TabLayout tabLayout;
AppBarLayout appBarLayout;
ViewPager viewPager;

tabLayout = findViewById(R.id.tab_layout);
appBarLayout = findViewById(R.id.appBar);
viewPager = findViewById(R.id.pager);

ViewPagerAdapter adapter= new ViewPagerAdapter(getSupportFragmentManager());
adapter.addFragment(new pendingTasks(), "Pending");
adapter.addFragment(new currentTasks(), "OnGoing");
adapter.addFragment(new completedTasks(), "Completed");
viewPager.setAdapter(adapter);
tabLayout.setupWithViewPager(viewPager);

class ViewPagerAdapter extends FragmentPagerAdapter {

    private final List < Fragment > fragmentList = new ArrayList < > ();
    private final List < String > fragmentListTitles = new ArrayList < > ();
    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return fragmentList.get(position);
    }

    @Override
    public int getCount() {
        return fragmentList.size();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return fragmentListTitles.get(position);
    }

    public void addFragment(Fragment fragment, String title) {
        fragmentList.add(fragment);
        fragmentListTitles.add(title);
    }
}