Differences between up and back button in Android

Question

I'm trying to create a basic "Find my nearest..." application as a learning exercise.

I have 3 activities:

1. A main activity, where the user enters his address, this passes an array of objects to the list activity via the intent.
2. A list activity, which shows the nearest 10 places. When an item is selected this passes an object to the detail activity via the intent.
3. A detail activity, showing the detail for the selected place.

I have most things working, however I have noticed a slight problem, arising the difference from the back button (at the bottom of my phone screen), and the up button (the on action bar). After researching the difference between the two, it would appear they should behave the same in this scenario. When hitting the back button, everything works as intended, and we still see the same list. However, when hitting the up button, it appears to startup / resume the list activity in a different way, and the data from the list has disappeared.

Would anyone be able to explain to me why this is happening and how to prevent it?

My code is below: (any criticisms about my Android / Java techniques are very welcome!)

MainActivity.java

public class MainActivity extends Activity {

    public final static String TRACKS = "com.example.firstapp.TRACKS";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    /** Called when the user clicks the 'Search' button */
    public void search(View view) {

        // Get Address Text
        // EditText editText = (EditText) findViewById(R.id.edit_address);
        // String address = editText.getText().toString();

        // Get tracks near to address
        Track[] trackArray = new Track[20];

        for(int i=0; i < trackArray.length; i++) {
            trackArray[i] = new Track("Track " + i, "Address" + i, (float) Math.random()*10);
        }

        // Fire intent to show results
        Intent intent = new Intent(this, TrackListActivity.class);
        intent.putExtra(TRACKS, trackArray);
        startActivity(intent);
    }

}

TrackListActivity.java

public class TrackListActivity extends ListActivity {

    public final static String TRACK = "com.mycroft.runtrackdir.TRACK";

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        Intent intent = getIntent();

        if (intent.hasExtra(MainActivity.TRACKS)) {
            Parcelable[] tempTrackArray = intent.getParcelableArrayExtra(MainActivity.TRACKS);        

            // Cast Parcelable to Track
            Track[] trackArray = new Track[tempTrackArray.length];
            for(int i=0; i < trackArray.length; i++) {
                trackArray[i] = (Track) tempTrackArray[i];
            }

            ArrayAdapter<Track> arrayAdapter = new ArrayAdapter<Track>(this, android.R.layout.simple_list_item_1, trackArray);
            setListAdapter(arrayAdapter);
        }
    }

    @Override
    protected void onListItemClick (ListView l, View v, int position, long id) {
        Track clickedTrack = (Track) l.getItemAtPosition(position);

        // Fire intent to show detail
        Intent intent = new Intent(this, TrackDetailActivity.class);
        intent.putExtra(TRACK, clickedTrack);
        startActivity(intent);
    }
}

TrackDetailActivity.java

public class TrackDetailActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Intent intent = getIntent();
        Track track = intent.getParcelableExtra(TrackListActivity.TRACK);

        setContentView(R.layout.activity_track_detail);

        TextView track_name = (TextView) findViewById(R.id.track_name);
        TextView track_address = (TextView) findViewById(R.id.track_address);
        TextView track_distance = (TextView) findViewById(R.id.track_distance);

        track_name.setText(track.name);
        track_address.setText(track.address);
        track_distance.setText(track.distance.toString());

    }
}

Manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.firstapp"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.firstapp.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="com.example.firstapp.TrackListActivity"
            android:label="@string/title_activity_track_list"
            android:parentActivityName="com.example.firstapp.MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.firstapp.MainActivity" />
        </activity>
        <activity
            android:name="com.example.firstapp.TrackDetailActivity"
            android:label="@string/title_activity_track_detail"
            android:parentActivityName="com.example.firstapp.TrackListActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.firstapp.TrackListActivity" />
        </activity>
    </application>

</manifest>

Answer 1

I think you should modify your Manifest.xml file and tell your Manifest file the relationships between your activities. This should solve the problem.

For Example:

<application ... >
...
<!-- The main/home activity (has no parent activity) -->
<activity
    android:name="com.example.myfirstapp.MainActivity" ...>
    ...
</activity>
<!-- A child of the main activity -->
<activity
    android:name="com.example.myfirstapp.DisplayMessageActivity"
    android:label="@string/title_activity_display_message"
    android:parentActivityName="com.example.myfirstapp.MainActivity" >
    <!-- Parent activity meta-data to support API level 7+ -->
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="com.example.myfirstapp.MainActivity" />
</activity>

You can also review this document.

EDIT:
You can also use Data Persistence, if you are receiving empty Intents in certain Activities. for Data persistence in Android, you can use Shared Preferences, SQLite Database, Serialization, etc

As you are learning Android Development, you can simply put the trackArray values in Shared Preferences of your application while starting List Activity from Main Activity and Load the data of trackArray from Shared Preferences in List Activity in onResume and populate it in onCreate. This way, your Action bar's up button will not make the data of ListView Activity disappear.

P.S: Shared pref's actual purpose is to store values that are less in size and are changed occasionally such as Setting's data of a specific Application. Therefore, please don't misuse it.