What is difference between @+id/android:list and @+id/list

I am wondering what's the difference between @+id/android:list and @+id/list. I know the last one which is a regular id assignment but the first looks different. What makes it special?

Where I saw it: I was studying on ListView, ListAdapter and things like that and the author define the ListView in layout xml file as below :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
   >
<ListView
    android:id="@+id/android:list"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    />
<TextView
    android:id="@+id/android:empty"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:text="@string/main_no_items"/>
</LinearLayout>

and also let me mention @+id/android:empty id as well.

And he also extends ListActivity class.

Here is the source of the article.

And also what's in my mind as questions are :

1. Should we extend ListActivity? Maybe I want an Activity which also contains other Views.
2. We use @+id/android:list just because we extend ListActivity or we can use the same convention if we extend Activity?

Thanks.

Solution 1:

Resource IDs in Android are specific to a package (which is good, or else you'd have lots of conflicts if your app is dealing with several packages at the same time).

@+id/list will create a resource ID in your app (=your package) with the name "list" and give it a unique ID. In code, that would be R.id.list.

@android:id/list will use the ID "list" from the package android (which, in code, would be android.R.id.list.

EDIT: Need to add the corrections David Hedlund pointed out: The proper reference would be @android:id/list. Also, + indicates you're defining a new ID - you obviously don't need that when you're referencing something that was defined in the Android API.

Solution 2:

I think the example code you posted has a typo, so it should be @android:id/list (without the +). From the ListActivity javadoc:

your own view MUST contain a ListView object with the id "@android:id/list"

@android:id/list is specific to ListActivity, so you do not need it if you are adding a ListView into any other kind of Activity. You should extend ListActivity if you want the user to do more than view the list. For example, you can override ListActivity.onListItemClick to respond to clicks on an item in the list.

Similarly, @id/android:empty (again, without the +), is a special case for ListActivity. This allows you to specify an alternative view that should be displayed when your list is empty. That View will not be displayed when the list is populated.