动态添加条目到列表

Question

我正在使用意图捕获图像来调用本机相机，并且在活动结果中我试图为捕获的图像提供标题并将其添加到列表中，但我没有成功这样做我得到错误时我将该项目添加到列表中。应用程序从设备关闭，无法找出错误。有人可以建议我将元素添加到List中的正确方法。它应该添加到适配器或底层ArrayList吗？请帮助我。

我的代码如下：

public class AttachmentsActivity extends ListActivity {

private AttachmentListAdapter m_adapter;
private ArrayList<MediaData> m_orders = null;
StringBuffer nameBuffer;
private String captureDateTime;
private Uri uri;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    m_orders = new ArrayList<MediaData>();
    Enumeration<MediaData> enumeration = DashboardManager.getInstance()
            .getAttachmentList().elements();
    while (enumeration.hasMoreElements()) {
        m_orders.add((MediaData) enumeration.nextElement());
    }

    this.m_adapter = new AttachmentListAdapter(this,
            android.R.layout.simple_list_item_1, m_orders);
    setListAdapter(this.m_adapter);
    this.m_adapter.notifyDataSetChanged();
}

@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    // TODO Auto-generated method stub
    menu.removeGroup(1);
    if (DashboardManager.getInstance().isEnableCamera()) {
        menu.add(1, 1, Menu.NONE, "Camera");
    }
    if (DashboardManager.getInstance().isEnableAudio()) {
        menu.add(1, 2, Menu.NONE, "Audio");
    }

    return true;
}

private class AttachmentListAdapter extends ArrayAdapter<MediaData> {

    private ArrayList<MediaData> items;
    private LayoutInflater mInflater;
    private Bitmap high;
    private Bitmap medium;
    private Bitmap low;
    private LinearLayout linearLayout;
    private TextView[] textViews;
    private Context context;
    private Vector tableHeaders;
    private int width;
    private StringBuffer firstLine;



    public AttachmentListAdapter(Context context, int textViewResourceId,
            ArrayList<MediaData> items) {
        super(context, textViewResourceId, items);
        this.context = context;
        System.out.println("Inside NotificationListAdapter");
        System.out.println(items);
        this.items = items;
        firstLine = new StringBuffer();
        mInflater = LayoutInflater.from(context);
        Display display = getWindowManager().getDefaultDisplay();

    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        MediaData mediaData = items.get(position);
        ViewHolder holder = null;
        if (mediaData != null) {
            if (convertView == null) {
                holder = new ViewHolder();
                // holder.content = (TextView) mInflater.inflate(
                // R.layout.column, null);
                holder.content = new TextView(context);
                holder.content.setGravity(Gravity.CENTER_VERTICAL);
                holder.content.setHeight(25);
                convertView = mInflater.inflate(R.layout.table_row, null);
                linearLayout = (LinearLayout) convertView
                        .findViewById(R.id.linearLayout);

                linearLayout.addView(holder.content);

            }
            firstLine.delete(0, firstLine.length());
            firstLine.append(mediaData.getType()).append(" ")
                    .append(mediaData.toString());

            holder.content.setText(firstLine.toString());
        }

        return linearLayout;
    }

}

static class ViewHolder {
    TextView content;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // TODO Auto-generated method stub
    switch (item.getItemId()) {

    case 1:
        nameBuffer = new StringBuffer();
        if (DashboardManager.getInstance().getWaterMarkingText().length() > 0) {
            nameBuffer.append(
                    DashboardManager.getInstance().getWaterMarkingText())
                    .append("_");
        }

        captureDateTime = Calander.getSubmitDateTime(new Date());
        nameBuffer
                .append(captureDateTime)
                .append("_")
                .append(MobileBaseService.getInstance()
                        .getApplicationUser().getUserId()).append(".jpg");

        String dirString = "/sdcard/pravaa/";
        File dir = new File(dirString);
        if(!dir.exists()) dir.mkdir();
        File attachment=new File(dirString+nameBuffer.toString());
        uri=Uri.fromFile(attachment);
        System.out.println("URI:" + attachment.toURI());
        int resultCode = 1337;

        startActivityForResult(AttachmentUtility.getCameraIntent(uri),
            resultCode);


    }
    return true;
}



protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (resultCode == Activity.RESULT_OK) {
        switch (requestCode) {
        case 1337:

                     // TODO Auto-generated method stub String
                     String dirString="/sdcard/pravaa/"; //Toast

                      File attachment=new File(dirString+nameBuffer.toString());


                      if(attachment.exists()){ 
                          try { 
                      FileInputStream fis;
                      fis = new FileInputStream(attachment);
                      ByteArrayOutputStream baos = new ByteArrayOutputStream();

                      Bitmap bi = BitmapFactory.decodeStream(fis);
                      bi.compress(CompressFormat.JPEG, 100, baos);
                      byte[]attData = baos.toByteArray();
                      fis.close();
                      if (attData.length>0) {
                                MediaData mediaData = new MediaData();
                                mediaData.setAttachmentUri(uri);
                                mediaData.setType(ApplicationStore.MEDIA_TYPE_IMAGE);
                                mediaData.setData(attData);
                                mediaData.setNewAttachment(true);
                                mediaData.setNote("");
                                mediaData.setTransactionId(MobileBaseService.getInstance().getNextTransactionId());
                                mediaData.setCaptureDataTime(captureDateTime);
                                mediaData.setCaption(mediaData.getCaptureDataTime() + "_" + MobileBaseService.getInstance().getApplicationUser().getUserId());
                                mediaData.setNumber(nameBuffer.toString());

                                if (DashboardManager.getInstance().isEnableGps() && ApplicationManager.getInstance().isGpsEnabled()) {
                                    mediaData.setGpsEnabled(true);
                                    Location location= ApplicationManager.getInstance().getLocation();

                                    if (location != null) {

                                        mediaData.getGpsData().setLattitude(location.getLatitude());
                                        mediaData.getGpsData().setLongitude(location.getLongitude());
                                        mediaData.getGpsData().setAltidude(location.getAltitude());
                                        mediaData.getGpsData().setAccuracy("Y");
                                        mediaData.getGpsData().setDateTime(Calander.getSubmitDateTime(new Date()));
                                    } else {
                                     //   ApplicationManager.getInstance().Log(Level.TRACE, "Could not obtain GPS for Attachment " + mediaData.getNumber());
                                        mediaData.getGpsData().setLattitude(0);
                                        mediaData.getGpsData().setLongitude(0);
                                        mediaData.getGpsData().setAltidude(0);
                                        mediaData.getGpsData().setAccuracy("N");
                                        mediaData.getGpsData().setDateTime("");
                                    }
                                } else {
                                    mediaData.getGpsData().setLattitude(0);
                                    mediaData.getGpsData().setLongitude(0);
                                    mediaData.getGpsData().setAltidude(0);
                                    mediaData.getGpsData().setAccuracy("N");
                                    mediaData.getGpsData().setDateTime(Calander.getSubmitDateTime(new Date()));
                                }
                                    **//Commenting this line of code does not cause the crash**
                                m_adapter.add(mediaData);


                      }
                      } catch (FileNotFoundException e) {
                     //e.printStackTrace();
                     } catch (IOException e) {

                    // TODO Auto-generated catch block
                    // e.printStackTrace();
                     }catch (Exception e) {

                     }
                     }
            break;
        }
    }
}
}

调用以下内容时出错： m_adapter.add（mediaData）; （第149行）和我的堆栈跟踪如下：

E/AndroidRuntime( 1367): java.lang.NullPointerException
E/AndroidRuntime( 1367):        at com.pravaa.dashboard.ui.components.Attachment
sActivity$AttachmentListAdapter.getView(AttachmentsActivity.java:149)
E/AndroidRuntime( 1367):        at android.widget.AbsListView.obtainView(AbsList
View.java:1427)
E/AndroidRuntime( 1367):        at android.widget.ListView.makeAndAddView(ListVi
ew.java:1802)
E/AndroidRuntime( 1367):        at android.widget.ListView.fillSpecific(ListView
.java:1347)
E/AndroidRuntime( 1367):        at android.widget.ListView.layoutChildren(ListVi
ew.java:1633)
E/AndroidRuntime( 1367):        at android.widget.AbsListView.onLayout(AbsListVi
ew.java:1280)
E/AndroidRuntime( 1367):        at android.view.View.layout(View.java:7035)
E/AndroidRuntime( 1367):        at android.widget.FrameLayout.onLayout(FrameLayo
ut.java:333)
E/AndroidRuntime( 1367):        at android.view.View.layout(View.java:7035)
E/AndroidRuntime( 1367):        at android.widget.LinearLayout.setChildFrame(Lin
earLayout.java:1249)
E/AndroidRuntime( 1367):        at android.widget.LinearLayout.layoutVertical(Li
nearLayout.java:1125)
E/AndroidRuntime( 1367):        at android.widget.LinearLayout.onLayout(LinearLa
yout.java:1042)
E/AndroidRuntime( 1367):        at android.view.View.layout(View.java:7035)
E/AndroidRuntime( 1367):        at android.widget.FrameLayout.onLayout(FrameLayo
ut.java:333)
E/AndroidRuntime( 1367):        at android.view.View.layout(View.java:7035)
E/AndroidRuntime( 1367):        at android.view.ViewRoot.performTraversals(ViewR
oot.java:1045)
E/AndroidRuntime( 1367):        at android.view.ViewRoot.handleMessage(ViewRoot.
java:1727)
E/AndroidRuntime( 1367):        at android.os.Handler.dispatchMessage(Handler.ja
va:99)
E/AndroidRuntime( 1367):        at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime( 1367):        at android.app.ActivityThread.main(ActivityThrea
d.java:4633)
E/AndroidRuntime( 1367):        at java.lang.reflect.Method.invokeNative(Native
Method)
E/AndroidRuntime( 1367):        at java.lang.reflect.Method.invoke(Method.java:5
21)
E/AndroidRuntime( 1367):        at com.android.internal.os.ZygoteInit$MethodAndA
rgsCaller.run(ZygoteInit.java:858)
E/AndroidRuntime( 1367):        at com.android.internal.os.ZygoteInit.main(Zygot
eInit.java:616)
E/AndroidRuntime( 1367):        at dalvik.system.NativeStart.main(Native Method)

错误发生在 AttachmentListAdapter的 getView（）方法中的以下行 holder.content.setText（firstLine.toString（））; STRONG>。 请帮助我。提前谢谢。

Answer 1

如果holder不是convertView，则不会为null分配值。要正确使用此“holder”-technique，请将持有者保存在convertView（作为标记）中，并在convertView不为空时检索它（this video中对此进行了解释）很好，看看吧！）。

   ViewHolder holder = null;
    if (mediaData != null) {
        if (convertView == null) {
            holder = new ViewHolder();
            // holder.content = (TextView) mInflater.inflate(
            // R.layout.column, null);
            holder.content = new TextView(context);
            holder.content.setGravity(Gravity.CENTER_VERTICAL);
            holder.content.setHeight(25);
            convertView = mInflater.inflate(R.layout.table_row, null);
            convertView.setTag(holder);
            linearLayout = (LinearLayout) convertView
                    .findViewById(R.id.linearLayout);

            linearLayout.addView(holder.content);

        } else {
            holder = convertView.getTag();
        }
        firstLine.delete(0, firstLine.length());
        firstLine.append(mediaData.getType()).append(" ")
                .append(mediaData.toString());

        holder.content.setText(firstLine.toString());
    }

还有一些其他奇怪的事情发生（例如，为什么linearLayout是一个成员变量），你应该看看。