Difference between @+id, @id and @android in Android

Any view that needs to be referenced in either Java code or XML layout requires a unique ID. An ID is defined inside the android:id attribute. Depending on the situation, it can appear in one of the two forms, either @+id/my_view_id or @id/my_view_id (note that the former has a plus symbol). The same holds true for referencing an already defined ID in the layout, such as when using a RelativeLayout.

An obvious question arises: When do I use which notation? This has been answered on several websites including StackOverflow. The rule actually sounds quite simple.

When defining a new ID, use the @+id notation.
When referencing an already defined ID, use the @id notation.

While the above is true, it is somewhat misleading at first. It may trick some people into thinking that one should use the @+id/my_view_id notation when defining a View, i.e. inside the android:id attribute, and the @id/my_view_id version anywhere else. This is completely wrong. In fact, it has nothing to do with anything View related (notice the use of the word ID, as opposed to View in the rule above). It is all about the source code appearance of the ID in the layout and not the context of its usage. That is, the first time an ID is used in the layout regardless of the attribute in which it appears, the “plus notation” must be used. After that, all references to that ID should be in the latter form.

The plus symbol tells the XML parser to generate an integer, which along with the ID (my_view_id in our example) is used to create a new entry in the R.java file. Without it, the parser will instead look for an existing entry with the same name. Absence of such mapping will trigger the (in)famous error “No resource found that matches the given name”, which I’m sure every developer has encountered at least a few times.

Some folks have developed a habit of always using the first form (with the plus symbol). This way they don’t have to worry about undefined ID problems or think about ordering Views. It works, but only because of the Android system being forgiving of duplicate IDs, by ignoring further instructions to create a new ID with the same name. However, this may change in the future, which will cause such layouts to become broken. Even if it doesn’t, it may be confusing to other developers. At the very least, it is rather irresponsible, which doesn’t fit well into the Android methodology.

Finally, @android notation is used when referencing resources defined by the Android system. You should never use a plus symbol with this notation.

Hope that makes it clear.


6 thoughts on “Difference between @+id, @id and @android in Android

  1. CBN

    Good explanation, but for goodness sakes:

    “doesn’t fit well into the Java ecosystem”

    This jargon is ridiculous at the best of times but here it’s just plain wrong. You mean:

    “doesn’t fit well into the Android methodology”

  2. Jack

    This still doesn’t make any sense to me. XML is declarative. How can there be a difference between “defining an id” and “referencing an already defined id”? It’s not an imperative program that has an order of execution.

    1. admin Post author

      This step is executed way before inflating XML files. In order to generate R.id.* entries aapt compiler has to scan all layout files searching for @+id/* and @id/* values. Whenever aapt sees @+id/bla it creates a new R.id.bla int field and assigns it a unique, newly generated number, and replaces @+id/bla with that number. After that, whenever it sees @id/bla, it merely replaces @id/bla with the value of the already present bla field. The scanning is done in a top-down approach. That’s why the first reference must have a plus sign in it. The resulting XML files will have numbers in place of @id tokens. The static R file is for java code access to id values.

  3. Pingback: '@android:id' notation in layout files | 我爱源码网

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