Each content provider exposes a public URI (wrapped as a Ui object) that uniquely identifies its data set. A content provider that controls multiple data sets (multiple tables) exposes a separate URI for each one. All URIs for providers begin with the string "content://". The content: scheme identifies the data as being controlled by a content provider.

If you're defining a content provider, it's a good idea to also define a constant for its URI, to simplify client code and make future updates cleaner. Android defines CONTENT URI constants for all the providers that come with the platform. For example, the URI for the table that matches phone numbers to people and the URI for the table that holds pictures of people (both controlled by the Contacts content provider) are:

To restrict a query to just one record, you can append the ID value for that record to the URI — that is, place a string matching the ID as the last segment of the path part of the URI. For example, if the ID is 23, the URI would be:

There are some helper methods, particularly ContentUris.withAppendedId() and Uri. withAppendedPath() that make it easy to append an ID to a URI. Both are static methods that return a Uri object with the ID added. So, for example, if you were looking for record 23 in the database of people contacts, you might construct a query as follows:

import android.provider.Contacts.People; import android.content.ContentUris; import android.net.Uri; import android.database.Cursor;

//Use the ContentUris method to produce the base URI for contact with ID==23 Uri myPerson = ContentUris.withAppendedId(People.CONTENT_URI, 23); // Alternatively, use the Uri method to produce the base URI. // It takes a string rather than an integer.

Uri myPerson = Uri.withAppendedPath(People.CONTENT_URI, "23"); // Then query for this specific record:

Cursor cur = managedQuery(myPerson, null, null, null, null);

The other arguments to the query() and managedQuery() methods delimit the query in more detail. They are:

• The names of the data columns that should be returned. A null value returns all columns. Otherwise, only columns that are listed by name are returned. All the content providers that come with the platform define constants for their columns. For example, the android.provider.Contacts.Phones class defines constants for the names of the columns in the phone table illustrated earlier &mdash ID,


• A filter detailing which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). A null value returns all rows (unless the URI limits the query to a single record).

• Selection arguments.

• A sorting order for the rows that are returned, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). A null value returns the records in the default order for the table, which may be unordered.

Let's look at an example query to retrieve a list of contact names and their primary phone numbers:

import android.provider.Contacts.People; import android.database.Cursor;

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