What is different between MainActivity.this vs getApplicationContext()

Question 1

Which context to use?

There are two types of Context:

Application context is associated with the application and will always be the same throughout the life of application; it does not change. So if you are using Toast, you can use application context or even activity context (both) because Toast can be displayed from anywhere within your application and is not attached to a specific window. But there are many exceptions. One such exception is when you need to use or pass the activity context.

Activity context is associated with the activity and can be destroyed if the activity is destroyed; there may be multiple activities (more than likely) with a single application. Sometimes you absolutely need the activity context handle. For example, should you launch a new Activity, you need to use activity context in its Intent so that the newly-launched activity is connected to the current activity in terms of activity stack. However, you may also use application's context to launch a new activity, but then you need to set flag Intent.FLAG_ACTIVITY_NEW_TASK in intent to treat it as a new task.

Let's consider some cases:

MainActivity.this refers to the MainActivity context which extends Activity class but the base class (Activity) also extends Context class, so it can be used to offer activity context.

getBaseContext() offers activity context.

getApplication() offers application context.

getApplicationContext() also offers application context.

For more information please check this link.

Question 2

MainActivity.this only works if you are in an inner class of MainActivity.
If you are in MainActivity itself, just use this.
If you are in another class entirely, you need to pass it an instance of a context from the Activity you are in.

Hope this helps..

Question 3

This explanation is probably missing some subtle nuances but it should give you a better understanding of why one works but the other doesn't.

The difference is that MainActivity.this refers to the current activity (context) whereas the getApplicationContext() refers to the Application class.

The important differences between the two are that the Application class never has any UI associations and as such has no window token.

Long story short: For UI items that need context, use the Activity.

Question 4

MainActivity.this refers to the the current activity (context) where the getApplicationContext() refers to the Application class.

The getApplicationContext() method return the context of the single, global Application object of the current process. This generally should only be used if you need a Context whose lifecycle is separate from the current context, that is tied to the lifetime of the process rather than the current component.

MainActivity.this will change when activity destroyed and re-created, getApplicationContext() will change when the application killed and re-started.

Question 5

mainActivity gives the context of the current Activity. context depends on activity's lifecycle. getApplicationContext() gives the context of the application and depends on the application's lifecycle.

Question 6

This is what the developer.android.com says:

Return the context of the single, global Application object of the current process. This generally should only be used if you need a Context whose lifecycle is separate from the current context, that is tied to the lifetime of the process rather than the current component.

In general, use ..Activity.this instead of getApplicationContext();

Further reading: developer.android.com/reference/android/content/Context.html#getApplicationContext()

Question 7

MainActivity.this has more information. You can think of

MainActivity.this = getApplicationContext() + UI related information.

Why getApplicationContext() did not work here

2. pd=ProgressDialog.show(getApplicationContext(), "", "Fething data");

is because the ProgressDialog needs UI information. ProgressDialog may or may not be full screen size. If it only takes a portion of the screen, what should the background be? Should it be black, white, or your MainActivity's layout?

Which version of context to use depends on your use cases.

Non-UI-related context usages:

context.getFilesDir()  // File I/O
context.getColor()     // Preset values, strings, colors, pictures...

// create intent
Intent intent = new Intent(context, SettingsActivity.class);

// start new activity
context.startActivity(intent);

...

UI-related context usages (i.e. XXXActivity.this):

// create Adapters for you UI element
myRecyclerViewAdapter = new myRecyclerViewAdapter(this);

// Make new dialogs, i.e. ProgressDialog, AlertDialog...
new AlertDialog.Builder(this)...

// Set current layout fullscreen to get rid of status bar
this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN)

// Set attributes such as colors based on theme
TypedValue typedValue = new TypedValue();
this.getTheme().resolveAttribute(R.attr.colorBackground, typedValue, true);
@ColorInt int colorBasedOnTheme = typedValue.data;

When you pass context to other classes, the Non-UI version context (i.e. getApplicationContext()) will not expire because it lives as long as your application itself. But the UI version context (i.e. XXXActivity.this) will expire because it only lives as long as your XXXActivity. And if an object holds a context(XXXActivity.this) out-lives the original XXXActivity, there will be problems: memory leak, null point exception, undefined behaviours, etc.