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Explicit intent and action performed

I am wondering how does the following code works (it starts an activity). I don't get how does the system figure out what is the action that should be preformed. No action is specified for the Intent. I would have expected a set_action.
Intent i = new Intent(this, ActivityTwo.class);
startActivity(i);
I am wondering how is it possible to have an Intent which action is not explicitely specified considering what I read in the documentation:
The primary pieces of information in an intent are:
action -- The general action to be performed, such as ACTION_VIEW, ACTION_EDIT, ACTION_MAIN, etc.
data -- The data to operate on, such as a person record in the contacts database, expressed as a Uri.
I hope it make sense. Thank you for your help.

There are two types of intents in Android, implicit intents and explicit intents.
1) Implicit intent
You set an Action, Category and data type and let Android find an activity that fits the specified characteristics (has an intent filter with the specified Action, Category and Data Type).
2) Explicit intent
As the docs says:
An explicit intent is one that you use to launch a specific app
component, such as a particular activity or service in your app. To
create an explicit intent, define the component name for the Intent
object—all other intent properties are optional.
You tell which activity/service to open explicitly. So the system doesn't need to figure out which one to open, you're already telling it to open a specific Activity/Service.
The one you read in the docs is an implicit intent, this is the explicit one:
Intent i = new Intent(this, ActivityTwo.class);

There are multiple constructor functions for Intent class.
If checking the source code for public Intent(Context packageContext, Class cls), the following info is mentioned:
Create an intent for a specific component. All other fields (action, data,
* type, class) are null, though they can be modified later with explicit
* calls. This provides a convenient way to create an intent that is
* intended to execute a hard-coded class name, rather than relying on the
* system to find an appropriate class for you;

Is it possible to set Action to Explicit Intents?

For this code, 'localIntent' seems Explicit Intent directed to 'SecondActivity.class'. But it also seems that it's action has been set as 'SAMPLE'. In this case, is this still Explicit Intent? And when this type of Intent usually is used for?
Intent localIntent = new Intent(getApplicationContext(), SecondActivity.class).setAction("SAMPLE");
startActivity(localIntent);

For Intent of Android, there are only two types like this:
Explicit Intents have specified a component (via setComponent(ComponentName) or setClass(Context, Class)), which
provides the exact class to be run. Often these will not include any
other information, simply being a way for an application to launch
various internal activities it has as the user interacts with the
application.
Implicit Intents have not specified a component; instead, they must include enough information for the system to determine which of the
available components is best to run for that intent.
So, whether you set component or set class, that will be explicit intent.(read more here)

Do implicit intents do a broadcast internally?

Following piece of code open an URL using implicit intent.
EditText editText = (EditText) findViewById(R.id.url_editText);
Intent myIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(editText.getText().toString()));
startActivity(myIntent);
It shows up all the applications that support this action Intent.ACTION_VIEW like Chrome, Firefox etc. Hence I am assuming this procedure internally does a broadcast with action Intent.ACTION_VIEW. Please correct me if I have misunderstood.
As per above assumption I tried the foloowing code
Intent myIntent = new Intent(Intent.ACTION_VIEW);
sendBroadcast(myIntent);
but it does not work. Nothing is shown. What is the difference. Can someone clear my confusion?

This has to do with IntentFilters. Before launching the Activity, it's asking the system to give it the list of everything that's an Activity and can handle the intent -- no broadcasting involved here.
As for sendBroadcast() -- it's essentially the same thing but for BroadcastReceivers. The mechanism is the same: match intent filters, deliver the Intent, but the Intent is delivered to all the receivers regardless of their quantity (as opposed to what startActivity() does -- because it can only result in starting a single activity, hence the need to choose one if there are multiple that match).
I don't think there are any BroadcastReceivers registered for Intent.ACTION_VIEW (since it's an action whose purpose is to start an activity, there's no logical reason to listen for it and start nothing, except count activity launches or something) but you can register one yourself and see what happens.

Hence I am assuming this procedure internally does a broadcast with action Intent.ACTION_VIEW.
No.
but it does not work. Nothing is shown
Of course.
What is the difference.
startActivity() != sendBroadast(). They are separate operations, just as addition and subtraction are separate mathematical operations.
If you wish to think of the Intent system as being a bit like a message bus, that bus has three totally separate channels:
activities (startActivity())
services (startService(), bindService())
broadcasts (sendBroadcast(), sendOrderedBroadcast(), etc.)

The difference between those two is just who receives the Intent. If you call sendBroadcast() the Intent will be sent to BroadcastReceivers. If you call startActivity() the Intent will be sent to Activities. That's the reason why this:
Intent myIntent = new Intent(Intent.ACTION_VIEW);
sendBroadcast(myIntent);
Doesn't start an Activity, because the Intent is only visible to BroadcastReceivers.
The same goes for startService(). By calling that method the Intent will only target Services. I guess the confusion comes from the word broadcast. It implies that it is sent everywhere and visible to everyone, but that is not the case. Broadcasts are only visible to BroadcastReceivers just like if you call startActivity() the Intent will only target Activities and nothing else.
You can find more information here.

What is the different between Explicit and implicit activity call in android?

What is the difference between explicit and implicit activity call in android? If you explain the answer with a simple example will be good.

For example:
implicit activity call
In intent filter you create action for you activity, so other app can call your activity via this action as following:
<activity android:name=".BrowserActivitiy" android:label="#string/app_name">
<intent-filter>
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.DEFAULT" />
<data android:scheme="http"/>
</intent-filter>
</activity>
And the other way to call implicit Intent is below:
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.example.com"));
startActivity(intent);
Explicit activity call
You make a call that indicate exactly which activity class:
Intent intent = new Intent(this, ActivityABC.class);
intent.putExtra("Value", "This value for ActivityABC");
startActivity(intent);
Hope this help you understand more about Explicit and implicit activity call in android.
You can get more detail about Android Intent here

Explicit Intents are used to call a specific component. When you know which component you want to launch and you do not want to give the user free control over which component to use.For example, you have an application that has 2 activities. Activity A and activity B. You want to launch activity B from activity A. In this case you define an explicit intent targeting activityB and then use it to directly call it.
Implicit Intents are used when you have an idea of what you want to do, but you do not know which component should be launched. Or if you want to give the user an option to choose between a list of components to use. If these Intents are send to the Android system it searches for all components which are registered for the specific action and the data type. If only one component is found, Android starts the component directly. For example, you have an application that uses the camera to take photos. One of the features of your application is that you give the user the possibility to send the photos he has taken. You do not know what kind of application the user has that can send photos, and you also want to give the user an option to choose which external application to use if he has more than one. In this case you would not use an explicit intent. Instead you should use an implicit intent that has its action set to ACTION_SEND and its data extra set to the URI of the photo.
An explicit intent is always delivered to its target, no matter what it contains; the filter is not consulted. But an implicit intent is delivered to a component only if it can pass through one of the component's filters

Every time I get confused among these in either interview. So, I have summarised it like this, may it help someone to keep this difference in mind.
Summary:
In Implicit Intents, the user implicitly tells the system WHAT should be done, without specifying who should do.
In Explicit Intents, the user explicitly tells the system WHOM to be triggered for whatever the work is.

See Intent Resolution here
http://developer.android.com/guide/topics/intents/intents-filters.html
Explicit intents (activities) refer to a specific class, and in general, are only available to your packages. Implicit intents refer to intent filters where apps publicly announce that they can handle certain types of data or can provide specific services, e.g. send an email. With implicit intents, the users chooses which activity (typically a package) to use to handle the intent or if a default handler is set, it is launched.

When to use which?
Explicit intent: When you know which component can handle your request. So you explicitly mention that component name in the intent.
Intent i = new Intent(context,DetailActivity.class); // DetailActivity.class is the component name
startActivity(i);
Implicit intent: When you don't know which application can handle your request then you mention the action in intent and let the OS decide which application/s is/are suitable for your task.
Example: Play music
Intent intent = new Intent();
intent.setAction(Intent.ACTION_VIEW);
intent.setData(file);
startActivity(intent);
How OS decides?
When there is implicit call with an intent then OS takes out the action and then it matches with all the intent-filters of all the registered activities of all application using PackageManager and then populates the result as a list. It is called intent resolution
So there is a possibility that no application is available in your device which can handle your request. In that case, you will get NullPointer Exception.
So a safer way to call implicit intent would be this
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(file);
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(intent);
}

There are two types of intents:
Explicit Intent:
While creating an Intent object when we explicitly specify and pass on the target component name directly in the intent, it’s an explicit intent.
Implicit Intent:
In this case we delegate the task of evaluating the registered components (registration is usually done using intent filters that we’ll cover later) to Android based on the intent data and the intended action (like send an email, capture a photo, pin location on a map, etc.) that we pass. So Android will automatically fire up the component from the same app or some other app that can handle the intent message/job. The idea in this case is that, let’s say we have to pin a location on a map, for that we don’t have to code our own activity to handle that. Instead just pass on the location data to an app like Google maps that can do the job on our app’s behalf.
source : http://codetheory.in/android-intents/

Implicit intent doesn't specify the component. Intent provides the information of a component
Intent intent=new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse("http://www.google.com"));
startActivity(intent);
whereas,
Explicit intent specify the component. The intent provides information about the class.
Intent i = new Intent(this, ClassB.class);
startActivity(i);

What is the purpose of "android.intent.category.DEFAULT"?

What is the purpose of using android.intent.category.DEFAULT in the Category field of Intent Filters?

Categories are used for implicit Intents. So, If your Activity can be started by an implicit Intent when no other specific category is assigned to activity, activity's Intent filter should include this category. (even if you have other categories in the Intent filter). If you are sure that your activity must be called with any other Category, don't use the Default.
Setting Category to Default doesn't mean that this Activity will be used by default when your app launches. The Activity just says to system that " Oh I could be started, even if the starter Intent's category is set to Nothing at all ! "

This category is mainly used for implicit intents. If your activity wishes to be started by an implicit intent it should include this catetory in its filter.
I think the term "default" should be understood as "default candidate". If the action on a piece of data resolves to multiple activities, then Android will present all candidates to the user and the user can select his preferred default.
Reference:
http://developer.android.com/guide/components/intents-filters.html
Extract from that page:
Android treats all implicit intents passed tostartActivity() as if they contained at least one category: "android.intent.category.DEFAULT" (the CATEGORY_DEFAULT constant). Therefore, activities that are willing to receive implicit intents must include "android.intent.category.DEFAULT" in their intent filters. (Filters with "android.intent.action.MAIN" and "android.intent.category.LAUNCHER" settings are the exception. They mark activities that begin new tasks and that are represented on the launcher screen. They can include "android.intent.category.DEFAULT" in the list of categories, but don't need to.)

Activities will need to support the CATEGORY_DEFAULT so that they can be found by Context.startActivity().
In order to receive implicit intents, you must include the CATEGORY_DEFAULT category in the intent filter. The methods startActivity() and startActivityForResult() treat all intents as if they declared the CATEGORY_DEFAULT category. If we do not declare it in our intent filter, no implicit intents will resolve to our activity.

It is actually to make sure your other activities can be called out when the app is running. LAUNCHER will make the activity that has it the first activity that starts. To use intents to get to the other activities, they have to be listed as "actual" activities by putting DEFAULT. That is from what I know so don't quote me if it's wrong.

It is used to declare some operation as default action (as its name suggest).
Lets consider we have a notepad app(referring to android notepad sample). The first page of app consists of a list of all notepad files. When one notepad file is selected one of the operations like edit note, delete note etc can be performed. But I want to make edit as my default action which means when i press center button of my keypad,edit window should be open.

category:
android.intent.category.DEFAULT
Matches any implicit Intent. This category must be included for your Activity to receive any implicit Intent.
https://codelabs.developers.google.com/codelabs/android-training-activity-with-implicit-intent/index.html?index=..%2F..%2Fandroid-training#6

https://developer.android.com/guide/components/intents-filters
To receive implicit intents, you must include the CATEGORY_DEFAULT category in the intent filter. The methods startActivity() and startActivityForResult() treat all intents as if they declared the CATEGORY_DEFAULT category. If you do not declare this category in your intent filter, no implicit intents will resolve to your activity.

Before an implicit Intent is accepted by an Activity the Intent must pass a category test: Each category in the Intent must match the exact same category in the Intent-filter of the Activity.
The category DEFAULT is automatically applied to all implicit intents (by default) so because of the reason above every Activity that want to receive any implicit intent at all has to include this category in its Intent-filter.
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