java.lang.NullPointerException: Attempt to invoke virtual method on ViewPager

Question

I have an error with my app. I want to put viewPager in fragment. But there are some problem when do the setCurrentItem on viewPager.

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setCurrentItem(int)' on a null object reference

this is error message

public void clickBottomToolbar(View view) {
    viewPager viewPager=new viewPager();
    int position=0;
    switch (view.getId()) {
        case R.id.map:
            position=0;
            break;
        case R.id.customerList:
            position=1;
            break;
        case R.id.captureNameCard:
            position=2;
            break;
        case R.id.captureDocument:
            position=3;
            break;
        case R.id.documentList:
            position=4;
            break;
    }
    viewPager.setPosition(position);
    FragmentTransaction fragmentTransaction=fm.beginTransaction();
    fragmentTransaction.replace(R.id.fragment,viewPager);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();
}

Upper is code in main_activitiy.java and Lower is viewPager.java

public class viewPager extends Fragment {
    ViewPager viewPager;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
        View view=inflater.inflate(R.layout.viewpager,container,false);
        viewPager=(ViewPager)view.findViewById(R.id.viewPager);
        viewPager.setAdapter(new viewPagerAdapter(view.getContext()));
        return view;
    }
    public void setPosition(int position){
        viewPager.setCurrentItem(position);
        return;
    }
}

This is viewpager.xml

<RelativeLayout
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
    <android.support.v4.view.ViewPager
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        xmlns:android="http://schemas.android.com/apk/res/android">
    </android.support.v4.view.ViewPager>
</RelativeLayout>