java - ListView.setAdapter NullPointerException

Question

当我 catch NPE 时，我正在做 Coursera 类(class)的作业。这个问题好几天都无法解决，不解决就无法继续。谷歌没有帮助。页面下方更新了代码这是我的代码:

主要 Activity

List<Selfie> selfies = new ArrayList<Selfie>();
private CustomAdapter ADAPTER = new CustomAdapter(this, selfies);

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

@Override
protected void onResume() {
    super.onResume();
    File youDir = new File("/sdcard/DailySelfie/");

    outer : for (File f : youDir.listFiles()) {
        //try to find selfie in files
        if (f.isFile()) {
            for (Selfie selfie : selfies) {
                if (selfie.getSelfieName() == f.getName()) {
                    continue outer;
                }
                else { continue;}
            }
            //code below works only if file doesn't exist in selfies.
            Bitmap myBitmap = BitmapFactory.decodeFile(f.getAbsolutePath());
            try {
                 selfies.add(new Selfie(f.getName(), myBitmap));
            }  catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

自拍:

public class Selfie {
private Bitmap selfieBitmap;
private String selfieName;
public Selfie(String selfieName, Bitmap selfieBitmap) {
    this.selfieName = selfieName;
    this.selfieBitmap = selfieBitmap;
}
public String getSelfieName() {
    return selfieName;
}
public Bitmap getSelfieBitmap() {
    return selfieBitmap;
}
}

自定义适配器:

private Context context;
private List<Selfie> selfies = new ArrayList<Selfie>();

public CustomAdapter(Context context, List<Selfie> selfies) {
    this.context = context;
    this.selfies = selfies;
}

@Override
public int getCount() {
    return selfies.size();
}

@Override
public Object getItem(int position) {
    return selfies.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup viewGroup) {
    Selfie selfie = selfies.get(position);
    if (convertView == null) {
        LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View rowView = new View(context);
        //get layout from row.xml
        rowView = inflater.inflate(R.layout.row, null);
        ImageView imageView = (ImageView) rowView.findViewById(R.id.imageView);
        imageView.setImageBitmap(selfie.getSelfieBitmap());
        TextView textView = (TextView) rowView.findViewById(R.id.text);
        textView.setText(selfie.getSelfieName());
    }
    return convertView;
}
}

亚行:

    04-27 13:17:58.792: E/AndroidRuntime(1979): FATAL EXCEPTION: main
04-27 13:17:58.792: E/AndroidRuntime(1979): java.lang.NullPointerException
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.AbsListView.obtainView(AbsListView.java:2179)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.ListView.makeAndAddView(ListView.java:1840)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.ListView.fillDown(ListView.java:675)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.ListView.fillFromTop(ListView.java:736)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.ListView.layoutChildren(ListView.java:1655)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.AbsListView.onLayout(AbsListView.java:2012)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.layoutHorizontal(LinearLayout.java:1660)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.onLayout(LinearLayout.java:1436)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.support.v7.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:502)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.View.layout(View.java:14289)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewGroup.layout(ViewGroup.java:4562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1976)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1730)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.Choreographer.doCallbacks(Choreographer.java:562)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.Choreographer.doFrame(Choreographer.java:532)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.os.Handler.handleCallback(Handler.java:730)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.os.Handler.dispatchMessage(Handler.java:92)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.os.Looper.loop(Looper.java:137)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at android.app.ActivityThread.main(ActivityThread.java:5103)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at java.lang.reflect.Method.invokeNative(Native Method)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at java.lang.reflect.Method.invoke(Method.java:525)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-27 13:17:58.792: E/AndroidRuntime(1979):     at dalvik.system.NativeStart.main(Native Method)

似乎我在 lv.setAdapter(ADAPTER); 处有一个 NPE。在 StackOverflow 上发现了同样的问题，但没有解决方案帮助我。更新:重写了我的代码，所以现在是

List<Selfie> selfies;
ListView lv;
private CustomAdapter ADAPTER;

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

    selfies =  new ArrayList<Selfie>();
    lv = (ListView) findViewById(R.id.listView);
    ADAPTER = new CustomAdapter(this, selfies);
}

@Override
protected void onResume() {
    super.onResume();
...
lv.setAdapter(ADAPTER);
}

仍然有同样的问题 - NPE。我尝试将“lv =.. ; ADAPTER =...”移至 onResume，尝试将其放在 onResume 的开头 - 它没有帮助:(

Answer 1

添加

private CustomAdapter ADAPTER = new CustomAdapter(this, selfies);

进入 onCreate 方法，因为 this 上下文在类方法中使用。重写为

 private CustomAdapter ADAPTER;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        ADAPTER = new CustomAdapter(this, selfies);
    }