NullPointerException异常的ListView控件

Question

我需要我的第一个刷卡Layout帮助和设置适配器ListView时... 我将显示在ListView一些文本，但它不工作:(

MainActivity.java：

package com.evolutio.blocklaunch; 

import java.util.ArrayList; 
import java.util.Arrays; 
import java.util.List; 
import java.util.Locale; 

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.app.NavUtils; 
import android.support.v4.view.ViewPager; 
import android.view.Gravity; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ArrayAdapter; 
import android.widget.ListAdapter; 
import android.widget.ListView; 
import android.widget.TextView; 

public class MainActivity extends FragmentActivity { 

/** 
* The {@link android.support.v4.view.PagerAdapter} that will provide 
* fragments for each of the sections. We use a 
* {@link android.support.v4.app.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; 

    private ListView mainListView ; 
    private ArrayAdapter<String> listAdapter ; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    // Create the adapter that will return a fragment for each of the three 

    // Find the ListView resource. 
    mainListView = (ListView) findViewById(R.id.mainListView); 

    // Create and populate a List of planet names. 
    String[] planets = new String[] { "Mercury", "Venus", "Earth", "Mars", 
             "Jupiter", "Saturn", "Uranus", "Neptune"};  
    ArrayList<String> planetList = new ArrayList<String>(); 
    planetList.addAll(Arrays.asList(planets)); 

    // Create ArrayAdapter using the planet list. 
    listAdapter = new ArrayAdapter<String>(this, R.layout.news, planetList); 

    // Add more planets. If you passed a String[] instead of a List<String> 
    // into the ArrayAdapter constructor, you must not add more items. 
    // Otherwise an exception will occur. 
    listAdapter.add("Ceres"); 
    listAdapter.add("Pluto"); 
    listAdapter.add("Haumea"); 
    listAdapter.add("Makemake"); 
    listAdapter.add("Eris"); 

    // Set the ArrayAdapter as the ListView's adapter. 
    mainListView.setAdapter(listAdapter); 

    // primary sections of the app. 
    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.main, menu); 
    return true; 
} 

/** 
* 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 DummySectionFragment (defined as a static inner class 
     // below) with the page number as its lone argument. 
     Fragment fragment = new DummySectionFragment(); 
     Bundle args = new Bundle(); 
     args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1); 
     fragment.setArguments(args); 
     return fragment; 
    } 

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

    @Override 
    public CharSequence getPageTitle(int position) { 
     Locale l = Locale.getDefault(); 
     int tabLayout = 0; 
     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; 
    } 
} 

/** 
* A dummy fragment representing a section of the app, but that simply 
* displays dummy text. 
*/ 
public static class DummySectionFragment extends Fragment { 
    /** 
    * The fragment argument representing the section number for this 
    * fragment. 
    */ 
    public static final String ARG_SECTION_NUMBER = "section_number"; 

    public DummySectionFragment() { 
    } 

    public static final String ARG_OBJECT = "object"; 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 

      Bundle args = getArguments(); 
      int position = args.getInt(ARG_OBJECT); 

      int tabLayout = 0; 
      switch (position) { 
      case 0: 
      tabLayout = R.layout.news; 
      break; 
      case 1: 
      tabLayout = R.layout.news; 
      break; 
      case 2: 
      tabLayout = R.layout.news; 
      break; 
      } 

      View rootView = inflater.inflate(tabLayout, container, false); 
      return rootView; 
     } 
} 

} 

---

XML的文件

activity_main.xml中

<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" > 

    <!-- 
    This title strip will display the currently visible page title, as well as the page 
    titles for adjacent pages. 
    --> 

    <android.support.v4.view.PagerTitleStrip 
     android:id="@+id/pager_title_strip" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:layout_gravity="top" 
     android:background="#33b5e5" 
     android:paddingBottom="4dp" 
     android:paddingTop="4dp" 
     android:textColor="#fff" /> 

</android.support.v4.view.ViewPager> 

Item.xml

<TextView xmlns:android="http://schemas.android.com/apk/res/android" 
android:id="@+id/rowTextView" 
android:layout_width="fill_parent" 
android:layout_height="wrap_content" 
android:padding="10dp" 
android:textSize="16sp" > 
</TextView> 

News.xml

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="vertical" 
android:layout_width="fill_parent" 
android:layout_height="fill_parent"> 

    <ListView android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:id="@+id/mainListView"> 
    </ListView> 
</LinearLayout> 

而且错误：

03-29 10:01:39.827: D/AndroidRuntime(6902): Shutting down VM 
03-29 10:01:39.827: W/dalvikvm(6902): threadid=1: thread exiting with uncaught exception (group=0x2b521930) 
03-29 10:01:39.827: E/AndroidRuntime(6902): FATAL EXCEPTION: main 
03-29 10:01:39.827: E/AndroidRuntime(6902): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.evolutio.blocklaunch/com.evolutio.blocklaunch.MainActivity}: java.lang.NullPointerException 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2357) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread.access$600(ActivityThread.java:153) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.os.Looper.loop(Looper.java:137) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread.main(ActivityThread.java:5226) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at java.lang.reflect.Method.invoke(Method.java:511) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at dalvik.system.NativeStart.main(Native Method) 
03-29 10:01:39.827: E/AndroidRuntime(6902): Caused by: java.lang.NullPointerException 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at com.evolutio.blocklaunch.MainActivity.onCreate(MainActivity.java:74) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.Activity.performCreate(Activity.java:5104) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261) 
03-29 10:01:39.827: E/AndroidRuntime(6902):  ... 11 more 
03-29 10:01:42.327: I/Process(6902): Sending signal. PID: 6902 SIG: 9 

Answer 1

你正在使用错误的布局MainActivity，因为你已经在News.xml添加ListView但MainActivity设置activity_main。你应该使用News.xml布局MainActivity与mainListView的ListView它初始化mainListView：

setContentView(R.layout.News); 

Answer 2

的logcat中说，问题是你的MainActivity线74，但我认为真正的问题是在第53行：

mainListView = (ListView) findViewById(R.id.mainListView);

我觉得mainListView为null，因为这个布局元素不存在于activity_main.xml中（也许它符合82装？）

您可以尝试到m OVE线53和线82后，74 ...

Answer 3

试试这个

 package com.kt.listviewexample; 

     import java.util.ArrayList; 
     import java.util.Arrays; 

     import android.app.Activity; 
     import android.content.Intent; 
     import android.os.Bundle; 
     import android.view.View; 
     import android.widget.AdapterView; 
     import android.widget.AdapterView.OnItemClickListener; 
     import android.widget.ArrayAdapter; 
     import android.widget.ListView; 
     import android.widget.Toast; 

    public class MainActivity extends Activity { 
/** Here two object are created mainListView and fruits of class ListView and String respectively 
* fruits is contains a set of data as array type 
*/ 
    private ListView mainListView ; 
    String[] fruits = { 
      "Mango", 
      "Pine-Apple", 
      "Guava", 
      "Apple", 
      "Banana", 
      "Grapes", 
      "Orange", 
      "watermilon", 
      "Dates", 
      "Olive", 

     }; 


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

    // called main UI 

    setContentView(R.layout.activity_main); 

    //Binding the object of ListView 

    mainListView = (ListView) findViewById(R.id.mainlistview); 

    /**A fruitslist object is created of ArrayList type Which holds all element of array(Mango ,Pine-Apple...) , 
    * in friutlist object 
    */ 

    ArrayList<String> fruitlist = new ArrayList<String>(); 
    fruitlist.addAll(Arrays.asList(fruits)); 

    /** Now all these data are bind with a ListView's object for appearance in android device or emulator 
    * with a parameter of current class , name of Xml file which contain TextView , id of that TextView 
    * and finally object of ArrayList in which we all those data 
    */ 


    mainListView.setAdapter(new ArrayAdapter<String>(this,R.layout.list_view , R.id.rowTextView , fruitlist)); 

    /** Call of listener when we click on every item of ListView 
    * 
    */ 
    mainListView.setOnItemClickListener(new OnItemClickListener() { 
     public void onItemClick(AdapterView<?> parent, View view,int position, long id) { 

      // When clicked, show a toast with the TextView text 
      if(position == 0) 

      { 
       // do this on first item click 

      } 
      if(position == 1) 
      { 
        // do this on second item click 

      } 
      else 
      { 

      Toast.makeText(getApplicationContext(), "You have selected " + fruits[position],Toast.LENGTH_SHORT).show(); 
     } 
     } 
}); 

} 


    }