Difference between "@id/" and "@+id/" in Android

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..

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

Its very simple:

"@+..." - create new

"@..." - link on existing

Source: https://developer.android.com/guide/topics/resources/layout-resource.html#idvalue