Using Intent Flags
When starting an activity, you can modify the default association of an activity to its task by including flags in the intent that you deliver to
startActivity(). The flags you can use to modify the default behavior are:
Start the activity in a new task. If a task is already running for the activity you are now starting, that task is brought to the foreground with its last state restored and the activity receives the new intent in onNewIntent().
This produces the same behavior as the "singleTask" launchMode value, discussed in the previous section.
If the activity being started is the current activity (at the top of the back stack), then the existing instance receives a call to onNewIntent(), instead of creating a new instance of the activity.
This produces the same behavior as the "singleTop" launchMode value, discussed in the previous section.
If the activity being started is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it are destroyed and this intent is delivered to the resumed instance of the activity (now on top), through onNewIntent()).
There is no value for the launchMode attribute that produces this behavior.
FLAG_ACTIVITY_CLEAR_TOP is most often used in conjunction with FLAG_ACTIVITY_NEW_TASK. When used together, these flags are a way of locating an existing activity in another task and putting it in a position where it can respond to the intent.