Android Activity with Fragments，findFragment始终返回null

Question

我使用活动处理Android应用，使用ViewPager和FragmentPagerAdapter处理3个片段。 该应用程序正常工作，但我仍然无法检索活动中的片段的引用。 方法findFragmentById和findFragmentbyTag始终返回null。 这是我的代码： FragmentPagerAdapter：

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class ViewPagerAdapter extends FragmentPagerAdapter {

    final int PAGE_COUNT = 3;

    public ViewPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public int getCount() {
        return PAGE_COUNT;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {

            // Open FragmentTab1.java
            case 0:
                FragmentTab1 fragmenttab1 = new FragmentTab1();
                return fragmenttab1;

            // Open FragmentTab2.java
            case 1:
                FragmentTab2 fragmenttab2 = new FragmentTab2();
                return fragmenttab2;

            // Open FragmentTab3.java
            case 2:
                FragmentTab3 fragmenttab3 = new FragmentTab3();
                return fragmenttab3;
        }
        return null;
    }
}

片段实施：

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.support.v4.app.Fragment;

public class FragmentTab1 extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Get the view from fragmenttab1.xml
        View view = inflater.inflate(R.layout.fragmenttab1, container, false);
        return view;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
    }
}

最后我的活动：

import android.support.v4.app.FragmentActivity;
import android.support.v4.app.Fragment;
import android.support.v4.view.ViewPager;
import android.os.Bundle;

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from activity_main.xml
        setContentView(R.layout.activity_main);

        // Locate the viewpager in activity_main.xml
        viewPager = (ViewPager) findViewById(R.id.pager);
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        // Set the ViewPagerAdapter into ViewPager
        viewPager.setAdapter(viewPagerAdapter);
    }

    @Override
    protected void onStart()
    {
        super.onStart();
        InitializeVariables();
        viewPager.setCurrentItem(1);
        Fragment fragment = (Fragment)getSupportFragmentManager().findFragmentByTag(makeFragmentName(viewPager.getId(), 0));  
    }