I'm trying the new Toolbar component and having some trouble with the navigation icon. I want to implement a custom icon for back navigation :
In my manifest i set a parent to my activity :
<activity android:name=".CardsActivity" android:parentActivityName=".MainActivity">
    <!-- Parent activity meta-data to support API level 7+ -->
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value=".MainActivity" />
</activity>
I declare the toolbar like this :
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.lollitest.MainActivity" >
    <android.support.v7.widget.Toolbar
        android:id="@+id/my_awesome_toolbar"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:minHeight="?attr/actionBarSize"
        android:layout_marginBottom="10dp"
        android:background="?attr/colorPrimary" />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/my_awesome_toolbar"
        android:text="@string/hello_world" />
</RelativeLayout>
Then in my activity i configure the Toolbar like this :
Toolbar toolbar = (Toolbar) findViewById(R.id.my_awesome_toolbar);
toolbar.setNavigationIcon(R.drawable.ic_good);
toolbar.setTitle("Title");
toolbar.setSubtitle("Sub");
toolbar.setLogo(R.drawable.ic_launcher);
setSupportActionBar(toolbar);
Which giving me :
 
The back icon is not the one i set with setNavigationIcon() ! Whatever drawable i give to the method the navigation icon is always the back arrow.
I have tried to remove the parent association in the manifest but the only effect is (obviously) to prevent the button to go back.
On contrary if i want the default back arrow icon and don't call setNavigationIcon() i don't have any icon at all.
What is the correct way to handle the navigation icon in toolbar (custom and default) ?
NOte : i'm running my test on Android 4.4
Currently you can use it, changing the order: (it seems to be a bug)
Toolbar toolbar = (Toolbar) findViewById(R.id.my_awesome_toolbar);
setSupportActionBar(toolbar);
toolbar.setNavigationIcon(R.drawable.ic_good);
toolbar.setTitle("Title");
toolbar.setSubtitle("Sub");
toolbar.setLogo(R.drawable.ic_launcher);
Specific to the navigation icon, this is the correct order
// get the actionbar as Toolbar and set it up
Toolbar toolbar = (Toolbar) findViewById(R.id.signIn_toolbar);
setSupportActionBar(toolbar);
Inform the Toolbar to provide back navigation. This will set the icon to the default material icon
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
Later override the icon with the custom one, in my case the Holo back icon
toolbar.setNavigationIcon(R.drawable.ic_chevron_left_white_36dp);
(The answer to user802421)
private void setToolbar() {
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    if (toolbar != null) {
        setSupportActionBar(toolbar);
        toolbar.setNavigationIcon(R.drawable.ic_action_back);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onBackPressed();
            }
        });
    }
}
toolbar.xml
<android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="@dimen/toolbar_height"
    android:background="?attr/colorPrimaryDark" />
I just found the solution. It is really very simple:
mDrawerToggle.setDrawerIndicatorEnabled(false);
Hope it will help you.
Use setNavigationIcon to change it. don't forget create ActionBarDrawerToggle first!
sample code work for me:
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    drawer = (DrawerLayout)findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();
    toolbar.setNavigationIcon(R.drawable.ic_menu);
I had simillar problem. After a big headache I found, that my ActionBarDrawerToggle was modifying the icon, also when it should not modify the icon (because I didn't give reference to toolbar to the toggle component). So  in my NavigationDrawerFragment class (that handles the opening and closing) in setUp(...) method I set mDrawerToggle.setHomeAsUpIndicator(R.drawable.app_icon); 
and finally it worked.
I tried to set up toolbar like @Gabriele Mariotti, but I had some problem with title. So then I set order to
toolbar.setTitle("Title")
setSupportActionBar(toolbar);
toolbar.setNavigationIcon(R.drawable.ic_good);
and it works.
I used the method below which really is a conundrum of all the ones above. I also found that onOptionsItemSelected is never activated.
    mDrawerToggle.setDrawerIndicatorEnabled(false);
    getSupportActionBar().setHomeButtonEnabled(true);
    Toolbar toolbar = (Toolbar) findViewById(R.id.tool_bar);
    if (toolbar != null) {
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onBackPressed();
            }
        });
    }
You can use invalidate() method to change toolbar state in any place.
Example:
Toolbar toolbar = (Toolbar)findViewById(R.id.my_awesome_toolbar);
setSupportActionBar(toolbar);
toolbar.setNavigationIcon(R.mipmap.arrow_white);
toolbar.invalidate();       // restore toolbar
Remove this line from activity if you have added
 @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }
Then set icon
 getSupportActionBar().setHomeAsUpIndicator(icon);
Try this:
  <android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:toolbar="http://schemas.android.com/apk/res-auto"
        android:id="@+id/tool_drawer"
        android:layout_width="match_parent"
        android:layout_height="?actionBarSize"
        toolbar:navigationIcon="@drawable/ic_navigation">
    </android.support.v7.widget.Toolbar>
work for me...
<android.support.v7.widget.Toolbar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/toolBar"
        android:background="@color/colorGreen"
        app:title="Title"
        app:titleTextColor="@color/colorBlack"
        app:navigationIcon="@drawable/ic_action_back"/>
In case you don't wish to set the toolbar as the action bar, you can use this:
        val toggle = ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close)
        toggle.isDrawerSlideAnimationEnabled = false
        toggle.isDrawerIndicatorEnabled = false
        toggle.setHomeAsUpIndicator(AppCompatResources.getDrawable(this, ...))
        drawer!!.addDrawerListener(toggle)
        toggle.setToolbarNavigationClickListener {
            setDrawerOpened(!isDrawerOpened())
        }
        toggle.syncState()
fun setDrawerOpened(open: Boolean) {
    if (open == drawerLayout.isDrawerOpen(GravityCompat.START))
        return
    if (open)
        drawerLayout.openDrawer(GravityCompat.START)
    else drawerLayout.closeDrawer(GravityCompat.START)
}
来源:https://stackoverflow.com/questions/26525229/toolbar-navigation-icon-never-set