java - 获取卡片 View 标签时出现空指针异常

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我创建了一个自定义列表，其中包含卡片 View 、按钮、 TextView 和开关。我已将标签设置为卡片 View 。

但是当我尝试在 TableView 持有者中检索此标记时，它会抛出空指针异常。

如果我在监听器中获取标签，它不会抛出任何异常。我不知道从哪里访问这个标签。

如果状态为 1，我想为 switch 设置 setChecked true，如果状态为 0，则 setChecked 应该为 false。

但我不知道在哪里可以做到这一点？

自定义警报适配器

public class TableListAdapter extends RecyclerView.Adapter<TableListAdapter.TableViewHolder> {


    public static TimeTableHelper db;
    public static TimeTableList timeTableList;

    public static int cardId,id,status=0;
    public static boolean editMode;
    public static List<TimeTable> tableList;
    public static TimeTable t = new TimeTable();
    public static TimeTable ci;

    public TableListAdapter(List<TimeTable> tableList,TimeTableList timeTableList) {
        this.tableList = tableList;
        this.timeTableList = timeTableList;
    }
    private Context context;

    public TableListAdapter(Context context) {
        this.context = context;
    }
    @Override
    public int getItemCount() {
          return tableList.size();
    }

    @Override
    public void onBindViewHolder(TableViewHolder tableViewHolder, int i) {


        ci = tableList.get(i);

        tableViewHolder.cv.setTag(i);

        tableViewHolder.aSwitch.setTag(i);

        Log.d("setId", String.valueOf(i));
        tableViewHolder.tableTitle.setText(ci.getTitle());

        ((GradientDrawable)tableViewHolder.color.getBackground()).setColor(ci.getTableColor());



    }

   @Override
   public TableViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {

        View  itemView = LayoutInflater.
                   from(viewGroup.getContext()).
                   inflate(R.layout.table_card, viewGroup, false);


       return new TableViewHolder(itemView);
   }

  public static class TableViewHolder extends RecyclerView.ViewHolder {

      protected TextView tableTitle;
      protected CardView cv;
      protected SwitchCompat aSwitch;
      protected Button color;


      public TableViewHolder(final View v) {
          super(v);
          tableTitle = (TextView) v.findViewById(R.id.tableTitle);
          cv = (CardView) v.findViewById(R.id.card_view);
          aSwitch = (SwitchCompat) v.findViewById(R.id.switch2);
          color = (Button) v.findViewById(R.id.selectColor);

          db = new TimeTableHelper(timeTableList);


          cv.setOnLongClickListener(new View.OnLongClickListener() {

              @Override
              public boolean onLongClick(final View v) {
                  // TODO Auto-generated method stub

                  final AlertDialog.Builder builder = new AlertDialog.Builder(timeTableList);

                  builder.setTitle("Delete Time Table")
                          .setMessage("Are you sure you want to Delete this Time Table?")
                          .setPositiveButton(android.R.string.yes, new DialogInterface.OnClickListener() {
                              public void onClick(DialogInterface dialog, int which) {

                                  cardId = (int)v.getTag();

                                  Log.d("cardId", String.valueOf(cardId));

                                  t = tableList.get(cardId);
                                  id = t.getId();

                                  t = db.getTable(id);
                                  db.deleteTable(t);

                                  Intent intent = new Intent(timeTableList,TimeTableList.class);
                                  timeTableList.finish();
                                  timeTableList.startActivity(intent);

                              }

                          })


                          .setNegativeButton(android.R.string.no, new DialogInterface.OnClickListener() {
                              public void onClick(DialogInterface dialog, int which) {
                                  // do nothing
                              }
                          })

                          .setIcon(R.drawable.ic_warning_black_36dp)
                          .show();
                  return true;
              }

          });


          cv.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(final View v) {

                  cardId = (int)v.getTag();

                  Log.d("cardId", String.valueOf(cardId));

                  t = tableList.get(cardId);
                  id = t.getId();

                  Log.d("Id",String.valueOf(id));

                  editMode = true;
                  Intent i = new Intent(timeTableList, NewTimeTable.class);
                  i.putExtra("editMode", editMode);
                  i.putExtra("tableId", id);
                  timeTableList.startActivity(i);
              }
          });


          cardId = (int) cv.getTag();

          Log.d("cardId", String.valueOf(cardId));

          t = tableList.get(cardId);
          id = t.getId();

          if(status == 1)
          {
              aSwitch.setChecked(true);
          }
          else
          {
              aSwitch.setChecked(false);
          }


          aSwitch.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
              @Override
              public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {



                  if (isChecked) {

                      status = 1;

                      t = db.getTable(id);
                      t.setStatus(status);
                      db.updateStatus(t);

                      Log.d("status", String.valueOf(status));

                      final List<TimeTable> events = db.getAllTables();
                      for (TimeTable cn : events) {
                          String log = "Id: " + cn.getId() + " ,Title: " + cn.getTitle() +
                                  "Status: " + cn.getStatus() + ",color: " + cn.getTableColor();
                          Log.d("Data ", log);
                      }

                  } else {

                      status = 0;

                      t = db.getTable(id);
                      t.setStatus(status);
                      db.updateStatus(t);
                      final List<TimeTable> events = db.getAllTables();
                      for (TimeTable cn : events) {
                          String log = "Id: " + cn.getId() + " ,Title: " + cn.getTitle() +
                                  "Status: " + cn.getStatus() + ",color: " + cn.getTableColor();
                          Log.d("Data ", log);
                      }
                      Log.d("status", String.valueOf(status));
                  }

              }
          });

      }
  }

    public void updateAdapaterList(List<TimeTable> newTimeTableList) {
        //Replace the current list with new list
        this.tableList = newTimeTableList;
        //notify the adapter that the data set has changed
        notifyDataSetChanged();
    }
}

谢谢。

Answer 1

您应该在 View 持有者中使用 ID、标签或任何类型的标识符，因为您的 View 持有者会在回收者 View 中重复使用。我相信这就是给你带来麻烦的原因。

您应该使用onBindViewHolder来分配每个 View 持有者的数据并将其链接到它。

为了在该方法内分配监听器和 ID，您应该执行如下操作:

public void onBindViewHolder(TableViewHolder tableViewHolder, int i) {
    final TimeTable ci = tableList.get(i);

    //...

    tableViewHolder.setOnSomethingListener() {
    // You can use the ci reference here because you declared it as final
    // When this view is reused and references another position, the listener will be updated on the onBindViewHolder, to reference the new position that it will be being associated with
    }

}

另外，顺便说一句，此类中的所有静态变量可能都不是静态的，因为如果您碰巧在其他 View 中拥有更多适配器实例，您最终不希望它们相互干扰。