如何让复选框在我的列表视图中工作？

Question

我一直在尝试按照 S.O 上的教程/post 学习如何让我的 checkbox 在我的 listview 中工作。我想用它做很多事情，但首先我想简单地检查项目的位置，但应用程序崩溃了，我收到 NullPointerException 错误。

错误是（这部分下面有更多内容）：

    java.lang.NullPointerException at com.example.chris.tutorialspoint.SelectPhoneContactAdapter.getView(SelectPhoneContactAdapter.java:104)

第 104 行是：

convertView.setTag(v);

但对我来说，我似乎一直在正确地遵循教程，但我不知道如何使这些 post 适应我的问题：Getting NullPointerException in custom ListView and Crash in ListView at AbsListView.obtainView for ListActivity。你能告诉我哪里出了问题吗？在我开始尝试使用这些复选框之前，一切都运行良好。

这是我的 customadapter 代码，SelectPhoneContactAdapter:

public class SelectPhoneContactAdapter extends BaseAdapter {

    //define a list made out of SelectPhoneContacts and call it theContactsList
    public List<SelectPhoneContact> theContactsList;
    //define an array list made out of SelectContacts and call it arraylist
    private ArrayList<SelectPhoneContact> arraylist;
    boolean itemChecked[];
    Context _c;

    //define a ViewHolder to hold our name and number info, instead of constantly querying
    // findviewbyid. Makes the ListView run smoother
    ViewHolder v;

    public SelectPhoneContactAdapter(final List<SelectPhoneContact> selectPhoneContacts, Context context) {
        theContactsList = selectPhoneContacts;
        _c = context;
        this.arraylist = new ArrayList<SelectPhoneContact>();
        this.arraylist.addAll(theContactsList);
        itemChecked = new boolean[theContactsList.size()];

    }


    @Override
    public int getCount() {
        System.out.println("the amount in arraylist :" + arraylist.size());
        return arraylist.size();
    }

    @Override
    public Object getItem(int i) {
        return theContactsList.get(i);
    }

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

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)

    static class ViewHolder {
        //        In each cell in the listview show the items you want to have
//        Having a ViewHolder caches our ids, instead of having to call and load each one again and again
        TextView title, phone;
        CheckBox check;
    }

    @Override
    public View getView(final int i, View convertView, ViewGroup viewGroup) {

        //we're naming our convertView as view
        View view = convertView;


        if (view == null) {

            v = new ViewHolder();
            System.out.println("getview position :" + i);

            //if there is nothing there (if it's null) inflate the view with the layout
            LayoutInflater li = (LayoutInflater) _c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = li.inflate(R.layout.phone_inflate_listview, null);


            //      So, for example, title is cast to the name id, in phone_inflate_listview,
//        phone is cast to the id called no etc
            v.title = (TextView) view.findViewById(R.id.name);
            v.phone = (TextView) view.findViewById(R.id.no);
            v.check = (CheckBox) view.findViewById(R.id.checkBoxContact);

            convertView.setTag(v);

            //or else use the view (what we can see in each row) that is already there
        } else {
            view = convertView;

        }


//        store the holder with the view
        final SelectPhoneContact data = (SelectPhoneContact) theContactsList.get(i);

        //in the listview for contacts, set the name
        v.title.setText(data.getName());
        //in the listview for contacts, set the number
        v.phone.setText(data.getPhone());
        v.check.setChecked(false);

        v.check.setChecked(itemChecked[i]);

        v.check
                .setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
                    @Override
                    public void onCheckedChanged(CompoundButton buttonView,
                                                 boolean isChecked) {
                        itemChecked[i] = isChecked;
                    }
                });

        // Return the completed view to render on screen
        return view;


    }
}

我的 getter 和 setter，SelectPhoneContact:

public class SelectPhoneContact {

    String phone;

    public String getPhone() {return phone;}

    public void setPhone(String phone) {
        this.phone = phone;
    }

    String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    Boolean selected;

    public boolean isSelected() {
        return selected;
    }

    public void setSelected(boolean selected){
        this.selected=selected;
    }
}

如有必要，我可以 post 更多代码。

Answer 1

考虑这段代码：

@Override
public View getView(final int i, View convertView, ViewGroup viewGroup) {
    ...
    View view = convertView;

    if (view == null) {
        ...
        view = li.inflate(R.layout.phone_inflate_listview, null);
        ...
        convertView.setTag(v);
    }
    ...
}

首先，您将 convertView 的值分配给 view 变量。当它为 null 时，您分支到 if 语句，在这里您通过 li.inflate().

将新值分配给 view

但是，您稍后会在此 if 语句中引用 convertView。尽管你在上面写了 view = convertView，但此时 convertView 仍然是 null。

有两种方法可以解决这个问题。第一个选项是简单地将 convertView.setTag(v) 更改为 view.setTag(v)。另一种是删除这一行：

View view = convertView;

只需将您引用 view 的任何地方更改为使用 convertView 即可。无需引入新的 View view 变量；您可以直接使用 convertView。

如何让复选框在我的列表视图中工作？

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