new to android - understanding source of content provider

Question

As I was working through the following tutorial, I came across this code :

public void onClickRetrieveStudents(View view) {
      // Retrieve student records
      String URL = "content://com.example.provider.College/students";

I am interested to see what kind of data this is, so I tried to go to the website http://com.example.provider.College/students to view the data, however it just gave some kind of error. Therefore my question is , is this URL some kind of xml document? what exactly is the format for this data... and how can I view it ?

Answer 1

I would recommend you familiarize yourself with the following documenation:

Content Providers:

http://developer.android.com/guide/topics/providers/content-providers.html

Essentially when you pass that "URL" to the ContentResolver (presumably you're doing somethign like this):

    // Queries the user dictionary and returns results
    mCursor = getContentResolver().query(
    UserDictionary.Words.CONTENT_URI,   // The content URI of the words table
    mProjection,                        // The columns to return for each row
    mSelectionClause                    // Selection criteria
    mSelectionArgs,                     // Selection criteria
    mSortOrder);                        // The sort order for the returned rows

You're asking android to resolve that URL to a ContentProvider which is set up to handle that URL. The URL is not "imaginary" so much as it's targets are Local objects and processes which exist and are defined by applications which use the ContentProvider mechanism to store and make data available to other applications.

The goal of that URL (which is converted to a URI in this case) is to specify which ContentProvider you want, and what you want from it.

ContentProviders are generally used by applications that want to manage a database and make that information available to other applications while minimizing access violations etc..

EDIT:
This code is from your tutorial. See added comments:

    /// this url points to the content provider.  
    //The content provider uses it to 
///reference a specific database which it has knowledge of  
//This URI doesn't represent an 
//actual FILE on your system, rather it represents a way for you to tell the content //provider what DATABASE to access and what you want from it.
    String URL = "content://com.example.provider.College/students";
          //  This line converts yoru "URL" into a URI 
          Uri students = Uri.parse(URL);
          /// This call returns a Cursor - a cursor is a object type which contains the results of your QUERY in an order manner.  IN this case it is a set of rows, each of which has a number of columns coresponding to your query and database, which can be iterated over to pull information from the DB..



    /// managedQuery takes, as an argument, the URI conversion of the URL - this is
// where you are actually calling to the contentprovider, asking it to do a query on the
// databse for some information
      Cursor c = managedQuery(students, null, null, null, "name");

//   This line moves to the first ROW in the cursor
          if (c.moveToFirst()) {
            // this does somethign as long as the while loop conditional is true.
             do{
    //  This line creates a pop up toast message with the information stored in the columns of the row you the cursor is currently on.
                Toast.makeText(this, 
                c.getString(c.getColumnIndex(StudentsProvider._ID)) + 
                ", " +  c.getString(c.getColumnIndex( StudentsProvider.NAME)) + 
                ", " + c.getString(c.getColumnIndex( StudentsProvider.GRADE)), 
                Toast.LENGTH_SHORT).show();
             } while (c.moveToNext());
          }

Your question in the comments was:

"all I need is an example of this file: String URL = "content://com.example.provider.College/students"; , what would the data look like ? "

The answer to this is that you have an Sqlite Database on your phone somewhere - generally (and in this case definitely) created by the application and/or content provider you are accessing. You also know that the content resolver accepts this URI and some other information and will return you a CURSOR.

This question addresses what a cursor is.

use of cursor in android

If you read the tutorial fully you will find this code::

public class StudentsProvider extends ContentProvider {

   static final String PROVIDER_NAME = "com.example.provider.College";
   static final String URL = "content://" + PROVIDER_NAME + "/students";
   static final Uri CONTENT_URI = Uri.parse(URL);

   static final String _ID = "_id";
   static final String NAME = "name";
   static final String GRADE = "grade";

You will also find, in the manifest of your tutorial:

  <provider android:name="StudentsProvider" 
       android:authorities="com.example.provider.College">
    </provider>

Which is the registration of your ContentProvider for the URI at question.

You will note that your URL and the "PROVIDER_NAME" and "URL" have eerie similarities. This is because the ContentProvider is utilizing these values to identify itself as the resolver for this partiuclar URI to the android system.

You should create the files as described in the tutorial, make the sample app function, and you will be able to start understanding this more clearly.

Answer 2

It's not real, and it's not a web url. That is an example of a hypothetical ContentURI.

As an example, you might consult the UserDictionary like so -

// Queries the user dictionary and returns results
mCursor = getContentResolver().query(
  UserDictionary.Words.CONTENT_URI,   // The content URI of the words table
  mProjection,                        // The columns to return for each row
  mSelectionClause                    // Selection criteria
  mSelectionArgs,                     // Selection criteria
  mSortOrder);                        // The sort order for the returned rows

You might also create your own.