ListView.setAdapter 空指针异常

Question

当我遇到 NPE 时，我正在为 coursera 课程做作业。好几天都解决不了这个问题，不修复就无法继续。 Google 没有帮助。 更新了页面下方的代码 这是我的代码：

主要活动

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)

我似乎在 lv.setAdapter(ADAPTER); 有一个 NPE。在 Whosebug 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

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。我试图将 "lv =.. ; ADAPTER =..." 移动到 onResume，试图将它放在 onResume 的开始 - 它没有帮助:(

Answer 1

检查此 returns 是否为 null 以外的任何内容：

 ListView lv = (ListView) findViewById(R.id.listView);

可能是找不到列表。

Answer 2

由于您在 onResume 中调用 ListView lv = (ListView) findViewById(R.id.listView);，setContentView 尚未被调用。 onResume 总是在 onCreate 之前调用，因此您的 ListView 当前不在上下文中。因此 NullPointerException。

将您的 Activity 代码更改为此

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    ListView lv = (ListView) findViewById(R.id.listView);

}

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

Answer 3

问题出在你设置的这个，当你初始化新的自定义....你必须初始化它onCreate()！（我的意思是 MainActivity class ）

Answer 4

添加

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

进入 onCreate 方法，因为 this 上下文在 class methods.i.e 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);
    }

Answer 5

我认为 NPE 与您的 ListView 有关。尝试构建您的应用程序，以便声明 class 个变量并在 onCreate() 中初始化它们。这样，您稍后就可以引用它们（就像您在 onResume() 中所做的那样）。

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

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

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

    lv.setAdapter(ADAPTER);
}

@Override
protected void onResume() {
    super.onResume();
    /*
    *I don't quite see the purpose in this below section, 
    *as you can account for a null ArrayList in your adapter class
    */
    if (ADAPTER.getCount() >= 0) {
    lv.setAdapter(ADAPTER);
    }

}

此外，为了更好地解释我在上面所做的评论，您可以通过将空数据视为 0 大小列表来解释适配器 getCount() 中的空数据：

getCount() {
    if (selfies == null || selfies.isEmpty()) {
        return 0;
    }
    return selfies.size;
}

编辑： 如果问题仅限于您的适配器，则可能是来自 getView() 的 NPE。您可以尝试以下代码：

@Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View rowView;
            Selfie selfie = selfies.get(position);

            if (convertView == null) {
                LayoutInflater inflater = (LayoutInflater) parent.getContext().
                        getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                rowView = inflater.inflate(R.layout.row, parent, false);
            } else {
                rowView = convertView;
            }
            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 rowView;
        }

Answer 6

你的自拍列表是空的，因为我没有看到任何添加？还有你的 class Selfie 只有 getter 但没有 setter，那么你如何将自拍设置到你的自拍列表中？

Answer 7

答案是，在我的 getView() 代码中，我返回了 convertView 但膨胀了 rowView。所以 convertView 保持为空。 PPartisan 向我展示了我的错误并给出了有效代码（如下）。要提高评级，我必须有 15 个声誉，但我没有。有人可以提高他的评分和正确答案吗？

    public View getView(int position, View convertView, ViewGroup parent) {
    View rowView;
    Selfie selfie = selfies.get(position);
    if (convertView == null) {
        LayoutInflater inflater = (LayoutInflater) parent.getContext().
                getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        rowView = inflater.inflate(R.layout.row, parent, false);
    } else {
        rowView = convertView;
    }
    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 rowView;
}

ListView.setAdapter 空指针异常

ListView.setAdapter NullPointerException

java

android

listview