Display back button on action bar

I think onSupportNavigateUp() is the best and Easiest way to do so, check the below steps. Step 1 is necessary, step two have alternative.

Step 1 showing back button: Add this line in onCreate() method to show back button.

assert getSupportActionBar() != null;   //null check
getSupportActionBar().setDisplayHomeAsUpEnabled(true);   //show back button

Step 2 implementation of back click: Override this method

@Override
public boolean onSupportNavigateUp(){  
    finish();  
    return true;  
}

thats it you are done
OR Step 2 Alternative: You can add meta to the activity in manifest file as

<meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="MainActivity" />

Edit: If you are not using AppCompat Activity then do not use support word, you can use

getActionBar().setDisplayHomeAsUpEnabled(true); // In `OnCreate();`

// And override this method
@Override 
public boolean onNavigateUp(){ 
     finish(); 
     return true; 
}

Thanks to @atariguy for comment.

The magic happens in onOptionsItemSelected.

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            // app icon in action bar clicked; go home
            Intent intent = new Intent(this, HomeActivity.class);
            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            startActivity(intent);
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

Official solution

Add those two code snippets to your SubActivity

@Override
public void onCreate(Bundle savedInstanceState) {
    ...
    getActionBar().setDisplayHomeAsUpEnabled(true);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

add meta-data and parentActivity to manifest to support lower sdk.

 <application ... >
    ...
    <!-- The main/home activity (it has no parent activity) -->
    <activity
        android:name="com.example.myfirstapp.MainActivity" ...>
        ...
    </activity>
    <!-- A child of the main activity -->
    <activity
        android:name="com.example.myfirstapp.SubActivity"
        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>
</application>

Reference here:http://developer.android.com/training/implementing-navigation/ancestral.html

Add these lines to onCreate()

android.support.v7.app.ActionBar actionBar = getSupportActionBar();
   actionBar.setHomeButtonEnabled(true);
   actionBar.setDisplayHomeAsUpEnabled(true);

and in onOptionItemSelected

@Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                //Write your logic here
                this.finish();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }

Hope this will help you..!

Try this code, considers it only if you need the back button.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //YOUR CODE
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    //YOUR CODE
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    onBackPressed();
    return true;
}