ActionBar Back Button

Activity.java

public class AktuellesDetail extends AppCompatActivity {

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

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

AndroidManifest.xml

<activity
  android:name=".content.Detail"
  android:label="@string/title_activity_detail" />