title: Having trouble understanding the navigation draw pre made code in android studio
body: <p>I have been trying to understand how to make an app using a navigation drawer to change fragments. Instead of creating an empty project I created one with the pre-made navigation drawer to maybe try and get a better idea of how it works, so I can develope my app with one.</p>
  
  <p>My problem is I can not figure out where the fragments get changed when something is selected in the navigation drawer. It seems that it is just changing text in the one fragment when an item is selected. What I want to do is make my own list of items for the navigation drawer and each one loads a different fragment...</p>
  
  <p>Here is the example code android studio created that I am trying to understand:</p>
  
  <p>MainActivity:</p>
  
  <pre><code>package com.example.myapplication1;
  
  import android.app.Activity;
  import android.support.v7.app.ActionBarActivity;
  import android.support.v7.app.ActionBar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentManager;
  import android.content.Context;
  import android.os.Build;
  import android.os.Bundle;
  import android.view.Gravity;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.view.View;
  import android.view.ViewGroup;
  import android.support.v4.widget.DrawerLayout;
  import android.widget.ArrayAdapter;
  import android.widget.TextView;
  
  public class MainActivity extends ActionBarActivity
          implements NavigationDrawerFragment.NavigationDrawerCallbacks {
  
      /**
       * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
       */
      private NavigationDrawerFragment mNavigationDrawerFragment;
  
      /**
       * Used to store the last screen title. For use in {@link #restoreActionBar()}.
       */
      private CharSequence mTitle;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          mNavigationDrawerFragment = (NavigationDrawerFragment)
                  getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
          mTitle = getTitle();
  
          // Set up the drawer.
          mNavigationDrawerFragment.setUp(
                  R.id.navigation_drawer,
                  (DrawerLayout) findViewById(R.id.drawer_layout));
      }
  
      @Override
      public void onNavigationDrawerItemSelected(int position) {
          // update the main content by replacing fragments
          FragmentManager fragmentManager = getSupportFragmentManager();
          fragmentManager.beginTransaction()
                  .replace(R.id.container, PlaceholderFragment.newInstance(position + 1))
                  .commit();
      }
  
      public void onSectionAttached(int number) {
          switch (number) {
              case 1:
                  mTitle = getString(R.string.title_section1);
                  break;
              case 2:
                  mTitle = getString(R.string.title_section2);
                  break;
              case 3:
                  mTitle = getString(R.string.title_section3);
                  break;
          }
      }
  
      public void restoreActionBar() {
          ActionBar actionBar = getSupportActionBar();
          actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
          actionBar.setDisplayShowTitleEnabled(true);
          actionBar.setTitle(mTitle);
      }
  
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          if (!mNavigationDrawerFragment.isDrawerOpen()) {
              // Only show items in the action bar relevant to this screen
              // if the drawer is not showing. Otherwise, let the drawer
              // decide what to show in the action bar.
              getMenuInflater().inflate(R.menu.main, menu);
              restoreActionBar();
              return true;
          }
          return super.onCreateOptionsMenu(menu);
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          // Handle action bar item clicks here. The action bar will
          // automatically handle clicks on the Home/Up button, so long
          // as you specify a parent activity in AndroidManifest.xml.
          switch (item.getItemId()) {
              case R.id.action_settings:
                  return true;
          }
          return super.onOptionsItemSelected(item);
      }
  
      /**
       * A placeholder fragment containing a simple view.
       */
      public static class PlaceholderFragment extends Fragment {
          /**
           * The fragment argument representing the section number for this
           * fragment.
           */
          private static final String ARG_SECTION_NUMBER = "section_number";
  
          /**
           * Returns a new instance of this fragment for the given section
           * number.
           */
          public static PlaceholderFragment newInstance(int sectionNumber) {
              PlaceholderFragment fragment = new PlaceholderFragment();
              Bundle args = new Bundle();
              args.putInt(ARG_SECTION_NUMBER, sectionNumber);
              fragment.setArguments(args);
              return fragment;
          }
  
          public PlaceholderFragment() {
          }
  
          @Override
          public View onCreateView(LayoutInflater inflater, ViewGroup container,
                  Bundle savedInstanceState) {
              View rootView = inflater.inflate(R.layout.fragment_main, container, false);
              TextView textView = (TextView) rootView.findViewById(R.id.section_label);
              textView.setText(Integer.toString(getArguments().getInt(ARG_SECTION_NUMBER)));
              return rootView;
          }
  
          @Override
          public void onAttach(Activity activity) {
              super.onAttach(activity);
              ((MainActivity) activity).onSectionAttached(
                      getArguments().getInt(ARG_SECTION_NUMBER));
          }
      }
  
  }
  </code></pre>
  
  <p>NavigationDrawerFragment:</p>
  
  <pre><code>package com.example.myapplication1;
  
  import android.support.v7.app.ActionBarActivity;;
  import android.app.Activity;
  import android.support.v7.app.ActionBar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.ActionBarDrawerToggle;
  import android.support.v4.view.GravityCompat;
  import android.support.v4.widget.DrawerLayout;
  import android.content.SharedPreferences;
  import android.content.res.Configuration;
  import android.os.Bundle;
  import android.preference.PreferenceManager;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.MenuInflater;
  import android.view.MenuItem;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.AdapterView;
  import android.widget.ArrayAdapter;
  import android.widget.ListView;
  import android.widget.Toast;
  
  /**
   * Fragment used for managing interactions for and presentation of a navigation drawer.
   * See the &lt;a href="https://developer.android.com/design/patterns/navigation-drawer.html#Interaction"&gt;
   * design guidelines&lt;/a&gt; for a complete explanation of the behaviors implemented here.
   */
  public class NavigationDrawerFragment extends Fragment {
  
      /**
       * Remember the position of the selected item.
       */
      private static final String STATE_SELECTED_POSITION = "selected_navigation_drawer_position";
  
      /**
       * Per the design guidelines, you should show the drawer on launch until the user manually
       * expands it. This shared preference tracks this.
       */
      private static final String PREF_USER_LEARNED_DRAWER = "navigation_drawer_learned";
  
      /**
       * A pointer to the current callbacks instance (the Activity).
       */
      private NavigationDrawerCallbacks mCallbacks;
  
      /**
       * Helper component that ties the action bar to the navigation drawer.
       */
      private ActionBarDrawerToggle mDrawerToggle;
  
      private DrawerLayout mDrawerLayout;
      private ListView mDrawerListView;
      private View mFragmentContainerView;
  
      private int mCurrentSelectedPosition = 0;
      private boolean mFromSavedInstanceState;
      private boolean mUserLearnedDrawer;
  
      public NavigationDrawerFragment() {
      }
  
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
  
          // Read in the flag indicating whether or not the user has demonstrated awareness of the
          // drawer. See PREF_USER_LEARNED_DRAWER for details.
          SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(getActivity());
          mUserLearnedDrawer = sp.getBoolean(PREF_USER_LEARNED_DRAWER, false);
  
          if (savedInstanceState != null) {
              mCurrentSelectedPosition = savedInstanceState.getInt(STATE_SELECTED_POSITION);
              mFromSavedInstanceState = true;
          }
  
          // Select either the default item (0) or the last selected item.
          selectItem(mCurrentSelectedPosition);
  
          // Indicate that this fragment would like to influence the set of actions in the action bar.
          setHasOptionsMenu(true);
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
              Bundle savedInstanceState) {
          mDrawerListView = (ListView) inflater.inflate(
                  R.layout.fragment_navigation_drawer, container, false);
          mDrawerListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
              @Override
              public void onItemClick(AdapterView&lt;?&gt; parent, View view, int position, long id) {
                  selectItem(position);
              }
          });
          mDrawerListView.setAdapter(new ArrayAdapter&lt;String&gt;(
                  getActionBar().getThemedContext(),
                  android.R.layout.simple_list_item_activated_1,
                  android.R.id.text1,
                  new String[]{
                          getString(R.string.title_section1),
                          getString(R.string.title_section2),
                          getString(R.string.title_section3),
                  }));
          mDrawerListView.setItemChecked(mCurrentSelectedPosition, true);
          return mDrawerListView;
      }
  
      public boolean isDrawerOpen() {
          return mDrawerLayout != null &amp;&amp; mDrawerLayout.isDrawerOpen(mFragmentContainerView);
      }
  
      /**
       * Users of this fragment must call this method to set up the navigation drawer interactions.
       *
       * @param fragmentId   The android:id of this fragment in its activity's layout.
       * @param drawerLayout The DrawerLayout containing this fragment's UI.
       */
      public void setUp(int fragmentId, DrawerLayout drawerLayout) {
          mFragmentContainerView = getActivity().findViewById(fragmentId);
          mDrawerLayout = drawerLayout;
  
          // set a custom shadow that overlays the main content when the drawer opens
          mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
          // set up the drawer's list view with items and click listener
  
          ActionBar actionBar = getActionBar();
          actionBar.setDisplayHomeAsUpEnabled(true);
          actionBar.setHomeButtonEnabled(true);
  
          // ActionBarDrawerToggle ties together the the proper interactions
          // between the navigation drawer and the action bar app icon.
          mDrawerToggle = new ActionBarDrawerToggle(
                  getActivity(),                    /* host Activity */
                  mDrawerLayout,                    /* DrawerLayout object */
                  R.drawable.ic_drawer,             /* nav drawer image to replace 'Up' caret */
                  R.string.navigation_drawer_open,  /* "open drawer" description for accessibility */
                  R.string.navigation_drawer_close  /* "close drawer" description for accessibility */
          ) {
              @Override
              public void onDrawerClosed(View drawerView) {
                  super.onDrawerClosed(drawerView);
                  if (!isAdded()) {
                      return;
                  }
  
                  getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
              }
  
              @Override
              public void onDrawerOpened(View drawerView) {
                  super.onDrawerOpened(drawerView);
                  if (!isAdded()) {
                      return;
                  }
  
                  if (!mUserLearnedDrawer) {
                      // The user manually opened the drawer; store this flag to prevent auto-showing
                      // the navigation drawer automatically in the future.
                      mUserLearnedDrawer = true;
                      SharedPreferences sp = PreferenceManager
                              .getDefaultSharedPreferences(getActivity());
                      sp.edit().putBoolean(PREF_USER_LEARNED_DRAWER, true).apply();
                  }
  
                  getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
              }
          };
  
          // If the user hasn't 'learned' about the drawer, open it to introduce them to the drawer,
          // per the navigation drawer design guidelines.
          if (!mUserLearnedDrawer &amp;&amp; !mFromSavedInstanceState) {
              mDrawerLayout.openDrawer(mFragmentContainerView);
          }
  
          // Defer code dependent on restoration of previous instance state.
          mDrawerLayout.post(new Runnable() {
              @Override
              public void run() {
                  mDrawerToggle.syncState();
              }
          });
  
          mDrawerLayout.setDrawerListener(mDrawerToggle);
      }
  
      private void selectItem(int position) {
          mCurrentSelectedPosition = position;
          if (mDrawerListView != null) {
              mDrawerListView.setItemChecked(position, true);
          }
          if (mDrawerLayout != null) {
              mDrawerLayout.closeDrawer(mFragmentContainerView);
          }
          if (mCallbacks != null) {
              mCallbacks.onNavigationDrawerItemSelected(position);
          }
      }
  
      @Override
      public void onAttach(Activity activity) {
          super.onAttach(activity);
          try {
              mCallbacks = (NavigationDrawerCallbacks) activity;
          } catch (ClassCastException e) {
              throw new ClassCastException("Activity must implement NavigationDrawerCallbacks.");
          }
      }
  
      @Override
      public void onDetach() {
          super.onDetach();
          mCallbacks = null;
      }
  
      @Override
      public void onSaveInstanceState(Bundle outState) {
          super.onSaveInstanceState(outState);
          outState.putInt(STATE_SELECTED_POSITION, mCurrentSelectedPosition);
      }
  
      @Override
      public void onConfigurationChanged(Configuration newConfig) {
          super.onConfigurationChanged(newConfig);
          // Forward the new configuration the drawer toggle component.
          mDrawerToggle.onConfigurationChanged(newConfig);
      }
  
      @Override
      public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
          // If the drawer is open, show the global app actions in the action bar. See also
          // showGlobalContextActionBar, which controls the top-left area of the action bar.
          if (mDrawerLayout != null &amp;&amp; isDrawerOpen()) {
              inflater.inflate(R.menu.global, menu);
              showGlobalContextActionBar();
          }
          super.onCreateOptionsMenu(menu, inflater);
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          if (mDrawerToggle.onOptionsItemSelected(item)) {
              return true;
          }
  
          switch (item.getItemId()) {
              case R.id.action_example:
                  Toast.makeText(getActivity(), "Example action.", Toast.LENGTH_SHORT).show();
                  return true;
          }
  
          return super.onOptionsItemSelected(item);
      }
  
      /**
       * Per the navigation drawer design guidelines, updates the action bar to show the global app
       * 'context', rather than just what's in the current screen.
       */
      private void showGlobalContextActionBar() {
          ActionBar actionBar = getActionBar();
          actionBar.setDisplayShowTitleEnabled(true);
          actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
          actionBar.setTitle(R.string.app_name);
      }
  
      private ActionBar getActionBar() {
          return ((ActionBarActivity) getActivity()).getSupportActionBar();
      }
  
      /**
       * Callbacks interface that all activities using this fragment must implement.
       */
      public static interface NavigationDrawerCallbacks {
          /**
           * Called when an item in the navigation drawer is selected.
           */
          void onNavigationDrawerItemSelected(int position);
      }
  }
  </code></pre>
  
  <p>If there are any other file contents thats might be useful, let me know and I will add the code here too. Thanks for the help!</p>