如何在 DrawerLayout 中正确使用 Android Fragments？

Question

https://github.com/codepath/android_guides/wiki/Fragment-Navigation-Drawer I was trying to build a Drawer in my project and it went okay (image-wise) , but I can't travel between activities. When I try to click on the drawer menu item, my app crashes.

I checked StackOverflow for an hour and couldn't find a solution.

My Console Log:

       W/System.err: java.lang.ClassCastException: com.example.ali.bitirmeprojesi.DiyetListele cannot be cast to android.support.v4.app.Fragment
    at com.example.ali.bitirmeprojesi.WelcomePage.selectDrawerItem(WelcomePage.java:115)
    at com.example.ali.bitirmeprojesi.WelcomePage$1.onNavigationItemSelected(WelcomePage.java:91)
    at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:170)
    at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:840)
    at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:158)W/System.err:     at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:991)
    at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:352)
    at android.view.View.performClick(View.java:6266)
    at android.view.View$PerformClick.run(View.java:24730)
    at android.os.Handler.handleCallback(Handler.java:789)
    at android.os.Handler.dispatchMessage(Handler.java:98)
    at android.os.Looper.loop(Looper.java:171)
    at android.app.ActivityThread.main(ActivityThread.java:6699)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:246)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:783)D/AndroidRuntime: Shutting down VME/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ali.bitirmeprojesi, PID: 11292
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
    at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:396)
    at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:444)
    at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:434)
    at com.example.ali.bitirmeprojesi.WelcomePage.selectDrawerItem(WelcomePage.java:122)
    at com.example.ali.bitirmeprojesi.WelcomePage$1.onNavigationItemSelected(WelcomePage.java:91)
    at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:170)
    at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:840)
    at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:158)
    at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:991)
    at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:352)
    at android.view.View.performClick(View.java:6266)
    at android.view.View$PerformClick.run(View.java:24730)
    at android.os.Handler.handleCallback(Handler.java:789)
    at android.os.Handler.dispatchMessage(Handler.java:98)
    at android.os.Looper.loop(Looper.java:171)
    at android.app.ActivityThread.main(ActivityThread.java:6699)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:246)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:783)

My Code which fails:

       public void selectDrawerItem(MenuItem menuItem) {
    // Create a new fragment and specify the fragment to show based on nav item clicked
    Fragment fragment = null;
    Class fragmentClass;
    switch(menuItem.getItemId()) {
        case R.id.adımsayar:
            fragmentClass = StepCounter.class;
            break;
        case R.id.diyetliste:
            fragmentClass = DiyetListele.class;
            break;
        case R.id.diyetoner:
            fragmentClass = DiyetOner.class;
            break;
        default:
            fragmentClass = StepCounter.class;
    }

    try {
        fragment = (Fragment) fragmentClass.newInstance();
    } catch (Exception e) {
        e.printStackTrace();
    }

    // Insert the fragment by replacing any existing fragment
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

    // Highlight the selected item has been done by NavigationView
    menuItem.setChecked(true);
    // Set action bar title
    setTitle(menuItem.getTitle());
    // Close the navigation drawer
    mDrawer.closeDrawers();
}

I am trying to use frags(menu items) like an Intent and I want to use the drawer layout in all activities because frags are better as I researched. But it fails for some reason.

Answer 1

try not to use "fragmentClass" to instantiate fragment, instead use fragment = new FragmentName(). your code will look like this below:

Fragment fragment = null;
    //Class fragmentClass;
    switch(menuItem.getItemId()) {
        case R.id.adımsayar:
            fragment= new StepCounterFragment();
            break;
        case R.id.diyetliste:
            fragment= new DiyetListeleFragment();
            break;
        case R.id.diyetoner:
            fragment = new DiyetOnerFragment();
            break;
        default:
            fragment = new StepCounterFragment();
    }
    // Insert the fragment by replacing any existing fragment
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.flContent, fragment).commit();

    // Highlight the selected item has been done by NavigationView
    menuItem.setChecked(true);
    // Set action bar title
    setTitle(menuItem.getTitle());
    // Close the navigation drawer
    mDrawer.closeDrawers();