When should i use @android:id/?

Question

I\'ve found some code examples with the @android:id/xyz attribute set instead of @+id/xyz.
When and why is it necessary to use Android ids in

Answer 1

@android is used to access the default resources of Android . For example see this:

android:background="@android:color/transparent"

Using + sign means this resource must be added in R.java file while not using + sign means the resource already exist in R.java.