在自定义 ListView 中获取 NullPointerException答案

【问题标题】：Getting NullPointerException in custom ListView在自定义 ListView 中获取 NullPointerException
【发布时间】：2012-05-07 14:24:03
【问题描述】：

我在运行我的应用程序时遇到这样的错误：

02-27 22:59:19.331: E/AndroidRuntime(2060): FATAL EXCEPTION: main
02-27 22:59:19.331: E/AndroidRuntime(2060): java.lang.NullPointerException
02-27 22:59:19.331: E/AndroidRuntime(2060):     
        at myPlanAdapter.getView(MyPlanActivity.java:145)
        02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.AbsListView.obtainView(AbsListView.java:1315)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1198)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.ListView.onMeasure(ListView.java:1109)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.View.measure(View.java:8171)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:381)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.View.measure(View.java:8171)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.View.measure(View.java:8171)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.View.measure(View.java:8171)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.ViewRoot.performTraversals(ViewRoot.java:801)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.os.Handler.dispatchMessage(Handler.java:99)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.os.Looper.loop(Looper.java:123)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at android.app.ActivityThread.main(ActivityThread.java:4627)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at java.lang.reflect.Method.invokeNative(Native Method)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at java.lang.reflect.Method.invoke(Method.java:521)
    02-27 22:59:19.331: E/AndroidRuntime(2060):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)

我的源代码是：

public class MyPlanActivity extends Activity implements OnItemClickListener,
        OnClickListener {
    private String TAG = "MyPlanActivity";
    private Button addButton = null;
    private ListView listView;
    myPlanAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        viewInit();
        adapter = new myPlanAdapter(getBaseContext());
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(this);

    }

    private void viewInit() {
        setContentView(R.layout.myplan);
        addButton = (Button) findViewById(R.id.AddMyPlanButtonId);
        listView = (ListView) findViewById(R.id.myPlanListViewId);
        addButton.setOnClickListener(this);

    }

    public void onClick(View v) {
        switch (v.getId()) {
        case R.id.AddMyPlanButtonId:
            startActivity(new Intent(this, AddMyPlan.class));
            break;

        default:
            break;
        }
    }

    public void onItemClick(AdapterView<?> paramAdapterView, View paramView,
            int paramInt, long paramLong) {
        // TODO Auto-generated method stub

    }

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

    private class myPlanAdapter extends ArrayAdapter<MyPlan> {
        Context context;
        MyPlanDataSource dataSource;

        public myPlanAdapter(Context context) {
            super(context, R.layout.myplan_list_view_entry);
            this.context = context;
            dataSource = new MyPlanDataSource(context);
        }

        private ArrayList<MyPlan> getAllPlan() {

            dataSource.open();
            ArrayList<MyPlan> allPlan = dataSource.getAllPlan();
            dataSource.close();
            Log.d(TAG,allPlan.get(0).getSubjectName());
            return allPlan;
        }

        @Override
        public int getCount() {
            Log.d(TAG,"Size: "+getAllPlan().size());
            return getAllPlan().size();
        }

        @Override
        public MyPlan getItem(int position) {
            return getAllPlan().get(position);
        }

        @Override
        public View getView(final int position, View convertView,
                ViewGroup parent) {
            View row = convertView;
            if (row == null)
            {
                LayoutInflater inflater = (LayoutInflater) context
                        .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                row = inflater.inflate(R.layout.myplan_list_view_entry, parent,
                        false);

                final PlanViewHolder viewHolder = new PlanViewHolder();

                viewHolder.subjectName = (TextView) findViewById(R.id.myPlanListSubNameTextViewId);
                viewHolder.targetGrade = (TextView) findViewById(R.id.myPlanListTargetGradeTextViewId);
                viewHolder.duration = (TextView) findViewById(R.id.myPlanListDurationTextViewId);
                viewHolder.examDate = (TextView) findViewById(R.id.myPlanListExamDateTextViewId);
                viewHolder.daysTogo = (TextView) findViewById(R.id.myplanListDaystogoBoardTextViewId);
                viewHolder.delete = (ImageView) findViewById(R.id.myPlanDeleteImageViewId);
                Log.d(TAG, "" + position);

                row.setTag(viewHolder);
            }
            final PlanViewHolder holder = (PlanViewHolder) row.getTag();


            holder.subjectName.setText((getAllPlan().get(position) 
                    .getSubjectName())); // line 145
            holder.targetGrade.setText("Grade : "
                    + getAllPlan().get(position).getTargetGrade());
            holder.duration.setText(getAllPlan().get(position).getExamTime());
            holder.examDate.setText(getAllPlan().get(position).getExamDate());



            holder.daysTogo.setText(getAllPlan().get(position).getDaysTogo());

            holder.delete.setOnClickListener(new OnClickListener() {

                public void onClick(View v) {
                    AlertDialog.Builder builder = new AlertDialog.Builder(
                            context);
                    builder.setTitle("DELETE")
                            .setMessage(
                                    "Are you sure you want to delete this Subject?")
                            .setCancelable(false)
                            .setPositiveButton("Yes",
                                    new DialogInterface.OnClickListener()
                                    {
                                        public void onClick(
                                                DialogInterface dialog, int id)
                                        {

                                            dataSource.open();
                                            dataSource
                                                    .deletePlan(getItem(position));
                                            dataSource.close();
                                            notifyDataSetChanged();
                                        }
                                    })
                            .setNegativeButton("No",
                                    new DialogInterface.OnClickListener()
                                    {
                                        public void onClick(
                                                DialogInterface dialog, int id)
                                        {
                                            dialog.cancel();
                                        }
                                    });
                    AlertDialog alert = builder.create();
                    alert.show();

                }
            });


            return row;
        }
    }

    static class PlanViewHolder {
        private TextView subjectName;
        private TextView targetGrade;
        private TextView duration;
        private TextView examDate;
        private TextView daysTogo;
        private ImageView delete;
    }

}

【问题讨论】：

尝试使用this 而不是getBaseContext()。
重新运行并双击您的 logcat 错误 ....

标签： android android-listview

【解决方案1】：

我认为您在错误的位置搜索您的行视图。尝试在您的膨胀行中找到它们view（在getView() 方法中）：

viewHolder.subjectName = (TextView) row.findViewById(R.id.myPlanListSubNameTextViewId);
viewHolder.targetGrade = (TextView) row.findViewById(R.id.myPlanListTargetGradeTextViewId);
viewHolder.duration = (TextView) row.findViewById(R.id.myPlanListDurationTextViewId);
viewHolder.examDate = (TextView) row.findViewById(R.id.myPlanListExamDateTextViewId);
viewHolder.daysTogo = (TextView) row.findViewById(R.id.myplanListDaystogoBoardTextViewId);
viewHolder.delete = (ImageView) findViewById(R.id.myPlanDeleteImageViewId);

【讨论】：

【解决方案2】：

编辑ViewHolder ids，你需要像这样从convertView(the view passed in) 获取这些ids：

viewHolder.subjectName = (TextView) row.findViewById(R.id.myPlanListSubNameTextViewId);

为他人做同样的事情。

【讨论】：

【解决方案3】：

由于这是您获得NullPointerException 的行，因此您需要弄清楚null 是什么：

holder.subjectName.setText(
  (getAllPlan()
   .get(position)
   .getSubjectName()));

可能是对getAllPlan()、get(position) 或getSubjectName() 的调用。

getAllPlan() 方法返回什么类型？这种类型的键是否与列表中的位置相同？通过将代码放在不同的行上，可以更容易地发现你在哪里得到了 null。

此外，getAllPlan().get(position) 将在 ArrayAdapter 中返回与 getItem(position) 相同的值。

【讨论】：

【解决方案4】：

对您的 getView() 方法进行这些更改：

 @Override
        public View getView(final int position, View convertView,
                ViewGroup parent)
        {
            final PlanViewHolder viewHolder;

            if (row == null)
            {
                LayoutInflater inflater = (LayoutInflater) context
                        .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                convertView = inflater.inflate(R.layout.myplan_list_view_entry, parent,
                        false);

                viewHolder = new PlanViewHolder();

                viewHolder.subjectName = (TextView) findViewById(R.id.myPlanListSubNameTextViewId);
                viewHolder.targetGrade = (TextView) findViewById(R.id.myPlanListTargetGradeTextViewId);
                viewHolder.duration = (TextView) findViewById(R.id.myPlanListDurationTextViewId);
                viewHolder.examDate = (TextView) findViewById(R.id.myPlanListExamDateTextViewId);
                viewHolder.daysTogo = (TextView) findViewById(R.id.myplanListDaystogoBoardTextViewId);
                viewHolder.delete = (ImageView) findViewById(R.id.myPlanDeleteImageViewId);
                Log.d(TAG, "" + position);

                convertView.setTag(viewHolder);
            }
            else 
                viewHolder=(PlanViewHolder)convertView.getTag();   
         ... 
         return convertView;
}

【讨论】：