public abstract class BaseFragment extends Fragment {

    private View rootView;


    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        rootView = inflater.inflate(initlayout(),container,false);
        inittablayout(rootView,getLayoutInflater());
        initviewpager(rootView);
        return rootView;
    }



    protected void initviewpager(View rootView) {
    }

    protected void inittablayout(View rootView,LayoutInflater inflater) {
    }

    protected abstract int initlayout();

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public  class HomePageFragment extends BaseFragment {
 


    private ViewPager viewPager_homefragment;
    private TabLayout tabLayout_homefragemnt;
    private TextView tabtitle_homepagefragment;


    private String tabtitle_homefragments[] = {"推荐", "零食", "男装", "女装", "母婴", "数码家电", "美妆护肤"};

    protected int initlayout() {
        return R.layout.fragment_homepage;
    }

    protected void initviewpager(View rootView) {
        viewPager_homefragment = rootView.findViewById(R.id.viewpager_homefragment);
        HomefragmentAdapter homefragmentAdapter = new HomefragmentAdapter(getChildFragmentManager(), tabtitle_homefragments);
        viewPager_homefragment.setAdapter(homefragmentAdapter);
    }


    protected void inittablayout(View rootView,LayoutInflater inflater) {

        tabLayout_homefragemnt = rootView.findViewById(R.id.tablayout_homefragment);

        //实现自定义tab
        for (int i = 0; i < 7; i++) {

            //实例化tab
            TabLayout.Tab tab = tabLayout_homefragemnt.newTab();

            View view = inflater.inflate(R.layout.tabview_homepagefragment, null);

            //为tab设置自定义视图
            tab.setCustomView(view);

            tabtitle_homepagefragment = view.findViewById(R.id.tabtitle_homepagefragment);
            //添加标题
            tabtitle_homepagefragment.setText(tabtitle_homefragments[i]);

            //添加tab
            tabLayout_homefragemnt.addTab(tab);
            // 设置第一个tab为大号字体
            if (tab.getPosition() == 0) {
                tabtitle_homepagefragment = tab.getCustomView().findViewById(R.id.tabtitle_homepagefragment);
                tabtitle_homepagefragment.setTextSize(15);
            }
        }




            viewPager_homefragment.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout_homefragemnt));
            tabLayout_homefragemnt.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
                @Override
                public void onTabSelected(TabLayout.Tab tab) {
                    //viewpager跳转到指定页面
                    viewPager_homefragment.setCurrentItem(tab.getPosition());
                    tabtitle_homepagefragment = tab.getCustomView().findViewById(R.id.tabtitle_homepagefragment);
                    tabtitle_homepagefragment.setTextSize(15);


                }


                @Override
                public void onTabUnselected(TabLayout.Tab tab) {

                    tabtitle_homepagefragment.setTextSize(13);
                }

                @Override
                public void onTabReselected(TabLayout.Tab tab) {

                }
            });

        }

logcat报错：

ava.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.addOnPageChangeListener(android.support.v4.view.ViewPager$OnPageChangeListener)' on a null object reference