I've always understood #+id to indicate the creation of a new ID (generally used with android:id), and #id to reference it elsewhere, as explained at length in a popular question here and in the official documentation:
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...
That this is not enforced is a bit odd; you can just use #+id everywhere, multiple times with the same id, and it does not seem to be an error or problem (unless you use the same id with multiple android:id declarations).
When using the graphical designer in Android Studio with constraint layout (it could in fact be with everything, I have not checked), whenever a constraint is added it will declare them like this:
app:layout_constraintTop_toBottomOf="#+id/rlistfrag"
Where rlistfrag is assigned to another element with android:id in the same file -- if it didn't exist already, the designed could not have created the constraint, so there can be no contextual ambiguity.1 According to the docs this amounts to twice declaring "a new resource name that must be created and added to our resources".
It seems the semantics here are not, by omission at least, exactly as described in the docs. Why does the designer do this and what are those omitted semantics?
Or could there? What all this implies to me is that elements may be processed in any order so the point is whenever an id is first encountered it will be created, even if it is not associated with an existing element.
There's a bit in the doc guide on layout resources that confirms the id will only be set the first time:
The plus symbol, +, indicates that this is a new resource ID and the aapt tool will create a new resource integer in the R.java class, if it doesn't already exist.
Which need not mean the elements aren't processed in order.
It is applied in all layout components. whenever you want to mention "id" you have to use "#+id"
According to this thread I need to use #+id/ for the first time to make resource be created.
But what if I forget that this resource was previously created and create it again with #+id/? I have some input and set nextFocusDown for the element that is still not declared.
<EditText
...
android:nextFocusDown="#+id/myinput2"/>
200 lines below I create this element with #+id because I forgot that it's already declared.
<EditText
...
android:id="#+id/myinput2"/>
It works like this but can it cause an issue?
Some quick definitions:
#+id/foo means "use the id foo, and create it if it doesn't exist"
#id/foo means "use the id foo" (which will be an error if the id foo doesn't exist)
Previously, there were reasons to prefer #id over #+id (the system could tell you if you tried to reference a view via an id that didn't exist), but now the system is smart enough that even writing android:layout_below="#+id/idthatdoesntexistanywhere" will be tagged as an error:
So just always use #+id.
It works like this but can it cause an issue?
No, it can't. Always use #+id/. By now, the Android build environment is smart enought to figure it out.
If you put two elements with the same id in the same layout file, it generates an IDE error (red underline) and you won't be able to compile.
What is the diffirence between the #id/ and #+id/?
In #+id/ the plus symbol + instructs to create a new resource name and add in to the R.java file but what about #id/? From the documentation of ID: 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/list"
But in the image below Eclipse doesn't suggest any kind of #android:id/.
Are #id/ and #android:id/ the same?
you refer to Android resources , which are already defined in Android system, with #android:id/.. while to access resources that you have defined/created in your project, you use #id/..
More Info
As per your clarifications in the chat, you said you have a problem like this :
If we use android:id="#id/layout_item_id" it doesn't work. Instead #+id/ works so what's the difference here? And that was my original question.
Well, it depends on the context, when you're using the XML attribute of android:id, then you're specifying a new id, and are instructing the parser (or call it the builder) to create a new entry in R.java, thus you have to include a + sign.
While in the other case, like android:layout_below="#id/myTextView" , you're referring to an id that has already been created, so parser links this to the already created id in R.java.
More Info Again
As you said in your chat, note that android:layout_below="#id/myTextView" won't recognize an element with id myTextViewif it is written after the element you're using it in.
the + sign is a short cut to add the id to your list of resource ids. Otherwise you need to have them in a xml file like this
<?xml version="1.0" encoding="utf-8"?>
<resources>
<item name="my_logo" type="id"/>
</resources>
In Short
android:id="#+id/my_button"
+id Plus sign tells android to add or create a new id in Resources.
while
android:layout_below="#id/my_button"
it just help to refer the already generated id..
Its very simple:
"#+..." - create new
"#..." - link on existing
Source: https://developer.android.com/guide/topics/resources/layout-resource.html#idvalue
The plus sign (+) before the resource type is needed only when you're defining a resource ID for the first time. When you compile the app, the SDK tools use the ID name to create a new resource ID in your project's R.java file that refers to the EditText element. With the resource ID declared once this way, other references to the ID do not need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not needed for concrete resources such as strings or layouts. See the sidebox for more information about resource objects.
From: https://developer.android.com/training/basics/firstapp/building-ui.html
From the Developer Guide:
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"
There's a bug with Eclipse where sometimes if you just created a new #+id/.., it won't be added immediately to the R.java file, even after clean-building the project. The solution is to restart Eclipse.
This I think should be solved as soon as possible, because it may (and from experience, will) confuse some developers into thinking that there's something wrong with their syntax, and try to debug it even if there's really nothing to debug.
Android uses some files called resources where values are stored for the XML files.
Now when you use #id/ for an XML object, It is trying to refer to an id which is already registered in the values files. On the other hand, when you use #+id/ it registers a new id in the values files as implied by the '+' symbol.
Hope this helps :).
#id/ and #android:id/ is not the same.
#id/ referencing ID in your application, #android:id/ referencing an item in Android platform.
Eclipse is wrong.
Difference between #+id and #id is:
#+id is used to create an id for a view in R.java file.
#id is used to refer the id created for the view in R.java file.
We use #+id with android:id="", but what if the id is not created and we are referring it before getting created(Forward Referencing).
In that case, we have use #+id to create id and while defining the view we have to refer it.
Please refer the below code:
<RelativeLayout>
<TextView
android:id="#+id/dates"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_toLeftOf="#+id/spinner" />
<Spinner
android:id="#id/spinner"
android:layout_width="96dp"
android:layout_height="wrap_content"
android:layout_below="#id/dates"
android:layout_alignParentRight="true" />
</RelativeLayout>
In the above code,id for Spinner #+id/spinner is created in other view and while defining the spinner we are referring the id created above.
So, we have to create the id if we are using the view before the view has been created.
If the view item performs the same operation, you can use the #+id for each entry in any layout because during the compilation of multiple #+id/foo the R.java file only creates one enumeration. So for example, if I have a save button on each page that performs the same operation, I use android:id="#+id/button_save" in each layout. The R.java file only has one entry for the button_save.
Difference between “#+id/” and “#id/” in Android
The first one is used for to create the ID of the particular ui component and the another one is used for to refer the particular component
There are several different ways of definition the ID:
android:id="#id/android:list"
android:id="#+id/android:list"
android:id="#+id/confirm"
What is the difference?
android:id="#id/android:list" refers to android.R.id.list. android.R contains resources that are bundled with the platform. There are lots of these standard resources, they are all listed in the JavaDoc.
#+ creates a resource, so android:id="#+id/confirm" creates a new id in your application's R class, ie. R.id.confirm. This is explained in the User Interface > Declaring Layout > ID of the Dev Guide.
I'm not sure you can do android:id="#+id/android:list", as it would mean you are trying to create a resource on the platform's resource class.
+id/label means that your resource will have an id value = label and that label belongs to your application's name space.
There are many resources bundled with android.jar file and when referring to these resources you would say android:id = "#android:id/list" which means your resource will have an id value = list and that this value belongs to android name space.
Refer the following for details:
http://groups.google.com/group/android-developers/browse_thread/thread/dc8023b221351aa7
The '+' means add it to your current project's R.java. Those without pluses are resources inherited from the framework.
The document says "#[+]id/myid" as the definition for android:id, from the notation it indicates that the plus is optional. But nowhere can I find a definition of what the plus sign means or not supplying it would mean.
What does it mean and why is it there?
#+id/foo means you are creating an id named foo in the namespace of
your application. You can refer to it using #id/foo. #android:id/foo
means you are referring to an id defined in the android namespace.
This namespace is the namespace of the framework. In this case, you
need to use #android:id/list and #android:id/empty because these are
the id the framework expects to find (the framework knows only about
the ids in the android namespace.)
Taken from one of the "Android Developer | Google Groups" discussions.
The documentation says,
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"