Android 已选择onPageSelected上的ViewPager和片段异常。

我正在尝试从FragmentPageAdapter更改文本视图的颜色。当我在主屏幕或设置/朋友屏幕上调用时，我得到一个空点异常。这是解决文本视图的错误方法吗？它们在main menu.xml中被清除，在onCreate方法中更改颜色/文本时不会发生异常

我的MainMenu.java类

package com.nordoid.*;
import android.graphics.Color;
import android.os.Bundle;
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.support.v4.view.ViewPager.OnPageChangeListener;
import android.widget.TextView;

public class MainMenu extends FragmentActivity implements OnPageChangeListener {
private MyAdapter mAdapter;
private ViewPager mPager;

// MainMenu bar
 TextView home;
 TextView settings;
 TextView friends;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.mainmenu);
    mAdapter = new MyAdapter(getSupportFragmentManager());

    mPager = (ViewPager) findViewById(R.id.mainmenu_pager);
    mPager.setAdapter(mAdapter);
    mPager.setOnPageChangeListener(this);
    mPager.setCurrentItem(1);

    home = (TextView) findViewById(R.id.mmenu_home);
    settings = (TextView) findViewById(R.id.mmenu_settings);
    friends = (TextView) findViewById(R.id.mmenu_friends);

    // init starting position color.
    //home.setTextColor(Color.WHITE);

    home.setText("HOME");
    settings.setText("SETTINGS");
    friends.setText("FRIENDS")

}
public static class MyAdapter extends FragmentPagerAdapter  {
    public MyAdapter(FragmentManager fm) {
        super(fm);
    }
    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
        case 0:
            return new Mfragment_settings();
        case 1:
            return new Mfragment_main();
        case 2:
            return new Mfragment_friends();
        default:
            return null;
        }
    }
}

public void onPageScrollStateChanged(int arg0) {

}

public void onPageScrolled(int arg0, float arg1, int arg2) {

}

public void onPageSelected(int arg0) {

      switch (arg0) {
        case 0: 
            onSettingsScreen();
            break;
        case 1:
            onHomeScreen();
            break;      
        case 2: 
            onFriendsScreen();
            break;  
        }
}

public void onHomeScreen () {
    home.setTextColor(Color.WHITE);
    System.out.println("2");
    friends.setTextColor(Color.GRAY);
    settings.setTextColor(Color.GRAY);
}

public void onSettingsScreen ()  {
    settings.setTextColor(Color.WHITE);

    friends.setTextColor(Color.GRAY);
    home.setTextColor(Color.GRAY);
    System.out.println("PAGE 1");
}

public void onFriendsScreen () {
    friends.setTextColor(Color.WHITE);
    System.out.println("3");
    home.setText(Color.GRAY);
    settings.setTextColor(Color.GRAY);
  }
  }

和我的mainmenu.xml文件

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/mfrag_home" >

<android.support.v4.view.ViewPager
    android:id="@+id/mainmenu_pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<LinearLayout
    android:id="@+id/linearLayout1"
    android:layout_width="wrap_content"
    android:layout_height="27dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentRight="true"
    android:layout_marginTop="45dp" >

    <TextView
        android:id="@+id/mmenu_settings"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="15dp"
        android:paddingTop="2dp"
        android:text="TextView"
        android:textColor="@color/White" />

    <TextView
        android:id="@+id/mmenu_home"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="45dp"
        android:paddingTop="2dp"
        android:text="TextView"
        android:textColor="@color/White" />

    <TextView
        android:id="@+id/mmenu_friends"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="49dp"
        android:paddingTop="2dp"
        android:text="TextView"
        android:textColor="@color/White" />

   </LinearLayout>

---

我认为您有一个例外，因为当调用mPager.setCurrentItem（1）时，textview home friends&settings为null（初始化发生在之后）

移动：

home = (TextView) findViewById(R.id.mmenu_home);
settings = (TextView) findViewById(R.id.mmenu_settings);
friends = (TextView) findViewById(R.id.mmenu_friends);

之前：

mPager = (ViewPager) findViewById(R.id.mainmenu_pager);

应该解决NullPointerException

mPager = (ViewPager) findViewById(R.id.mainmenu_pager);