Question

I couldn’t come up with a better title for this question, so please feel free to amend it if you can.

So here is my question. I decided to write a small app for myself and along the way I am also trying to learn Android Development better. I know why there is a +(plus) sign in the android:id attribute as it means “creating a new id resource”. But I don’t know why Android needs to put + in the ones used for referencing such as below:

Here I have two buttons named StartButton and StopButton and as they are new resources, the android:id contains +:

  <Button
    android:id="@+id/StartButton"
    android:layout_width="150dp"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="53dp"
    android:textSize="18dp"
    android:text="Start"
    android:onClick="StartButton_OnClick" />

<Button
    android:id="@+id/StopButton"
    android:layout_width="150dp"
    android:layout_height="wrap_content"
    android:layout_alignTop="@+id/StartButton"
    android:layout_toRightOf="@+id/StartButton"
    android:text="Stop"
    android:textSize="18dp"
    tools:context=".CallerBlockerActivity"
    android:onClick="StopButton_OnClick" />

But why android:layout_alignTop contains + puzzles me since id/StartButton is defined when @id/StopButton reaches it. Can some one please explain the necessity of this syntax?

Answer 1

It doesn’t matter either way — there only needs to be a + for the first reference to an ID. If the ID is already defined, the + is ignored anyway. You could place a + before every reference in the XML if you wanted to, and everything would work as expected. It’s only necessary for the first reference, though.