"Back button" using getSupportActionbar and appcompat v7 toolbar

Not sure if this works in OP's case, but in many cases this is probably the simplest option to implement Back button with the AppCompat Toolbar.

Skip all the setHomeButtonEnabled, setDisplayHomeAsUpEnabled and onOptionsItemSelected stuff, and related issues.

Instead, when initialising the Toolbar, simply set 1) navigation icon and 2) navigation OnClickListener for it:

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

if (enableBackNavigation) {
    toolbar.setNavigationIcon(R.drawable.ic_back);
    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onBackPressed();
        }
    });
}

You can do it like this:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);  
    toolbar = (Toolbar)findViewById(R.id.toolbar);
    if (toolbar != null) {
      setSupportActionBar(toolbar);
      getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }

    setUpNavigationDrawer();

    getFragmentManager().addOnBackStackChangedListener(backStackListener); // listen to the backstack of the fragment manager
}

Define the onBackSTackChangedListener:

private FragmentManager.OnBackStackChangedListener backStackListener = new FragmentManager.OnBackStackChangedListener() {
   @Override
   public void onBackStackChanged() {
       setNavIcon();
   };
}

Set the icon according to your fragment's backstack:

protected void setNavIcon() {
    int backStackEntryCount = getFragmentManager().getBackStackEntryCount();
    drawerToggle.setDrawerIndicatorEnabled(backStackEntryCount == 0);
}

Detect when the drawer icon is pressed:

public boolean onOptionsItemSelected(MenuItem item) {
    if (drawerToggle.isDrawerIndicatorEnabled() && drawerToggle.onOptionsItemSelected(item)) {
        return true;
    }

    switch (item.getItemId()) {
      case x:
         return true;
      default:
         return false;
    }
}

And handle the up button:

public boolean onSupportNavigateUp() {
    onBackPressed();
    return true;
}

This works for me. Good luck.

Add this method in onCreate():

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

Then override the onOptionItemSelected() as below:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            onBackPressed();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}