onNavigationItemSelected not working in NavigationView

I have also faced the same problem some time back, and at the end i realized that i have not wrote the 2nd line of the following code

navigationView = (NavigationView) findViewById(R.id.navigation_view);        
navigationView.setNavigationItemSelectedListener(this);

you make sure you have written the same otherwise your listener will not work

Use this code:

navigationView = (NavigationView) findViewById(R.id.navigationView); 
navigationView.bringToFront();

Don't use

NavigationUI.setupWithNavController(navigationView, navController);

instead of it do this

navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                Toast.makeText(MainActivity.this, "clicked", Toast.LENGTH_SHORT).show();
                return false;
            }
        });

Have a look at your MainActivity.java.

You have implemented the callbacks for NavigationView.OnNavigationItemSelectedListener in MainActivity as below,

@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
    // blah blah
}

Also check the setupDrawerContent method.

private void setupDrawerContent(NavigationView navigationView) {
    navigationView.setNavigationItemSelectedListener(
            new NavigationView.OnNavigationItemSelectedListener() {
                @Override
                public boolean onNavigationItemSelected(MenuItem menuItem) {
                    menuItem.setChecked(true);
                    drawerLayout.closeDrawers();
                    return true;
                }
            });
}

In this method you are creating a local OnNavigationItemSelectedListener.

So you are not using the OnNavigationItemSelectedListener that you have overridden in MainActivity.

The solution is to use this as argument for setNavigationItemSelectedListener. By doing this all your clicks will go the onNavigationItemSelected of MainActivity rather than going to the local onNavigationItemSelected.

private void setupDrawerContent(NavigationView navigationView) {
    navigationView.setNavigationItemSelectedListener(this);
}

Also move the code in the local onNavigationItemSelected to the onNavigationItemSelected of MainActivity.

So your onNavigationItemSelected will be something like this,

@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
    // Handle navigation view item clicks here.
    int id = menuItem.getItemId();
    menuItem.setChecked(true);
    drawerLayout.closeDrawers();

    if (id == R.id.nav_home) {
        // Handle the home action
        Toast.makeText(this, "Home", Toast.LENGTH_SHORT).show();
    } else if (id == R.id.nav_the_wetlands) {
        Toast.makeText(this, "The Wetlands", Toast.LENGTH_SHORT).show();
        TheWetlandsFragment theWetlandsFragment = new TheWetlandsFragment();
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.relativelayout_for_fragment, theWetlandsFragment, theWetlandsFragment.getTag()).commit();
    } else if (id == R.id.nav_the_mistbelt_forests) {
        Toast.makeText(this, "The Mistbelt Forests", Toast.LENGTH_SHORT).show();
    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

Also change your activity_main_drawer_view.xml as follows to solve the multiple selection issue you have in the Navigation Drawer,

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_home"
            android:icon="@drawable/ic_dashboard"
            android:title="Home" />
    </group>

    <item android:title="Information">
        <group android:checkableBehavior="single">
            <item
                android:id="@+id/nav_the_wetlands"
                android:icon="@drawable/ic_event"
                android:title="The Wetlands" />
            <item
                android:id="@+id/nav_the_mistbelt_forests"
                android:icon="@drawable/ic_event"
                android:title="The Mistbelt Forests" />
            <item
                android:id="@+id/nav_the_grasslands"
                android:icon="@drawable/ic_event"
                android:title="The Grasslands" />
        </group>
    </item>

    <item android:title="Quick Go To">
        <group android:checkableBehavior="single">
            <item
                android:id="@+id/nav_accommodation"
                android:icon="@drawable/ic_event"
                android:title="Accommodation" />
            <item
                android:id="@+id/nav_cuisine"
                android:icon="@drawable/ic_forum"
                android:title="Cuisine" />
            <item
                android:id="@+id/nav_leisure_activites"
                android:icon="@drawable/ic_forum"
                android:title="Leisure &amp; Activites" />
            <item
                android:id="@+id/nav_agri_tourism"
                android:icon="@drawable/ic_forum"
                android:title="Agri-tourism" />
            <item
                android:id="@+id/nav_education"
                android:icon="@drawable/ic_forum"
                android:title="Education" />
            <item
                android:id="@+id/nav_arts_crafts_decor"
                android:icon="@drawable/ic_forum"
                android:title="Arts, Crafts &amp; DeCor" />
            <item
                android:id="@+id/nav_selective_shopping"
                android:icon="@drawable/ic_forum"
                android:title="Selective Shopping" />
            <item
                android:id="@+id/nav_for_children"
                android:icon="@drawable/ic_forum"
                android:title="For Children" />
        </group>
    </item>

    <item android:title="Midlands Animals">
        <group android:checkableBehavior="single">
            <item
                android:id="@+id/nav_midlands_birding_checklist"
                android:icon="@drawable/ic_dashboard"
                android:title="Midlands Birding Checklist" />
            <item
                android:id="@+id/nav_midlands_mammals_checklist"
                android:icon="@drawable/ic_dashboard"
                android:title="Midlands Mammals Checklist" />
        </group>
    </item>

</menu>

Good luck.