Question

Please help me clear this doubt I have in my head once and for all, considering the following two cases.

Case 1:

public class Activity {
  WebView mWebview;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    initWebView();
  }

  public void initWebView() {
    mWebView = (WebView) findViewById(R.id.webview);
  }

}

1. Does findViewById() instantiate a
   new WebView object the second time
   it is called?
2. If the answer is ‘yes’, is the old
   WebView object (that was previously
   assigned to mWebView) automatically
   destroyed? (i.e. gets lost by being
   placed in a queue for garbage
   collection)

Case 2:

public class Activity {
  WebView mWebview;
  MyPictureListener mPictureListener;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    initWebView();
  }

  public void initWebView() {
    mWebView = (WebView) findViewById(R.id.webview);
    mPictureListener = new MyPictureListener(mWebView);
    mWebView.setPictureListener(mPictureListener);
  }

}

1. When initWebView() is called a
   second time, is the old
   MyPictureListener object (that was
   previously assigned to
   mPictureListener) automatically
   destroyed? (i.e. gets lost by being
   placed in a queue for garbage
   collection)

Answer 1

Case 1: No matter how many times you call findViewById(R.id.webview), the object returned is the same WebView that was created when the layout was inflated by the UI framework. When you assign it to your variable, you’re just saying “I want a handle to that Object,” and that’s all it is. If you set mWebView to null, the Object still exists with references to it from the Activity’s UI, you just can’t access it from mWebView anymore.

Case 2: Yes (insofar as ‘automatically destroyed’ works in Java). Your mWebView has a handle (a pointer) to a MyPictureListener Object, when you assign it to a new instance of that class, the reference to the old Object is lost, and the old Object is now a candidate for garbage collection.

This is really a [grossly generalized] mini-explanation of how pointers work, which is a concept I’ve heard some people say that Java doesn’t use. That’s completely erroneous, and getting comfortable with pointers as they are used in C++ will give you much better insight into these kinds of questions in the future.