not Singleton pattern (uppercase S)), but single instance of some class, if you don't want to use libraries like Dagger and so, you can instantiate that class in your Application class, and then reference it from wherever you want.

Passing references to Activity Intent

For quite some time I've had troubles passing variables from one Activity to another, and I've usually had to resolve to some pretty ugly Static-class-hacks to make it work.
Generally something along the lines of a static method that I call with the type of the Activity, along with the variables the Activity requires. These gets stored in a static variable, and retrieved in the constructor of said activity.
Like I said, pretty ugly. And there's no such thing as "myActivity.StartActivity(new Activity);". All of the overloads for StartActivity takes either an Intent, or a typeof(MyOtherActivity).
So my question is, have I completely misunderstood the concept of Activities, or am I simply missing a completely obvious way to pass arguments to them?
#Edit: The reason I want to pass an actual reference to an object, instead of simply a copy of the object, is because I'm trying to pass a View Model from an overlying Activity, down to the new Activity. And of course any changes made to this view model, should be reflected on the parent activity, which will only be possible if the the two activy's viewmodels points to the same instance.
I'm writing the app using Xamarin.Android, but the code is nearly identical between C# and Java, so answers in either those languages is fine.

The problem is that Android can kill the process hosting your app at any time (if it is in the background). When the user then returns to your app, Android will create a new process to host your app and will recreate the Activity at the top of the stack. In order to do this, Android keeps a "serialized" version of the Intent so that it can recreate the Intent to pass it to the Activity. This is why all "extras" in an Intent need to be Parcelable or Serializable.
This is also why you cannot pass a reference to an object. When Android recreates the process, none of these objects will exist anymore.
Another point to consider is that different activities may run in different processes. Even activities from the same application may be in different processes (if the manifest specifies this). Since object references don't work across process boundaries, this is another reason why you cannot pass a reference to an object in an Intent.

You can also use The Application class to store objects globally and retrieve them:
using Android.Runtime;
namespace SomeName
{
[Application]
public class App : Application
{
public string Name { get; set;}
public App (IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer)
{
}
public override void OnCreate ()
{
base.OnCreate ();
Name = "";
}
}
}
And you can access the data with:
App application = (App)Application.Context;
application.Name = "something";
I choose to do this on the Application calss because this class is called on the App startup so you don't have to initiate it manually.
Keep in mind that variables which are scoped to the Application have their lifetime scoped to the application by extension.
This class will be Garbage Collected if the Android feels it is necessary so you have to modify the code to include this case also.
You can use SharedPreferences or a Database to save your variables in case they get deleted and retrieve them from the App class for faster results.
Don't be overly wasteful in how you use this approach though, as attaching too much information on this class it can lead to a degradation in performance. Only add information that you know will be needed by different parts of the application, and where the cost of retrieving that information exceeds the cost of storing it as an application variable.
Investigate which information you need to hold as application wide state, and what information can simply be retrieved straight from your database. There are cost implications for both and you need to ensure you get the balance right.
And don't forget to release resources as needed on OnStop and OnDestroy
I rarely use intents, i find this way better.

What is the best way to get Context throughout an Activity?

I save to SharedPreferences within a single Activity several times. Each time I do reads or writes, I pass the Context. Following are the two options I am thinking about - which one is better (or is there a better way)
Everytime I need to pass context to a function in another class (for ex a class writing to my SharedPreferences), I am passing the this getActivity().getApplicationContext().
Should I just save a variable once in the onCreate() of the Activity (i.e. Context ctxt = getActivity().getApplicationContext()) and then use the variable ctxt throughout the Activity whenever I need to pass context.
Which way is technically better? Key is - should I be accessing getApplicationContext() each time. I am guessing that is slow (& unnecessary) and also probably causes crashes (I've been seeing some logs from my users that I am unable to reproduce but the stack traces always point to lines containing getApplicationContext())

The Activity itself can be used as the Context. Simply pass this. You can pass ActivityName.this inside an inner class.
getApplicationContext() returns "the context of the single, global Application object of the current process."

check this post its what you want, and you just use the Activity as Context and there is no problem calling getApplicationContext() anytime you want, you just might need to find alternatives to that and maybe use getBaseContext() or the activity itself

Accessing strings.xml from different contexts

Does it matter whether I access strings from strings.xml file from different contexts?
I accessed it in many ways such as: getApplicationContext().getString(R.string.name), this.getString(R.string.name),MainActivity.this.getApplicationContext().getString(R.string.name), or simply getString(R.string.name).
I understand that the application context lasts the whole application lifetime, but I honestly do not know when I should use it. Also, I cannot understand what is the difference between using this and MainActivity.this.
Please explain how I should access strings from both activities and Java classes.

The key argument for using the Application Context or the Activity Context is the dependency to the lifecycle of the whole App or a component (Activity in this case). If you wanna access the strings.xml it doesn't really matter which of both you use. Because you just use the Context get a reference to the ressource, which goes away when the Activity will be destroyed. A more importent case would be if you pass the Context to another class f.e. Because then there is a dependency to this specific Context and can lead to memory leaks if you don't clean it. There is a nice explanation about this with the usage of a BroadcastReceiver in the docs.
There is no difference between this and AnyClass.this because it points to the same class object. But you have to use AnyClass.this if you wanna access the outer class in an inner class. Or if you have an anonymous class and need to access fields of the outer class. You could see it as more strict because you refer to a specific class.

Is it possible, and what is the best strategy, to pass objects by reference from one Activity to the next?

I have a simple Android application that uses an instance of a class, let's call it DataManager, to manage access to the domain classes in a Façade-like way. I initially designed it as a singleton which could be retrieved using static methods but I eventually got irritated with the messiness of my implementation and refactored to what I reckoned was a simpler and cleaner idea.
Now the idea is that for each file that is opened, one DataManager is created, which they handles both file I/O and modification of the domain classes (e.g. Book). When starting a new Activity, I pass this one instance as a Serializable extra (I haven't got on to using Parcelable yet, but expect I will when I have the basic concept working), and then I grab the DataManager from the Intent in the onCreate() method of the new Activity.
Yet comparison of the objects indicates that the object sent from one activity is not identical (different references) to the object retrieved from the Bundle in the second Activity. Reading up on Bundle (on StackOverflow, etc.) suggests that Bundles cannot do anything other than pass-by-value.
So what is likely to be the cleanest and safest strategy for passing an object between Activities? As I see it I could
Forget about passing by reference and live with each Activity having its own DataManager object. Pass back the new DataManager every time I close an activity so that the underlying activity can use it. (The simple solution, I think.)
Go back to using a singleton DataManager and use a static method to get it from each Activity. (Not keen on using singletons again.)
Extend Application to create a sort of global reference to DataManager. (Again, not keen on the idea of globals.)
Is that a fair summary? Is there some other healthy strategy I could use?

Another approach would be to create a service. The first activity would start the service and bind to it, when you launch a new intent, unbind the first activity and when second activity starts, bind to the service.
This way you don't have to ever stop the service or worry about passing data between activities.

Java does not have pass by reference so that option is out, I would suggest dependency injection for passing data between the activities. Otherwise definetely the singleton would be the way to go.

The prescribed one is Going by implementing Parcellable interface, thats the way to pass Objects between Activities.. and the 2nd and better choice is to make a Singleton to be sure its single Object.

Create your DataManager as a Singleton that implements Service. Bind the service to your application in the manifest xml (see the link), and you will have a persistent singleton your activities can access without issues.
Passing parcellable arguments can quickly get very messy if you need to get a lot of data. The singleton approach, although usually considered an anti-pattern, works like a charm in cases like these. Just remember to not create multiple singletons that interact with one another.

I would suggest using an Application Subclass. It allows you to hold a single reference to the DataManger class and is persistent as long as your app lives.
A singleton with a static field will also work, but there are some place in the documentation where it says that the content of static fields is not a safe place to store your data. As I understand static fields should persist as long as your ClassLoader stays in memory. Therefore a singleton should only vanish if the whole process leaves the memory and in that case the application class will also leave the memory, but it will call the onDestroy methods of the Application and that enables you to safely close your DataManager and persist important data to memory.
That said to your two variations.
The Android way to go would be to make your DataManager a ContentProvider. This will make it possible to access your Data from every Activity without holding a global reference. I'm not sure how you would build a caching Content Provider that stays in memory and is not reinstantiated too often.