back button close the activity instead of going back to main activity

Question

in AndroidManifest.xml I put this

<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 4.0 and lower -->
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="com.example.myfirstapp.MainActivity" />
</activity>

In onCreate() method I put this. I didn't override onOptionsItemSelected.

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

The problem is when I press the Back button(on the action bar) The activity get closed instead of going back! Help me to resolve this. Why I go for this method is I can easily place a back button on the left.