What is the difference between @id and @+id?

Question

I have just started using android, and have about 5 layout files finished. However, I just realized that I have been using @id and @+id interchangeably, but I'm not sure what the exact difference between the two are.

Answer 1

You need to use @+id when you are defining your own Id for a View.

Exactly from docs:

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.

Here is a practical example:

<Button 
   android:id="@+id/start"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
/>

<Button 
   android:id="@+id/check"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:layout_below="@id/start"
/>

So here, you created two IDs, start and check. Then, in your application you are able to connect to them with findViewById(R.id.start).

And this android:layout_below="@id/start" refer to existing id.start and means that your Button with id check will be positioned below Button with id start.

Answer 2

All the other answers forgot to mention this one little thing.

When using @id/ to refer to an already generated android resource, make sure that the resource you are referring to is defined earlier and not later.

That is Instead of this:

<Button 
 android:id="@+id/check"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_below="@id/start" 
 />
<Button 
 android:id="@+id/start"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 />

Use this:

<Button 
 android:id="@+id/start"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 />
<Button 
 android:id="@+id/check"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_below="@id/start"
 />

In the first example you are referring to a resource @id/start which is generated after you are accessing it. Although this would work in case of native android, but if you are going to use this code in react-native or ionic or any other hybrid platform, it would generate resource not found error.

So be careful to generate the resource id before using it as @id/

Answer 3

android:id="@+id/my_button"

+id Plus sing tells android to add or create a new id in Resources.

android:layout_below="@id/my_button"

it just help to refer the already generated id..

Answer 4

Sometimes you have to use + sign. E.g. when you use <include ... /> and the included file looks like this:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.FloatingActionButton xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    (...)
    app:layout_anchor="@+id/view_pager"
    app:layout_anchorGravity="top|right|end"
 />

If you don't add + in "@+id/view_pager" you will get error while building project:

Error:(9, 24) No resource found that matches the given name (at 'layout_anchor' with value '@id/view_pager').

It happend to me me in project with libraries.

Answer 5

In order to access a widget (or component) in Java or to make others dependent on it, we need a unique value to represent it. That unique value is provided by android:id attribute which essentially adds id provided as a suffix to @+id/ to the id resource file for others to query. An id for Toolbar can be defined like this,

android:id=”@+id/toolbar

The following id can now be tracked by findViewById(…) which looks for it in the res file for id, or simply R.id directory and returns the type of View in question. The other one, @id, behaves the same as findViewById(…) — looks for the component by the id provided but is reserved for layouts only. The most general use of it is to place a component relative to the component it returns.

android:layout_below=”@id/toolbar”