告诉两个具有相同ID的Android GUI对象之间的区别

Question

Say we have two XML files, of which these are snippets:

firstxml.xml

<?xml version="1.0" encoding="utf-8"?>
<Button
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:id="@+id/button1 />

secondxml.xml

<?xml version="1.0" encoding="utf-8"?>
<Button
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:id="@+id/button1 />

In our onCreate() method:

//Assume all packages necessary are imported
public class Example
{
    public void onCreate(Bundle sis)
    {
      super.onCreate(sis);
      setContentView(R.layout.firstxml);

      Button thisButton = (Button) findViewbyId(R.id.button1);

    }
}

Which button is called and instantiated when this code runs? Since the two buttons are in two different files, will the button in firstxml.xml be called because it is the content view of the function?

Thanks in advance for your answers!

Answer 1

When you set the firstxml to be the content view, that layout gets inflated. findViewById will find the id's in the current layout that's inflated rather than on every layout.

setContentView method description kind of answers your question.

Answer 2

The code will call and instantiate the button1 set on your firstxml.xml file.

Note For future readers. If the layout file doesnot contain button1 it will throw null pointer exception and the editor wont know that there is an error. This is because R.id.button1 is static field in your R.java class. Also provide better naming convention to your variables for maintainability.

Answer 3

Which button is called and instantiated when this code runs? 

Since you used the firstxml.xml as the layout of the activity you are calling the reference of that button from that xml not the button from the secondxml.xml .

And in your R.java it will only generate one instance of that id.

sample:

public static final class id {
    public static final int button1=0x7f090009;
}

so when you used that reference of that button it will first find/check the layout's id if it exist if it does not exist then it will throw NPE .

Answer 4

You are referencing the the first XML layout file R.layout.firstxml in the activity's onCreate(Bundle...) method, so the search here will be made.

Any call to findViewById(int id); will search in your inflated layout R.layout.firstxml .

The R.java file is automatically generated when you are defining/adding a new view to your layout.

You can use the same id multiple times, but not in the same layout !