Question

我是android编程的新手，我看过一个从服务器获取文件名并将其存储到列表中并构建一个listview来显示所有文件的教程，但这给了我空指针错误。谁能帮我改正代码，然后告诉我我的错误是什么，以便我学习。这是代码

public class VideoGalleryList extends AppCompatActivity {

    URL url1;
    List serverDir;
    ListView galleryList;
    public final String TAG="data.dot";


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

    protected void findViews() {
        galleryList = findViewById(R.id.gallerylist);
        List arraylist=GetfromServer();
        ArrayAdapter arrayAdapter= new ArrayAdapter(this,android.R.layout.simple_list_item_1,arraylist);
        galleryList.setAdapter(arrayAdapter);

    }
    public List GetfromServer(){
        try {
            url1=new URL("http://myipaddress/videos/");
            ApacheURLLister lister = new ApacheURLLister();
            serverDir=lister.listAll(url1);
            Log.d(TAG,serverDir.toString());
        }
        catch (Exception e){
            e.printStackTrace();
        }
        return serverDir;

    }

}

我的错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.a49ersense, PID: 2022
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.a49ersense/com.example.a49ersense.VideoGalleryList}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:392)
        at android.widget.ListView.setAdapter(ListView.java:585)
        at com.example.a49ersense.VideoGalleryList.findViews(VideoGalleryList.java:34)
        at com.example.a49ersense.VideoGalleryList.onCreate(VideoGalleryList.java:27)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

Answer 1

我希望这对您有用。

在您的findViews()方法中初始化您的列表

serverDir = new ArrayList();

并使用GetfromServer()这样的方法

public List GetfromServer(){
    try {
        url1=new URL("http://myipaddress/videos/");
        ApacheURLLister lister = new ApacheURLLister();
        serverDir.addAll(lister.listAll(url1));
        Log.d(TAG,serverDir.toString());
    }
    catch (Exception e){
        e.printStackTrace();
    }
    return serverDir;
}

Answer 2

首先检查空指针异常

    List arraylist=GetfromServer();
      if(arraylist!= null){
    ArrayAdapter arrayAdapter= new ArrayAdapter(this,android.R.layout.simple_list_item_1,arraylist);
    galleryList.setAdapter(arrayAdapter)
      };

 or 

  List arraylist = new ArrayList<>();
  arraylist=GetfromServer();
ArrayAdapter arrayAdapter= new ArrayAdapter(this,android.R.layout.simple_list_item_1,arraylist);
    galleryList.setAdapter(arrayAdapter)

因为您的函数getFromServer函数可能返回空值，所以您首先要初始化变量

List.size（）空指针异常

2 个答案: