page.title=Notepad Exercise 1
parent.title=Notepad Tutorial
parent.link=index.html
@jd:body
<p><em>In this exercise, you will construct a simple notes list that lets the
user add new notes but not edit them. The exercise demonstrates:</em></p>
<ul>
<li><em>The basics of <code>ListActivities</code> and creating and handling menu
options. </em></li>
<li><em>How to use a SQLite database to store the notes.</em></li>
<li><em>How to bind data from a database cursor into a ListView using a
SimpleCursorAdapter.</em></li>
<li><em>The basics of screen layouts, including how to lay out a list view, how
you can add items to the activity menu, and how the activity handles those menu
selections. </em></li>
</ul>
<div style="float:right;white-space:nowrap">
<span style="color:#BBB;">
[<a href="notepad-ex1.html" style="color:#BBB;">Exercise 1</a>]</span>
[<a href="notepad-ex2.html">Exercise 2</a>]
[<a href="notepad-ex3.html">Exercise 3</a>]
[<a href="notepad-extra-credit.html">Extra Credit</a>]
</div>
<h2>Step 1</h2>
<p>Open up the <code>Notepadv1</code> project in Eclipse.</p>
<p><code>Notepadv1</code> is a project that is provided as a starting point. It
takes care of some of the boilerplate work that you have already seen if you
followed the <a href="{@docRoot}resources/tutorials/hello-world.html">Hello,
World</a> tutorial.</p>
<ol>
<li>
Start a new Android Project by clicking <strong>File</strong> >
<strong>New</strong> > <strong>Android Project</strong>.</li>
<li>
In the New Android Project dialog, select <strong>Create project from existing source</strong>.</li>
<li>
Click <strong>Browse</strong> and navigate to where you copied the <code>NotepadCodeLab</code>
(downloaded during <a href="{@docRoot}resources/tutorials/notepad/index.html#preparing">setup</a>)
and select <code>Notepadv1</code>.</li>
<li>
The Project Name and other properties should be automatically filled for you.
You must select the Build Target—we recommend selecting a target with the
lowest platform version available. Also add an integer to the Min SDK Version field
that matches the API Level of the selected Build Target.</li>
<li>
Click <strong>Finish</strong>. The <code>Notepadv1</code> project should open and be
visible in your Eclipse package explorer.</li>
</ol>
<p>If you see an error about <code>AndroidManifest.xml</code>, or some
problems related to an Android zip file, right click on the project and
select <strong>Android Tools</strong> > <strong>Fix Project Properties</strong>.
(The project is looking in the wrong location for the library file,
this will fix it for you.)</p>
<h2>Step 2</h2>
<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Accessing and modifying data</h2>
<p>For this
exercise, we are using a SQLite database to store our data. This is useful
if only <em>your</em> application will need to access or modify the data. If you wish for
other activities to access or modify the data, you have to expose the data using a
{@link android.content.ContentProvider ContentProvider}.</p>
<p>If you are interested, you can find out more about
<a href="{@docRoot}guide/topics/providers/content-providers.html">content providers</a> or the
whole
subject of <a href="{@docRoot}guide/topics/data/data-storage.html">Data Storage</a>.
The NotePad sample in the <code>samples/</code> folder of the SDK also has an example of how
to create a ContentProvider.</p>
</div>
</div>
<p>Take a look at the <code>NotesDbAdapter</code> class — this class is provided to
encapsulate data access to a SQLite database that will hold our notes data
and allow us to update it.</p>
<p>At the top of the class are some constant definitions that will be used in the application
to look up data from the proper field names in the database. There is also a database creation
string defined, which is used to create a new database schema if one doesn't exist already.</p>
<p>Our database will have the name <code>data</code>, and have a single table called
<code>notes</code>, which in turn has three fields: <code>_id</code>, <code>title</code> and
<code>body</code>. The <code>_id</code> is named with an underscore convention used in a number of
places inside the Android SDK and helps keep a track of state. The <code>_id</code>
usually has to be specified when querying or updating the database (in the column projections
and so on). The other two fields are simple text fields that will store data.
</p>
<p>The constructor for <code>NotesDbAdapter</code> takes a Context, which allows it to communicate with aspects
of the Android operating system. This is quite common for classes that need to touch the
Android system in some way. The Activity class implements the Context class, so usually you will just pass
<code>this</code> from your Activity, when needing a Context.</p>
<p>The <code>open()</code> method calls up an instance of DatabaseHelper, which is our local
implementation of the SQLiteOpenHelper class. It calls <code>getWritableDatabase()</code>,
which handles creating/opening a database for us.</p>
<p><code>close()</code> just closes the database, releasing resources related to the
connection.</p>
<p><code>createNote()</code> takes strings for the title and body of a new note,
then creates that note in the database. Assuming the new note is created successfully, the
method also returns the row <code>_id</code> value for the newly created note.</p>
<p><code>deleteNote()</code> takes a <var>rowId</var> for a particular note, and deletes that note from
the database.</p>
<p><code>fetchAllNotes()</code> issues a query to return a {@link android.database.Cursor} over all notes in the
database. The <code>query()</code> call is worth examination and understanding. The first field is the
name of the database table to query (in this case <code>DATABASE_TABLE</code> is "notes").
The next is the list of columns we want returned, in this case we want the <code>_id</code>,
<code>title</code> and <code>body</code> columns so these are specified in the String array.
The remaining fields are, in order: <code>selection</code>,
<code>selectionArgs</code>, <code>groupBy</code>, <code>having</code> and <code>orderBy</code>.
Having these all <code>null</code> means we want all data, need no grouping, and will take the default
order. See {@link android.database.sqlite.SQLiteDatabase SQLiteDatabase} for more details.</p>
<p class="note"><b>Note:</b> A Cursor is returned rather than a collection of rows. This allows
Android to use resources efficiently -- instead of putting lots of data straight into memory
the cursor will retrieve and release data as it is needed, which is much more efficient for
tables with lots of rows.</p>
<p><code>fetchNote()</code> is similar to <code>fetchAllNotes()</code> but just gets one note
with the <var>rowId</var> we specify. It uses a slightly different version of the
{@link android.database.sqlite.SQLiteDatabase} <code>query()</code> method.
The first parameter (set <em>true</em>) indicates that we are interested
in one distinct result. The <var>selection</var> parameter (the fourth parameter) has been specified to search
only for the row "where _id =" the <var>rowId</var> we passed in. So we are returned a Cursor on
the one row.</p>
<p>And finally, <code>updateNote()</code> takes a <var>rowId</var>, <var>title</var> and <var>body</var>, and uses a
{@link android.content.ContentValues ContentValues} instance to update the note of the given
<var>rowId</var>.</p>
<h2 style="clear:right;">Step 3</h2>
<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Layouts and activities</h2>
<p>Most Activity classes will have a layout associated with them. The layout
will be the "face" of the Activity to the user. In this case our layout will
take over the whole screen and provide a list of notes.</p>
<p>Full screen layouts are not the only option for an Activity however. You
might also want to use a <a
href="{@docRoot}resources/faq/commontasks.html#floatingorfull">floating
layout</a> (for example, a <a
href="{@docRoot}resources/faq/commontasks.html#dialogsandalerts">dialog
or alert</a>),
or perhaps you don't need a layout at all (the Activity will be invisible
to the user unless you specify some kind of layout for it to use).</p>
</div>
</div>
<p>Open the <code>notepad_list.xml</code> file in <code>res/layout</code>
and
take a look at it. (You may have to
hit the <em>xml</em> tab, at the bottom, in order to view the XML markup.)</p>
<p>This is a mostly-empty layout definition file. Here are some
things you should know about a layout file:</p>
<ul>
<li>
All Android layout files must start with the XML header line:
<code><?xml version="1.0" encoding="utf-8"?></code>. </li>
<li>
The next definition will often (but not always) be a layout
definition of some kind, in this case a <code>LinearLayout</code>. </li>
<li>
The XML namespace of Android should always be defined in
the top level component or layout in the XML so that <code>android:</code> tags can
be used through the rest of the file:
<p><code>xmlns:android="http://schemas.android.com/apk/res/android"</code></p>
</li>
</ul>
<h2 style="clear:right;">Step 4</h2>
<p>We need to create the layout to hold our list. Add code inside
of the <code>LinearLayout</code> element so the whole file looks like this: </p>
<pre>
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<ListView android:id="@android:id/list"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<TextView android:id="@android:id/empty"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/no_notes"/>
</LinearLayout>
</pre>
<ul>
<li>
The <strong>@</strong> symbol in the id strings of the <code>ListView</code> and
<code>TextView</code> tags means
that the XML parser should parse and expand the rest of
the id string and use an ID resource.</li>
<li>
The <code>ListView</code> and <code>TextView</code> can be
thought as two alternative views, only one of which will be displayed at once.
ListView will be used when there are notes to be shown, while the TextView
(which has a default value of "No Notes Yet!" defined as a string
resource in <code>res/values/strings.xml</code>) will be displayed if there
aren't any notes to display.</li>
<li>The <code>list</code> and <code>empty</code> IDs are
provided for us by the Android platform, so, we must
prefix the <code>id</code> with <code>android:</code> (e.g., <code>@android:id/list</code>).</li>
<li>The View with the <code>empty</code> id is used
automatically when the {@link android.widget.ListAdapter} has no data for the ListView. The
ListAdapter knows to look for this name by default. Alternatively, you could change the
default empty view by using {@link android.widget.AdapterView#setEmptyView(View)}
on the ListView.
<p>
More broadly, the <code>android.R</code> class is a set of predefined
resources provided for you by the platform, while your project's
<code>R</code> class is the set of resources your project has defined.
Resources found in the <code>android.R</code> resource class can be
used in the XML files by using the <code>android:</code> name space prefix
(as we see here).</p>
</li>
</ul>
<h2 style="clear:right;">Step 5</h2>
<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Resources and the R class</h2>
<p>The folders under res/ in the Eclipse project are for resources.
There is a <a href="{@docRoot}resources/faq/commontasks.html#filelist">specific structure</a>
to the
folders and files under res/.</p>
<p>Resources defined in these folders and files will have
corresponding entries in the R class allowing them to be easily accessed
and used from your application. The R class is automatically generated using the contents
of the res/ folder by the eclipse plugin (or by aapt if you use the command line tools).
Furthermore, they will be bundled and deployed for you as part of the application.</p>
</p>
</div>
</div>
<p>To make the list of notes in the ListView, we also need to define a View for each row:</p>
<ol>
<li>
Create a new file under <code>res/layout</code> called
<code>notes_row.xml</code>. </li>
<li>
Add the following contents (note: again the XML header is used, and the
first node defines the Android XML namespace)<br>
<pre style="overflow:auto">
<?xml version="1.0" encoding="utf-8"?>
<TextView android:id="@+id/text1"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/></pre>
<p>
This is the View that will be used for each notes title row — it has only
one text field in it. </p>
<p>In this case we create a new id called <code>text1</code>. The
<strong>+</strong> after the <strong>@</strong> in the id string indicates that the id should
be automatically created as a resource if it does not already exist, so we are defining
<code>text1</code> on the fly and then using it.</p>
</li>
<li>Save the file.</li>
</ol>
<p>Open the <code>R.java</code> class in the
project and look at it, you should see new definitions for
<code>notes_row</code> and <code>text1</code> (our new definitions)
meaning we can now gain access to these from the our code. </p>
<h2 style="clear:right;">Step 6</h2>
<p>Next, open the <code>Notepadv1</code> class in the source. In the following steps, we are going to
alter this class to become a list adapter and display our notes, and also
allow us to add new notes.</p>
<p><code>Notepadv1</code> will inherit from a subclass
of <code>Activity</code> called a <code>ListActivity</code>,
which has extra functionality to accommodate the kinds of
things you might want to do with a list, for
example: displaying an arbitrary number of list items in rows on the screen,
moving through the list items, and allowing them to be selected.</p>
<p>Take a look through the existing code in <code>Notepadv1</code> class.
There is a currently an unused private field called <code>mNoteNumber</code> that
we will use to create numbered note titles.</p>
<p>There are also three override methods defined:
<code>onCreate</code>, <code>onCreateOptionsMenu</code> and
<code>onOptionsItemSelected</code>; we need to fill these
out:</p>
<ul>
<li><code>onCreate()</code> is called when the activity is
started — it is a little like the "main" method for an Activity. We use
this to set up resources and state for the activity when it is
running.</li>
<li><code>onCreateOptionsMenu()</code> is used to populate the
menu for the Activity. This is shown when the user hits the menu button,
and
has a list of options they can select (like "Create
Note"). </li>
<li><code>onOptionsItemSelected()</code> is the other half of the
menu equation, it is used to handle events generated from the menu (e.g.,
when the user selects the "Create Note" item).
</li>
</ul>
<h2>Step 7</h2>
<p>Change the inheritance of <code>Notepadv1</code> from
<code>Activity</code>
to <code>ListActivity</code>:</p>
<pre>public class Notepadv1 extends ListActivity</pre>
<p>Note: you will have to import <code>ListActivity</code> into the
Notepadv1
class using Eclipse, <strong>ctrl-shift-O</strong> on Windows or Linux, or
<strong>cmd-shift-O</strong> on the Mac (organize imports) will do this for you
after you've written the above change.</p>
<h2>Step 8</h2>
<p>Fill out the body of the <code>onCreate()</code> method.</p>
<p>Here we will set the title for the Activity (shown at the top of the
screen), use the <code>notepad_list</code> layout we created in XML,
set up the <code>NotesDbAdapter</code> instance that will
access notes data, and populate the list with the available note
titles:</p>
<ol>
<li>
In the <code>onCreate</code> method, call <code>super.onCreate()</code> with the
<code>savedInstanceState</code> parameter that's passed in.</li>
<li>
Call <code>setContentView()</code> and pass <code>R.layout.notepad_list</code>.</li>
<li>
At the top of the class, create a new private class field called <code>mDbHelper</code> of class
<code>NotesDbAdapter</code>.
</li>
<li>
Back in the <code>onCreate</code> method, construct a new
<code>NotesDbAdapter</code>
instance and assign it to the <code>mDbHelper</code> field (pass
<code>this</code> into the constructor for <code>DBHelper</code>)
</li>
<li>
Call the <code>open()</code> method on <code>mDbHelper</code> to open (or create) the
database.
</li>
<li>
Finally, call a new method <code>fillData()</code>, which will get the data and
populate the ListView using the helper — we haven't defined this method yet. </li>
</ol>
<p>
<code>onCreate()</code> should now look like this:</p>
<pre>
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.notepad_list);
mDbHelper = new NotesDbAdapter(this);
mDbHelper.open();
fillData();
}</pre>
<p>And be sure you have the <code>mDbHelper</code> field definition (right
under the mNoteNumber definition): </p>
<pre> private NotesDbAdapter mDbHelper;</pre>
<h2>Step 9</h2>
<div class="sidebox-wrapper">
<div class="sidebox">
<h2>More about menus</h2>
<p>The notepad application we are constructing only scratches the
surface with <a href="{@docRoot}resources/faq/commontasks.html#addmenuitems">menus</a>. </p>
<p>You can also <a href="{@docRoot}resources/faq/commontasks.html#menukeyshortcuts">add
shortcut keys for menu items</a>, <a
href="{@docRoot}resources/faq/commontasks.html#menukeyshortcuts">create
submenus</a> and even <a href="{@docRoot}resources/faq/commontasks.html#addingtoothermenus">add
menu items to other applications!</a>. </p>
</div>
</div>
<p>Fill out the body of the <code>onCreateOptionsMenu()</code> method.</p>
<p>We will now create the "Add Item" button that can be accessed by pressing the menu
button on the device. We'll specify that it occupy the first position in the menu.</p>
<ol>
<li>
In <code>strings.xml</code> resource (under <code>res/values</code>), add
a new string named "menu_insert" with its value set to <code>Add Item</code>:
<pre><string name="menu_insert">Add Item</string></pre>
<p>Then save the file and return to <code>Notepadv1</code>.</p>
</li>
<li>Create a menu position constant at the top of the class:
<pre>public static final int INSERT_ID = Menu.FIRST;</pre>
</li>
<li>In the <code>onCreateOptionsMenu()</code> method, change the
<code>super</code> call so we capture the boolean return as <code>result</code>. We'll return this value at the end.</li>
<li>Then add the menu item with <code>menu.add()</code>.</li>
</ol>
<p>The whole method should now look like this:
<pre>
@Override
public boolean onCreateOptionsMenu(Menu menu) {
boolean result = super.onCreateOptionsMenu(menu);
menu.add(0, INSERT_ID, 0, R.string.menu_insert);
return result;
}</pre>
<p>The arguments passed to <code>add()</code> indicate: a group identifier for this menu (none,
in this case), a unique ID (defined above), the order of the item (zero indicates no preference),
and the resource of the string to use for the item.</p>
<h2 style="clear:right;">Step 10</h2>
<p>Fill out the body of the <code>onOptionsItemSelected()</code> method:</p>
<p>This is going
to handle our new "Add Note" menu item. When this is selected, the
<code>onOptionsItemSelected()</code> method will be called with the
<code>item.getId()</code> set to <code>INSERT_ID</code> (the constant we
used to identify the menu item). We can detect this, and take the
appropriate actions:</p>
<ol>
<li>
The <code>super.onOptionsItemSelected(item)</code> method call goes at the
end of this method — we want to catch our events first! </li>
<li>
Write a switch statement on <code>item.getItemId()</code>.
<p>In the case of <var>INSERT_ID</var>, call a new method, <code>createNote()</code>,
and return true, because we have handled this event and do not want to
propagate it through the system.</p>
</li>
<li>Return the result of the superclass' <code>onOptionsItemSelected()</code>
method at the end.</li>
</ol>
<p>
The whole <code>onOptionsItemSelect()</code> method should now look like
this:</p>
<pre>
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case INSERT_ID:
createNote();
return true;
}
return super.onOptionsItemSelected(item);
}</pre>
<h2>Step 11</h2>
<p>Add a new <code>createNote()</code> method:</p>
<p>In this first version of
our application, <code>createNote()</code> is not going to be very useful.
We will simply
create a new note with a title assigned to it based on a counter ("Note 1",
"Note 2"...) and with an empty body. At present we have no way of editing
the contents of a note, so for now we will have to be content making one
with some default values:</p>
<ol>
<li>Construct the name using "Note" and the counter we defined in the class: <code>
String noteName = "Note " + mNoteNumber++</code></li>
<li>
Call <code>mDbHelper.createNote()</code> using <code>noteName</code> as the
title and <code>""</code> for the body
</li>
<li>
Call <code>fillData()</code> to populate the list of notes (inefficient but
simple) — we'll create this method next.</li>
</ol>
<p>
The whole <code>createNote()</code> method should look like this: </p>
<pre>
private void createNote() {
String noteName = "Note " + mNoteNumber++;
mDbHelper.createNote(noteName, "");
fillData();
}</pre>
<h2>Step 12</h2>
<div class="sidebox-wrapper">
<div class="sidebox">
<h2>List adapters</h2>
<p>Our example uses a {@link android.widget.SimpleCursorAdapter
SimpleCursorAdapter} to bind a database {@link android.database.Cursor Cursor}
into a ListView, and this is a common way to use a {@link android.widget.ListAdapter
ListAdapter}. Other options exist like {@link android.widget.ArrayAdapter ArrayAdapter} which
can be used to take a List or Array of in-memory data and bind it in to
a list as well.</p>
</div>
</div>
<p>Define the <code>fillData()</code> method:</p>
<p>This
method uses <code>SimpleCursorAdapter,</code> which takes a database <code>Cursor</code>
and binds it to fields provided in the layout. These fields define the row elements of our list
(in this case we use the <code>text1</code> field in our
<code>notes_row.xml</code> layout), so this allows us to easily populate the list with
entries from our database.</p>
<p>To do this we have to provide a mapping from the <code>title</code> field in the returned Cursor, to
our <code>text1</code> TextView, which is done by defining two arrays: the first a string array
with the list of columns to map <em>from</em> (just "title" in this case, from the constant
<code>NotesDbAdapter.KEY_TITLE</code>) and, the second, an int array
containing references to the views that we'll bind the data <em>into</em>
(the <code>R.id.text1</code> TextView).</p>
<p>This is a bigger chunk of code, so let's first take a look at it:</p>
<pre>
private void fillData() {
// Get all of the notes from the database and create the item list
Cursor c = mDbHelper.fetchAllNotes();
startManagingCursor(c);
String[] from = new String[] { NotesDbAdapter.KEY_TITLE };
int[] to = new int[] { R.id.text1 };
// Now create an array adapter and set it to display using our row
SimpleCursorAdapter notes =
new SimpleCursorAdapter(this, R.layout.notes_row, c, from, to);
setListAdapter(notes);
}</pre>
<p>Here's what we've done:</p>
<ol>
<li>
After obtaining the Cursor from <code>mDbHelper.fetchAllNotes()</code>, we
use an Activity method called
<code>startManagingCursor()</code> that allows Android to take care of the
Cursor lifecycle instead of us needing to worry about it. (We will cover the implications
of the lifecycle in exercise 3, but for now just know that this allows Android to do some
of our resource management work for us.)</li>
<li>
Then we create a string array in which we declare the column(s) we want
(just the title, in this case), and an int array that defines the View(s)
to which we'd like to bind the columns (these should be in order, respective to
the string array, but here we only have one for each).</li>
<li>
Next is the SimpleCursorAdapter instantiation.
Like many classes in Android, the SimpleCursorAdapter needs a Context in order to do its
work, so we pass in <code>this</code> for the context (since subclasses of Activity
implement Context). We pass the <code>notes_row</code> View we created as the receptacle
for the data, the Cursor we just created, and then our arrays.</li>
</ol>
<p>
In the future, remember that the mapping between the <strong>from</strong> columns and <strong>to</strong> resources
is done using the respective ordering of the two arrays. If we had more columns we wanted
to bind, and more Views to bind them in to, we would specify them in order, for example we
might use <code>{ NotesDbAdapter.KEY_TITLE, NotesDbAdapter.KEY_BODY }</code> and
<code>{ R.id.text1, R.id.text2 }</code> to bind two fields into the row (and we would also need
to define text2 in the notes_row.xml, for the body text). This is how you can bind multiple fields
into a single row (and get a custom row layout as well).</p>
<p>
If you get compiler errors about classes not being found, ctrl-shift-O or
(cmd-shift-O on the mac) to organize imports.
</p>
<h2 style="clear:right;">Step 13</h2>
<p>Run it!
<ol>
<li>
Right click on the <code>Notepadv1</code> project.</li>
<li>
From the popup menu, select <strong>Run As</strong> >
<strong>Android Application</strong>.</li>
<li>
If you see a dialog come up, select Android Launcher as the way of running
the application (you can also use the link near the top of the dialog to
set this as your default for the workspace; this is recommended as it will
stop the plugin from asking you this every time).</li>
<li>Add new notes by hitting the menu button and selecting <em>Add
Item</em> from the menu.</li>
</ol>
<h2 style="clear:right;">Solution and Next Steps</h2>
<p>You can see the solution to this class in <code>Notepadv1Solution</code>
from
the zip file to compare with your own.</p>
<p>Once you are ready, move on to <a href="notepad-ex2.html">Tutorial
Exercise 2</a> to add the ability to create, edit and delete notes.</p>