视图ID是否在活动上下文本地？

Question

What is the scope of the a view's identifier?

Is it local to its corresponding activity?

For example, if in activity_main.xml I specify

<EditText android:id="@+id/edit_message" />

Within my DisplayMessageActivity.java would I legally be able to access R.id.edit_message ?

When I test this I get an exception, but none of the messages are clearly telling me it's due to accessing an "id out of context", or something along those lines. I'm getting errors like java.lang.NullPointerException

This question arose as I was following the beginner tutorial:

https://developer.android.com/training/basics/firstapp/building-ui.html

EDIT Thanks both for clarifying the difference between the scope of R.id and that of the widget with id R.id.edit_message (in terms of accessing it by fiewViewById() ).

I actually intended to ask "would I be able to call findViewById(R.id.edit_message) in Activity A to access a widget in Activity B". And from your answers it appears the answer to that question is "no".

In my code, I did not include a view with id R.id.edit_message in the ViewGroup of Activity B. That's probably why I got the errors.

Answer 1

What is the scope of the a view's identifier?

The identifier is an integer. It does not have a scope.

Within my DisplayMessageActivity.java would I legally be able to access R.id.edit_message?

If we assume that you mean "would I be able to call findViewById(R.id.edit_message) to access a widget?", you can certainly call it. Whether you get null or not will depend on whether the activity has a widget in its view hierarchy with that ID.

However, Activity A cannot call findViewById() to retrieve a widget from Activity B. The id is an integer and has no scope. However, findViewById() has a scope: the view hierarchy of whatever you call it on (eg, an activity, a ViewGroup ).

Answer 2

Any id that you enter using the XML is processed and placed into a unique integer value in the R.java file (which you should not edit). ID values, in particular, go into R.id , just like the layout XML files you have would be identified by R.layout .

To answer your question, the scope is global. At least wherever you can

import your.package.name.R;

For example, you have two Activities that each have an EditText with android:id="@+id/edit_message" .
In that scenario, you can use findViewById(R.id.edit_message) in both Activities, but the value of R.id.edit_message is the same.

As far as "scoping" goes, findViewById() is the shorthand of YourActivity.this.findViewById() , so you would be searching the content view of the current Activity.

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For the reason you get the exception. Hard to say. One possibility is that you used the wrong XML layout for setContentView() and findViewById() returned you null. You can read on the documentation for why that is.