Toolbar - add the up button

I guess what you are looking for is something like this:

Toolbar toolbar = (Toolbar) findViewById(R.id.app_bar_detail);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);

Or in case of using in Fragment:

Toolbar toolbar = (Toolbar) view.findViewById(R.id.app_bar_detail);
((ActionBarActivity) getActivity()).setSupportActionBar(toolbar);
((ActionBarActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);

This will show up your Action Bar inside of your toolbar, but don't worry everything will fit together well. The last you have to do if you dont want any shadow under your action bar or any background of it is change your theme in vaules/styles.xml.

<style name="AppThmeme.Base" parent="Theme.AppCompat.NoActionBar">

If you're wondering why clicking on up button doesn't work with fragments, you need to set up a navigation listener as well, not sure why Google hasn't enabled it by default:

protected fun setupToolbar(toolbar: Toolbar) {
    (activity as AppCompatActivity).run {
        setSupportActionBar(toolbar)
        supportActionBar?.setDisplayHomeAsUpEnabled(true)
        toolbar.setNavigationOnClickListener { onBackPressed() }
    }
}

In case when previous activity is always same for an activity then up/back button can be achieved with the help of parentActivityName attribute. It can be mentioned in AndroidManifest.xml file as shown below:

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

Let's say DetailActivity was opened from MainActivity. So when you're on DetailActivity then tool bar will automatically show a left pointing arrow as shown in the screenshot below:

When we click on left pointing arrow then MainActivity gets displayed.

If you want to do this in XML, you can use...

<android.support.v7.widget.Toolbar
            app:navigationIcon="?homeAsUpIndicator"
            ...