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幫助！我真的很欣賞Stack Overflow，它是過去幾個月的貢獻者。我有很多問題，我在這裏找到答案......但這個我似乎無法找到任何地方...我是Java和Android的noob，我一直在想這個幾天。出於某種原因，我有一個名爲fileList的ListView對象，它返回null ...一切都很好編譯，但是當我嘗試使用fileList的時候我得到了一個NullPointerException異常...我已經能夠將它隔離到它的聲明中：爲什麼（ListView）findViewById返回null？
ListView fileList = (ListView)findViewById(R.id.open_ListView);
但我不能爲我的生活理解這條線有什麼問題！我在下面包含了很多代碼，理論上它應該有任何和所有可能與此錯誤有關的代碼。
請任何幫助，這將非常感謝！謝謝！
這是違規的代碼部分。它只是開關塊的OPEN_DIALOG部分，其他所有開關都能很好地顯示它們的新Dialog。我已加星出錯的行...
@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;
}
這是我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>
這是我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>
YAY！這工作！非常感謝！ ...然後，當你發佈這個，並且我嘗試了它之後，我意識到我一直在爲其他所有事情做這些......並且從未注意到......呃！謝謝你，謝謝你，謝謝你！ – 2011-05-30 00:51:19