Android将一个片段导航到另一个片段正在工作，但看不到其他片段的背景

我试图将一个片段导航到另一个片段。在这里，我用一个按钮来完成这个过程。当我运行这个程序时，最后会出现一个空白屏幕。

这些是我的步骤，

MainActivity.java

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
public class MainActivity extends FragmentActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final int page=2;
        final ViewPager pager = (ViewPager) findViewById(R.id.viewPager);
        pager.setAdapter(new MyPagerAdapter(getSupportFragmentManager()));
        pager.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (pager.getCurrentItem()==page) {
                    return  true;
                }
                return false;
            }
        });
    }
    private class MyPagerAdapter extends FragmentPagerAdapter {
        public MyPagerAdapter(FragmentManager fm) {
            super(fm);
        }
        @Override
        public Fragment getItem(int pos) {
            switch(pos) {
                case 0: return FirstFragment.newInstance("FirstFragment, Instance 1");
                case 1: return SecondFragment.newInstance("SecondFragment, Instance 1");
                case 2: return ThirdFragment.newInstance("ThirdFragment, Instance 1");
                default: return ThirdFragment.newInstance("ThirdFragment, Default");
            }
        }
        @Override
        public int getCount() {
            return 4;
        }
    }
}

我想把第三个片段导航到第四个片段。

activity_main.xml

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

third_frag.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/holo_red_light" >
<TextView
    android:id="@+id/tvFragThird"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:textSize="26dp"
    android:text="TextView" />
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:id="@+id/printButton"
        android:text="Next"/>
</RelativeLayout>

ThirdFragment.java

public class ThirdFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.third_frag, container, false);
        TextView tv = (TextView) v.findViewById(R.id.tvFragThird);
        tv.setText(getArguments().getString("msg"));
        Button button=(Button)v.findViewById(R.id.printButton);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FourthFragment fourthFragment=new FourthFragment();
                Bundle bundle=new Bundle();
                bundle.putInt(String.valueOf(FirstFragment.newInstance("Terance")),1);
                fourthFragment.setArguments(bundle);
                FragmentManager fragmentManager=getFragmentManager();
                FragmentTransaction transaction = fragmentManager.beginTransaction();
                transaction.replace(R.id.viewPager, fourthFragment);
                transaction.addToBackStack(null);
                transaction.commit();
            }
        });
        return v;
    }
    public static ThirdFragment newInstance(String text) {
        ThirdFragment f = new ThirdFragment();
        Bundle b = new Bundle();
        b.putString("msg", text);
        f.setArguments(b);
        return f;
    }
}

fourth_frag.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/holo_blue_dark">
    <TextView
        android:id="@+id/tvFragFourth"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:textSize="26dp"
        android:text="TextView" />
</RelativeLayout>

FourthFragment.java

public class FourthFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fourth_frag, container, false);
        TextView tv = (TextView) v.findViewById(R.id.tvFragFourth);
        tv.setText("Fourth Fragment");
        return v;
    }
    public static FourthFragment newInstance(String text) {
        text="Terance";
        FourthFragment f = new FourthFragment();
        Bundle b = new Bundle();
        b.putString("msg", text);
        f.setArguments(b);
        return f;
    }
}

有四个片段类，我只提到了两个用于此目的的片段类。当到达第三个片段时，不能向左或向右滑动。然后我用一个按钮转到第四个片段。但它不起作用。

对此有什么想法吗？

谢谢。