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/