XML : How can I change from Fragment to Fragment with an ImageButton?

Im trying to go from MainFragment to AddPostFragment with an ImageButton. How is this possible...I tried a lot of things... I want to use a fragment to keep my navigation drawer on top instead of a new activity.

MainFragment:

  public class MainFragment extends Fragment implements View.OnClickListener{    ImageButton floatButton;    @Nullable  @Override  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {      View rootView = inflater.inflate(R.layout.fragment_main, container, false);        floatButton = (ImageButton)rootView.findViewById(R.id.postButtonMain);      floatButton.setOnClickListener(new View.OnClickListener() {          @Override          public void onClick(View v) {              Intent post = new Intent(getActivity(), AddPostFragment.class);              startActivity(post);          }      });        return rootView;  }    @Override  public void onClick(View v) {    }    

}

AddPostFragment:

  public class AddPostFragment extends Fragment {    @Nullable  @Override  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {      View rootView = inflater.inflate(R.layout.frag_add_post, container, false);        return rootView;  }    

}

XML MainFragment:

  <?xml version="1.0" encoding="utf-8"?>    

  <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceLarge"      android:text="Home"      android:id="@+id/frag_home"      android:layout_gravity="center_horizontal|top" />    <ImageButton      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:id="@+id/postButtonMain"      android:src="@drawable/rsz_add_car"      android:background="?android:attr/selectableItemBackground"      android:layout_alignParentBottom="true"      android:layout_alignParentRight="true"      android:layout_alignParentEnd="true"      android:layout_gravity="right|bottom" />    

XML AddPostFragment:

  <?xml version="1.0" encoding="utf-8"?>    

  <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceLarge"      android:text="Add Post"      android:id="@+id/add_post"      android:layout_gravity="center_horizontal|top"      android:layout_alignParentTop="true"      android:layout_centerHorizontal="true" />    <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceMedium"      android:text="Airport"      android:id="@+id/text_airport"      android:layout_marginTop="88dp"      android:layout_below="@+id/add_post"      android:layout_toLeftOf="@+id/add_post"      android:layout_toStartOf="@+id/add_post" />    <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceMedium"      android:text="Date"      android:id="@+id/text_date"      android:layout_below="@+id/text_airport"      android:layout_alignRight="@+id/text_airport"      android:layout_alignEnd="@+id/text_airport"      android:layout_marginTop="36dp" />    <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceMedium"      android:text="Time"      android:id="@+id/text_time"      android:layout_centerVertical="true"      android:layout_alignRight="@+id/text_date"      android:layout_alignEnd="@+id/text_date" />    <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceMedium"      android:text="Persons"      android:id="@+id/text_persons"      android:layout_marginTop="43dp"      android:layout_below="@+id/add_time"      android:layout_toLeftOf="@+id/add_time"      android:layout_toStartOf="@+id/add_time" />    <TextView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:textAppearance="?android:attr/textAppearanceMedium"      android:text="Address"      android:id="@+id/text_address"      android:layout_marginTop="48dp"      android:layout_below="@+id/text_persons"      android:layout_alignLeft="@+id/text_persons"      android:layout_alignStart="@+id/text_persons" />    <EditText      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:inputType="time"      android:ems="10"      android:id="@+id/add_time"      android:layout_centerVertical="true"      android:layout_toRightOf="@+id/text_time"      android:layout_alignRight="@+id/add_post"      android:layout_alignEnd="@+id/add_post" />    <EditText      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:inputType="date"      android:ems="10"      android:id="@+id/add_date"      android:layout_alignTop="@+id/text_date"      android:layout_toRightOf="@+id/text_date"      android:layout_alignRight="@+id/add_time"      android:layout_alignEnd="@+id/add_time" />    <EditText      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:inputType="number"      android:ems="10"      android:id="@+id/add_persons"      android:layout_above="@+id/text_address"      android:layout_toRightOf="@+id/text_persons"      android:layout_alignRight="@+id/add_post"      android:layout_alignEnd="@+id/add_post" />    <android.support.v7.widget.SearchView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:id="@+id/add_search"      android:layout_above="@+id/text_date"      android:layout_toRightOf="@+id/text_airport"      android:layout_toEndOf="@+id/text_airport" />    <EditText      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:id="@+id/add_address"      android:layout_alignBottom="@+id/text_address"      android:layout_alignRight="@+id/add_post"      android:layout_alignEnd="@+id/add_post"      android:layout_toRightOf="@+id/text_address"      android:layout_toEndOf="@+id/text_address" />    

MainMenu with navigation drawer:

  public class MainMenu extends AppCompatActivity      implements NavigationView.OnNavigationItemSelectedListener {    Fragment someFragment;      /**   * ATTENTION: This was auto-generated to implement the App Indexing API.   * See https://g.co/AppIndexing/AndroidStudio for more information.   */  private GoogleApiClient client;    @Override  protected void onCreate(Bundle savedInstanceState) {      super.onCreate(savedInstanceState);      setContentView(R.layout.activity_main_menu);      Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);      setSupportActionBar(toolbar);        DrawerLayout 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();        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);      navigationView.setNavigationItemSelectedListener(this);          FragmentManager fm = getFragmentManager();      fm.beginTransaction().replace(R.id.content_main, new MainFragment()).commit();        }      @Override  public void onBackPressed() {      DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);      if (drawer.isDrawerOpen(GravityCompat.START)) {          drawer.closeDrawer(GravityCompat.START);      } else {          super.onBackPressed();      }  }    @Override  public boolean onCreateOptionsMenu(Menu menu) {      // Inflate the menu; this adds items to the action bar if it is present.      getMenuInflater().inflate(R.menu.main_menu, menu);      return true;  }    @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.      int id = item.getItemId();        //noinspection SimplifiableIfStatement        return super.onOptionsItemSelected(item);  }    @SuppressWarnings("StatementWithEmptyBody")  @Override  public boolean onNavigationItemSelected(MenuItem item) {      FragmentManager fm = getFragmentManager();        // Handle navigation view item clicks here.      int id = item.getItemId();        if (id == R.id.nav_home) {          // Handle the home action          fm.beginTransaction().replace(R.id.content_main, new MainFragment()).commit();      } else if (id == R.id.nav_demand) {          fm.beginTransaction().replace(R.id.content_main, new DemandFragment()).commit();      } else if (id == R.id.nav_posts) {          fm.beginTransaction().replace(R.id.content_main, new PostsFragment()).commit();      } else if (id == R.id.nav_messages) {          fm.beginTransaction().replace(R.id.content_main, new MessagesFragment()).commit();      } else if (id == R.id.nav_settings) {          fm.beginTransaction().replace(R.id.content_main, new SettingsFragment()).commit();      } else if (id == R.id.nav_logout) {        }        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);      drawer.closeDrawer(GravityCompat.START);      return true;  }