Main Activity Class (Cont'd)

MainActivity.java

public class MainActivity extends AppCompatActivity {
    MenuItem mMenuItem;
    DrawerLayout mDrawerLayout;

    private ActionBarDrawerToggle mActionBarDrawerToggle;
  
    @Override
    protected void onCreate(@Nullable final Bundle savedInstanceState) {
        mDrawerLayout = (DrawerLayout) findViewById(R.id.dl_main);
        final NavigationView navigationView = (NavigationView) findViewById(R.id.nv_main);
        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(@NonNull final MenuItem menuItem) {
                        mMenuItem = menuItem;
                        menuItem.setChecked(true);
                        mDrawerLayout.closeDrawers();

                        return true;
                    }
                });

        mActionBarDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, toolbar, R.string.drawer_open, R.string.drawer_close) {
            @Override
            public void onDrawerOpened(final View drawerView) {
                super.onDrawerOpened(drawerView);
            }

            @Override
            public void onDrawerClosed(final View drawerView) {
                super.onDrawerClosed(drawerView);

                if (null != mMenuItem) {
                    switchNavigationMenu(mMenuItem.getItemId());
                }
            }

            @Override
            public void onDrawerSlide(final View drawerView, final float slideOffset) {
                super.onDrawerSlide(drawerView, slideOffset);

                if (null != drawerView && drawerView == navigationView) {
                    super.onDrawerSlide(drawerView, 0);
                } else {
                    super.onDrawerSlide(drawerView, slideOffset);
                }
            }
        };

        mDrawerLayout.addDrawerListener(mActionBarDrawerToggle);
    }
    
    @Override
    protected void onPostCreate(@Nullable final Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        mActionBarDrawerToggle.syncState();
        showHomeFragment();
    }

    @Override
    public void onBackPressed() {
        if (mDrawerLayout.isDrawerVisible(GravityCompat.START)) {
            mDrawerLayout.closeDrawers();
        } else {
            finish();
        }
    }

    public void switchNavigationMenu(final int itemId) {
        switch (itemId) {
            case R.id.menu_home: {
                showHomeFragment();
                break;
            }
            case R.id.menu_detail: {
                showDetailFragment();
                break;
            }
        }
    }
    
    private void showHomeFragment() {
        final String tag = HomeFragment.class.getSimpleName();

        if (getSupportFragmentManager().findFragmentByTag(tag) == null) {
            final Fragment fragment = new HomeFragment();
            final FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fl_main, fragment, tag);
            fragmentTransaction.commit();
        }
    }

    private void showDetailFragment() {
        final String tag = DetailFragment.class.getSimpleName();

        if (getSupportFragmentManager().findFragmentByTag(tag) == null) {
            final Fragment fragment = new DetailFragment();
            final FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fl_main, fragment, tag);
            fragmentTransaction.commit();
        }
    }
}