Question

我的ListView自定义适配器存在一些问题。我不确定我的问题是xml文件，数据还是ListView的实例化。我在设置适配器时收到NullPointerException。适配器类：

public class InboxAdapter extends BaseAdapter {

    private Context context;

    private List<MailInbox> mails;

    public InboxAdapter(Context context, List<MailInbox> mails)
    {
        this.context = context;
        this.mails = mails;
    }

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

    @Override
    public Object getItem(int item) {
        return mails.get(item);
    }

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

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

        MailInbox entry = mails.get(position);

        if(convertView == null)
        {
            LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.mail_list_row, null);
        }

        ImageView mailImg = (ImageView)convertView.findViewById(R.id.mail_thumb);
        boolean read = entry.IsRead;

        if(read)
        {
            //Set "read" image
            mailImg.setImageResource(R.drawable.message);
        } else
        {
            //Set "unread" image
            mailImg.setImageResource(R.drawable.message);
        }

        TextView author = (TextView)convertView.findViewById(R.id.mail_text);
        TextView body = (TextView)convertView.findViewById(R.id.mail_detail);
        TextView date = (TextView)convertView.findViewById(R.id.mail_date);

        author.setText(entry.AuthorName);
        body.setText(entry.MessageBody);
        date.setText(entry.CreatedAt);

        return convertView;
    }
}

我使用它的Activity：

public class InboxActivity extends SherlockActivity 
{
    private ListView list;
    private GetMessagesConnection mMessages;

    public String kidID;

    public InboxActivity() {
        // TODO Auto-generated constructor stub
    }

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.inbox_layout);        

        Bundle extras = getIntent().getExtras();
        if(extras != null)
        {
            kidID = extras.getString("kidID");
        }

        new GetMails().execute();
    }

    private void setUpList()
    {
        list = (ListView)findViewById(R.id.mail_list);      

        ArrayList<MailInbox> mails = new ArrayList<MailInbox>();

        for(int i = 0; i < mMessages.myMail.size(); i++)
        {
            MailInbox inbox = new MailInbox();

            inbox.AuthorName = mMessages.myMail.get(i).AuthorName;
            inbox.CreatedAt = mMessages.myMail.get(i).CreatedAt;
            inbox.MessageBody = mMessages.myMail.get(i).MessageBody;
            inbox.IsRead = mMessages.myMail.get(i).IsRead;

            System.out.println("Author: " + inbox.AuthorName);
            System.out.println("Time: " + inbox.CreatedAt);
            System.out.println("Message: " + inbox.MessageBody);
            System.out.println("Read: " + inbox.IsRead);

            mails.add(inbox);
        }

        int length = mails.size();
        System.out.println("Size: " + length);

        InboxAdapter adapter = new InboxAdapter(this, mails);

        // It crashes here with a Nullpointer Exception
        list.setAdapter(adapter);
    }

    private class GetMails extends AsyncTask<Void, Void, Void> implements DialogInterface.OnCancelListener
    {
        private ProgressDialog dialog;

        protected void onPreExecute()
        {
                dialog = ProgressDialog.show(InboxActivity.this, "", "Henter beskeder...", true);
        }

        protected Void doInBackground(Void... unused)
        {
                mMessages = new GetMessagesConnection();

                mMessages.kidID = kidID;

                mMessages.connection();

                return null;
        }

        protected void onPostExecute(Void unused)
        {           
            dialog.dismiss();
            setUpList();
        }

        public void onCancel(DialogInterface dialog)
        {
            cancel(true);
            dialog.dismiss();
        }
    }
}

* mail_list *：

<?xml version="1.0" encoding="utf-8"?>
<ListView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mail_list"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_weight="2" >    
</ListView>

* mail_list_row *：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="48dip"
    android:background="@drawable/listselector"
    >
    <ImageView
        android:id="@+id/mail_thumb"
        android:layout_width="30dip"
        android:layout_height="30dip"
        android:layout_centerVertical="true"
        android:layout_alignParentLeft="true"
        android:contentDescription="@string/imgdesc"
        />
    <TextView
        android:id="@+id/mail_text"
        android:layout_toRightOf="@+id/mail_thumb"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="14dip"
        android:layout_marginLeft="8dip"
        android:layout_centerVertical="false"
        android:singleLine="true"
        android:ellipsize="end"
        android:textStyle="bold"
        android:textColor="@android:color/white"
        />
    <TextView 
        android:id="@+id/mail_detail"
        android:layout_toRightOf="@+id/mail_thumb"
        android:layout_below="@+id/mail_text"
        android:layout_width="100dip"
        android:layout_height="wrap_content"
        android:textSize="12dip"
        android:layout_marginLeft="8dip"
        android:layout_centerVertical="false"
        android:singleLine="true"
        android:ellipsize="end"
        android:textColor="@color/grey"     
        />
    <TextView 
        android:id="@+id/mail_date"
        android:layout_toRightOf="@+id/mail_detail"
        android:layout_below="@+id/mail_text"
        android:layout_width="100dip"
        android:layout_height="wrap_content"
        android:textSize="12dip"
        android:singleLine="true"
        android:ellipsize="end"
        android:textColor="@color/grey"
        android:layout_alignParentRight="true"      
        />
</RelativeLayout>

我不知道可能是null。我一直试图解决这个问题几个小时。错误日志：

05-14 19:55:45.525: E/AndroidRuntime(2509): FATAL EXCEPTION: main
05-14 19:55:45.525: E/AndroidRuntime(2509): java.lang.NullPointerException
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.cim.daycare.InboxActivity.setUpList(InboxActivity.java:75)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.cim.daycare.InboxActivity.access$2(InboxActivity.java:36)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.cim.daycare.InboxActivity$GetMails.onPostExecute(InboxActivity.java:101)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.cim.daycare.InboxActivity$GetMails.onPostExecute(InboxActivity.java:1)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.os.AsyncTask.finish(AsyncTask.java:417)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.os.AsyncTask.access$300(AsyncTask.java:127)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.os.Looper.loop(Looper.java:123)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at android.app.ActivityThread.main(ActivityThread.java:3683)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at java.lang.reflect.Method.invokeNative(Native Method)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at java.lang.reflect.Method.invoke(Method.java:507)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
05-14 19:55:45.525: E/AndroidRuntime(2509):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

在活动InboxActivity中，您将内容视图设置为R.layout.inbox_layout（我在您的问题中看不到此文件），并搜索标识为{{1的ListView }}。 R.id.mail_list文件中存在ListView id。您确定mail_list.xml的内容视图不是InboxActivity：

R.layout.mail_list

此外：

请不要为//... public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.mail_list); Bundle extras = getIntent().getExtras(); //...声明构造函数，在InboxActivity方法中初始化您想要的东西（这就是android世界中的工作原理）。
您在onCreate extras时处理案件了吗？（如果null为extras，则null也将为kidID，如果您不处理此问题，这可能会让您在null课程遇到麻烦场景）。

Answer 2

您可以将实例添加到列表中，例如list = (ListView)findViewById(R.id.mail_list);，这意味着您正在检查该ID的当前布局，但您当前的布局设置为setContentView(R.layout.inbox_layout);

什么是inbox_layout？您还没有向我们展示XML？您的意思是将其设置为setContentView(R.layout.mail_list);吗？

编辑：此外，您可能希望在自定义适配器中使用ViewHolder，以避免重复调用findViewByID(...)。看一下本教程：http://xjaphx.wordpress.com/2011/06/16/viewholder-pattern-caching-view-efficiently/

自定义ListView适配器抛出NullPointerException

2 个答案: