使用Android Studio实现Viewpager失败

当我尝试为

活动创建
查看页面
时，我遇到了这个
错误
：

FATAL EXCEPTION: main
java.lang.NullPointerException
at com.blutech.viewpager.PlaceHolderFragment.newInstance(PlaceHolderFragment.java:44)
at com.blutech.viewpager.LayoutSlideActivity$SectionsPagerAdapter.getItem(LayoutSlideActivity.java:94)
at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:97)
at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
at android.view.View.measure(View.java:15848)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5008)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at android.view.View.measure(View.java:15848)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5008)
at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302)
at android.view.View.measure(View.java:15848)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5008)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2189)
at android.view.View.measure(View.java:15848)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1905)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1104)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1284)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
at android.view.Choreographer.doCallbacks(Choreographer.java:562)
at android.view.Choreographer.doFrame(Choreographer.java:532)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
at android.os.Handler.handleCallback(Handler.java:730)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5103)
at java.lang.reflect.Method.invokeNative(NativeMethod)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)

这是我使用的片段：

public class PlaceHolderFragment extends Fragment {

private static final String ARG_SECTION_NUMBER = "section_number";
TextView text_header,text_content;


@Override
public View onCreateView(LayoutInflater inflater,ViewGroup parent,Bundle savedInstanceState)
{
    super.onCreateView(inflater, parent, savedInstanceState);
    View v=inflater.inflate(R.layout.layout_slide_page,parent,false);
    text_header=(TextView)v.findViewById(R.id.section_label);
    text_content=(TextView)v.findViewById(R.id.section_content);
    return v;
}

public PlaceHolderFragment()
{
    super();
}

public static PlaceHolderFragment newInstance(int sectionNumber)
{
    PlaceHolderFragment fragment = new PlaceHolderFragment();
    switch(sectionNumber)
    {
        case 0:
            fragment.text_header.setText(R.string.title_section1);
            fragment.text_content.setText(R.string.content_section1);
            break;
        case 1:
            fragment.text_header.setText(R.string.title_section2);
            fragment.text_content.setText(R.string.content_section2);
            break;
        case 2:
            fragment.text_header.setText(R.string.title_section3);
            fragment.text_content.setText(R.string.content_section3);
            break;
    }
    return fragment;
}
}

活动内容如下：

 public class LayoutSlideActivity extends ActionBarActivity {

/**
 * The {@link android.support.v4.view.PagerAdapter} that will provide
 * fragments for each of the sections. We use a
 * {@link FragmentPagerAdapter} derivative, which will keep every
 * loaded fragment in memory. If this becomes too memory intensive, it
 * may be best to switch to a
 * {@link android.support.v4.app.FragmentStatePagerAdapter}.
 */
SectionsPagerAdapter mSectionsPagerAdapter;

/**
 * The {@link ViewPager} that will host the section contents.
 */
ViewPager mViewPager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.layout_slide);



    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.layout_slide, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}





/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

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

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).
        return PlaceHolderFragment.newInstance(position);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return getString(R.string.title_section1).toUpperCase(l);
            case 1:
                return getString(R.string.title_section2).toUpperCase(l);
            case 2:
                return getString(R.string.title_section3).toUpperCase(l);
        }
        return null;
    }
}
}

错误源于第44行的片段：

 fragment.text_header.setText(R.string.title_section2);

下一个错误行在
FragmentPagerAdapter
的
getItem（int位置）
方法中

 return PlaceHolderFragment.newInstance(position);

将开关盒移动到
onCreateView
，在newInstance中只需设置节号。例：

public class PlaceHolderFragment extends Fragment {

private static final String ARG_SECTION_NUMBER = "section_number";
TextView text_header,text_content;
int sectionNumber;


@Override
public View onCreateView(LayoutInflater inflater,ViewGroup parent,Bundle savedInstanceState)
{
    super.onCreateView(inflater, parent, savedInstanceState);
    View v=inflater.inflate(R.layout.layout_slide_page,parent,false);
    text_header=(TextView)v.findViewById(R.id.section_label);
    text_content=(TextView)v.findViewById(R.id.section_content);
   switch(sectionNumber)
    {
        case 0:
            text_header.setText(R.string.title_section1);
            text_content.setText(R.string.content_section1);
            break;
        case 1:
            text_header.setText(R.string.title_section2);
            text_content.setText(R.string.content_section2);
            break;
        case 2:
            text_header.setText(R.string.title_section3);
            text_content.setText(R.string.content_section3);
            break;
    }
    return v;
}

public PlaceHolderFragment()
{
    super();
}

public static PlaceHolderFragment newInstance(int sectionNumber)
{
    PlaceHolderFragment fragment = new PlaceHolderFragment();
    fragment.setSectionNumber(sectionNumber);
    return fragment;
}
public void setSectionNumber(int sectionNumber){
   this.sectionNumber=sectionNumber;
}
}

问题是在创建视图时未调用
newInstance
，因此文本标题和文本内容未初始化。
ViewPager工作，但newInstance和getItem似乎在位置上做了一些奇怪的事情，这些是位置，行为如下所述。我从未看到主页，而它是
title\u section1
像您一样使用回调，我使用了一个静态变量，它没有被正确更新，当我将它作为参数传递给片段时，它起作用