Q:

How to open Activity via intent-filter and what is the use of intent-filter?

I'm working with activities I know how to open activities via intent, but I want to know how can I open activity via intent-filter and what is the role of intent-filter to open activities.

How many ways to open the activity?

A:

Activity can even be launched via IntentFilter 
try this out
Basically when you install your app, Android system will register the activity with corresponding action, when you declare your activity with custom action, Android system stores the activity with the respective activity. When you launch the intent with your custom action. The system will find the receiving activity and launch it it there is only one activity matching it, if there are more than one Activity receiving that action, System will ask the user to choose which activity to complete the action.  
declare activity in manifest as 
       <activity
            android:name=".YourActivity" >
            <intent-filter>
                <action android:name="your.custom.ACTION" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity> 

then you can start this activity by just calling 
startActivity(new Intent("your.custom.ACTION"));

