android - 为什么 (ListView)findViewById 返回 null？

Question

帮助!我真的很欣赏 Stack Overflow 和它在过去几个月里的贡献者。我有很多问题，我在这里找到了答案......但是这个我似乎无法在任何地方找到......我是 Java 和 Android 的菜鸟，我一直在努力弄清楚这个出去几天。出于某种原因，我有一个名为 fileList 的 ListView 对象，它返回 null ...一切都编译得很好，但是当我尝试使用 fileList 时我得到了一个 NullPointerException ...我已经能够将它隔离到它的声明中:

ListView fileList = (ListView)findViewById(R.id.open_ListView);

但我终究无法理解这行代码有什么问题!我在下面包含了很多代码，理论上它应该包含可能以任何方式与此错误相关的所有代码。

如有任何帮助，我们将不胜感激!谢谢!

---

这是有问题的代码部分。它只是开关 block 的 OPEN_DIALOG 部分，所有其他开关都可以很好地显示它们的新对话框。我已经用星标标记了违规行...

@Override
protected Dialog onCreateDialog(int id) 
{
    Dialog newDialog = new Dialog(Minervalia.this);
    switch(id)
    {
    case DIALOG_FILE_NEW:
        newDialog.setContentView(R.layout.new_file);
        newDialog.setTitle("New File");
        newDialog.setCancelable(true);

        Button okBtn = (Button) newDialog.findViewById(R.id.ok_btn);
        Button cancelBtn = (Button) newDialog.findViewById(R.id.cancel_btn);
        final EditText widthEt = (EditText) newDialog.findViewById(R.id.width_edit);
        final EditText heightEt = (EditText) newDialog.findViewById(R.id.height_edit);

        okBtn.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                file_width = Integer.parseInt(widthEt.getText().toString());
                file_height = Integer.parseInt(heightEt.getText().toString());
                onCreate(null);
                dismissDialog(DIALOG_FILE_NEW);
            }
        });

        cancelBtn.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                dismissDialog(DIALOG_FILE_NEW);
            }
        });
        return newDialog;
    case DIALOG_OPEN:
        newDialog.setContentView(R.layout.open_file);
        newDialog.setTitle("Open File");
        newDialog.setCancelable(true);

// ********** This next line returns null! Why?
        ListView fileList = (ListView)findViewById(R.id.open_ListView);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.list_item, loadFileList());
        fileList.setAdapter(adapter);
        fileList.setTextFilterEnabled(true);

        fileList.setOnItemClickListener(new OnItemClickListener() 
        {
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) 
            {
                //  When clicked, show a toast with the TextView text
                Toast.makeText(getApplicationContext(), ((TextView) view).getText(), Toast.LENGTH_SHORT).show();
            }
        });

        return newDialog;
    case DIALOG_SAVE:
        newDialog.setContentView(R.layout.save_file);
        newDialog.setTitle("Save File");
        newDialog.setCancelable(true);

//--==[[ Define the important TextViews for our Save Dialog ]]==--\\            
        TextView pathTxt = (TextView) newDialog.findViewById(R.id.save_path_info);
        EditText fnTxt = (EditText) newDialog.findViewById(R.id.save_filename_edit);

//--==[[ Define the Radio Buttons for our Save Dialog ]]==--\\          
        RadioButton JPEGoption = (RadioButton) newDialog.findViewById(R.id.save_JPEGoption);
        RadioButton PNGoption = (RadioButton) newDialog.findViewById(R.id.save_PNGoption);

        file_type = TYPE_JPEG; // Set this as the default option

        JPEGoption.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                file_type = TYPE_JPEG;
            }
        });

        PNGoption.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                file_type = TYPE_PNG;
            }
        });

        Button save_okBtn = (Button) newDialog.findViewById(R.id.save_ok_btn);
        Button save_cancelBtn = (Button) newDialog.findViewById(R.id.save_cancel_btn);

        path = pathTxt.getText().toString();

        fnTxt.addTextChangedListener(new TextWatcher() 
        { 
            public void afterTextChanged(Editable s) 
            { 
            } 
            public void beforeTextChanged(CharSequence s, int start, int count, int after) 
            { 
            } 
            public void onTextChanged(CharSequence s, int start, int before, int count) 
            { 
                filename = s.toString(); 
            } 
        }); 
        Toast.makeText(this, path, Toast.LENGTH_SHORT).show();
        Toast.makeText(this, filename, Toast.LENGTH_SHORT).show();
        save_okBtn.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                try 
                {
                    String fullName = path + filename;
                    Bitmap.CompressFormat compForm = Bitmap.CompressFormat.JPEG; // make sure our format is initialized
                    if(file_type == TYPE_JPEG) 
                    {
                        fullName = fullName + ".jpg";
                        compForm = Bitmap.CompressFormat.JPEG;
                    }
                    if(file_type == TYPE_PNG)
                    {
                        fullName = fullName + ".png";
                        compForm = Bitmap.CompressFormat.PNG;
                    }
                    File thisImage = new File(fullName);
                    FileOutputStream out = new FileOutputStream(thisImage);
                    mBitmap.compress(compForm, 90, out);

                    new SingleMediaScanner(mContext, thisImage);
                    out.flush();
                    out.close();
                } 
                catch (Exception e) 
                {
                    e.printStackTrace();
                }

                dismissDialog(DIALOG_SAVE);
            }
        });

        save_cancelBtn.setOnClickListener(new OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                dismissDialog(DIALOG_SAVE);
            }
        });
        return newDialog;
    }
    return null;
}

private String[] loadFileList()
{
    String[] mFileList = new String[0]; // generate empty array to avoid NullPointerException
    try
    {
        filePath.canWrite();
    }
    catch(SecurityException e)
    {
// Why isn't TAG recognized?...
//           Log.e(TAG, "unable to write on the sd card " + e.toString());
    }
    if(filePath.exists())
    {
        FilenameFilter filter = new FilenameFilter()
        {
            public boolean accept(File dir, String filename)
            {
                File sel = new File(dir, filename);
                return filename.contains(".jpg") || filename.contains(".png") || sel.isDirectory();
            }
        };
        mFileList = filePath.list(filter);
    }
    else
    {
        mFileList = new String[0];
    }
    return mFileList;
}

这是我的 open_file.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <ListView 
        android:id="@+id/open_ListView" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content">
    </ListView>
    <LinearLayout
        android:id="@+id/open_ButtonLinearLayout"
        android:orientation="horizontal" 
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"
        android:gravity="center_horizontal">
        <Button 
            android:id="@+id/open_ok_btn"
            android:text="Open" 
            android:layout_width="150dp" 
            android:layout_height="wrap_content">
        </Button>       
        <Button 
            android:id="@+id/open_cancel_btn" 
            android:text="Cancel"
            android:layout_width="150dp" 
            android:layout_height="wrap_content">
        </Button>       
    </LinearLayout>
</LinearLayout>

这是我的 list_item.xml:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:padding="10dp"
    android:textSize="16sp" >
</TextView>

Answer 1

试试这个，因为它在对话框的布局内，而不是 Activity 的布局内:

ListView fileList = (ListView)newDialog.findViewById(R.id.open_ListView);