（TabHost）findViewById（...）返回NULL，Android的

Question

我设置

tabHost = (TabHost) findViewBiId(android.R.id.tabhost); 

而且它出来NULL。

我有一个MainActivity扩展ActionBarActivity并实现ActionBar.TabListener，如下...

MainActivity

public class MainActivity extends ActionBarActivity implements ActionBar.TabListener { 

     SectionsPagerAdapter mSectionsPagerAdapter; 
     ViewPager mViewPager; 

     public static TabHost tabHost; 

     private SharedPreferences mSharedPreferences; 

     protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     tabHost = (TabHost) findViewById(android.R.id.tabhost); 

     // Set up the action bar. 
     final ActionBar actionBar = getSupportActionBar(); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

     mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager()); 

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

     mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() { 
      @Override 
      public void onPageSelected(int position) { 
       actionBar.setSelectedNavigationItem(position); 
      } 
     }); 

     for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) { 
      actionBar.addTab(
        actionBar.newTab() 
          .setText(mSectionsPagerAdapter.getPageTitle(i)) 
          .setTabListener(this)); 
     } 
    } 

    ... other methods ... 
} 

但是，当我尝试使用tabHost，我得到一个NullPointerException。检查tabHost表明它实际上是空的。

即使是一个简单的tabHost.getCurrentTab()崩溃的整个事情。我希望能够以编程方式切换选项卡，但它不起作用。我已经通过了几十个线程，似乎没有任何工作。

SectionsPagerAdapter

import java.util.Locale; 

import android.content.Context; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentPagerAdapter; 
import android.widget.TabHost; 

public class SectionsPagerAdapter extends FragmentPagerAdapter { 

    protected Context mContext; 

    public SectionsPagerAdapter(Context context, FragmentManager fm) { 
     super(fm); 
     mContext = context; 
    } 

    @Override 
    public Fragment getItem(int position) { 
     switch(position){ 
      case 0: 
       return new PunchCardFragment(); 
      case 1: 
       return new CalendarFragment(); 
     } 
     return null; 
    } 

    @Override 
    public int getCount() { 
     return 2; 
    } 

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

摇篮

apply plugin: 'com.android.application' 

android { 
    compileSdkVersion 21 
    buildToolsVersion "21.1.2" 

    defaultConfig { 
     applicationId "..." 
     minSdkVersion 14 
     targetSdkVersion 21 
     versionCode 1 
     versionName "1.0" 
    } 
    buildTypes { 
     release { 
      minifyEnabled false 
      proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro' 
     } 
    } 
} 

dependencies { 
    compile fileTree(dir: 'libs', include: ['*.jar']) 
    compile 'com.android.support:appcompat-v7:21.0.3' 
} 

activity_main XML

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:id="@+id/pager" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:context=".MainActivity"/> 

Answer 1

您activity_main.xml不声明android:id="@+id/tabhost"，只是android:id="@+id/pager"与ViewPager。
findViewById只给你，在setContentView(R.layout.activity_main)