Any View object may have an integer ID associated with it, to uniquely
identify the View within the tree. When the application is compiled, this ID is
referenced as an integer, but the ID is typically assigned in the layout XML
file as a string, in the id attribute. This is an XML attribute common to all
View objects (defined by the View class) and you will use it very often. The
syntax for an ID, inside an XML tag
is:
android:id="@+id/my_button"
The
at-symbol (@) at the beginning of the string indicates that the XML parser
should parse and expand the rest of the ID string and identify it as an ID
resource. The plus-symbol (+) means that this is a new resource name that must
be created and added to our resources (in the R.java file). There are a number
of other ID resources that are offered by the Android framework. When
referencing an Android resource ID, you do not need the plus-symbol, but must
add the android package namespace, like
so:
android:id="@android:id/empty"
android:id="@+id/btn",表示在R.java文件里面新增一个id为btn的控件索引，最常用的一种声明控件id的方式。android:id="@android:id/tabhost",表示引用的是系统已有的ID，在对应的sdk目录下的ids.xml里面。一般外部不去调用，是组件内部调用的时候使用。.android:id="@id/btn" ,表示引用一个已经存在的ID，在R.java里面的，比如我们自己建了一个ids.xml，里面声明了一组id，其中一个是btn，那么你就可以这样引用了。
@+id/和android:id有什么区别？,布布扣,bubuko.com
原文地址：http://www.cnblogs.com/KevinJasmine/p/3753031.html
