由于在空对象引用上调用虚方法，无法启动列表视图活动。(Unable to start a List View activity due to invoking virtual method on null object reference. Tips?)
抱歉这个愚蠢而重复的问题。 我是Android编程的新手，正在制作一个可以扫描QR码并将数据保存到SQLite数据库并在另一个活动的ListView中显示的应用程序。 此外，ListView具有“搜索”功能，该功能使用户能够键入文本，该文本将从列表视图中的项目中显示类似的结果（如果在列表视图中找到）。 最后，当用户点击项目时，删除功能会从ListView中删除该项目。
该应用程序工作正常，打开ListView活动显示ListView与内容，直到我集成搜索和删除功能。 该应用程序声明我试图在空对象引用上调用虚方法。 我已经查明了如何解决它，但找不到任何东西。 现在它显示了这个错误：
07-31 18:52:31.688 2900-2900/app.num.barcodescannerproject E/AndroidRuntime: FATAL EXCEPTION: main Process: app.num.barcodescannerproject, PID: 2900 java.lang.RuntimeException: Unable to start activity ComponentInfo{app.num.barcodescannerproject/app.num.barcodescannerproject.ListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void app.num.barcodescannerproject.CustomCursorAdapter.changeCursor(android.database.Cursor)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442) at android.app.ActivityThread.access$800(ActivityThread.java:156) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:211) at android.app.ActivityThread.main(ActivityThread.java:5389) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void app.num.barcodescannerproject.CustomCursorAdapter.changeCursor(android.database.Cursor)' on a null object reference at app.num.barcodescannerproject.ListActivity.onCreate(ListActivity.java:41) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2332) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442) at android.app.ActivityThread.access$800(ActivityThread.java:156) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:211) at android.app.ActivityThread.main(ActivityThread.java:5389) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815) 07-31 18:52:32.968 2900-2900/app.num.barcodescannerproject I/Process: Sending signal. PID: 2900 SIG: 9下面是Activity的代码，它将带有数据的Intent发送到ListView Activity：
public void saveOnClick (View saveButton){ String prescriptionName = scanResult.getText().toString(); if (prescriptionName.length() != 0) { Intent newIntent = new Intent (this, ListActivity.class); newIntent.putExtra("tag_person_name", prescriptionName); startActivity(newIntent); finish(); } }这是ListActivity的代码，具有ListView，搜索和删除功能：
@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); databaseHelper = new PersonDatabaseHelper(this); listView = (ListView) findViewById(R.id.list_data); databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener); customAdapter = new CustomCursorAdapter(ListActivity.this, databaseHelper.getAllData()); listView.setAdapter(customAdapter); Cursor cursor = databaseHelper.getAllData(); EditText myFilter = (EditText) findViewById(R.id.editText); myFilter.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) { customAdapter.getFilter().filter(s.toString()); } }); customAdapter.setFilterQueryProvider(new FilterQueryProvider() { public Cursor runQuery(CharSequence constraint) { return databaseHelper.fetchDataByName(constraint.toString()); } }); } //public void onClickEnterData(View btnAdd) { // startActivityForResult(new Intent(this, ResultActivity.class), ENTER_DATA_REQUEST_CODE); //} // @Override //protected void onActivityResult(int requestCode, int resultCode, Intent data) { // super.onActivityResult(requestCode, resultCode, data); // if (requestCode == ENTER_DATA_REQUEST_CODE && resultCode == RESULT_OK) { // databaseHelper.insertData(data.getExtras().getString("tag_person_name")); // customAdapter.changeCursor(databaseHelper.getAllData()); // } // } private OnItemClickListener listContentOnItemClickListener = new OnItemClickListener() { @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { // TODO Auto-generated method stub Cursor cursor = (Cursor) parent.getItemAtPosition(position); final int item_id = cursor.getInt(cursor.getColumnIndex(PersonDatabaseHelper.PERSON_TABLE_COLUMN_ID)); String item_name = cursor.getString(cursor.getColumnIndex(PersonDatabaseHelper.PERSON_TABLE_COLUMN_NAME)); AlertDialog.Builder myDialog = new AlertDialog.Builder(ListActivity.this); myDialog.setTitle("Delete?"); TextView dialogTxt_id = new TextView(ListActivity.this); ViewGroup.LayoutParams dialogTxt_idLayoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT); dialogTxt_id.setLayoutParams(dialogTxt_idLayoutParams); dialogTxt_id.setText(String.valueOf(item_id)); TextView dialogC1_id = new TextView(ListActivity.this); ViewGroup.LayoutParams dialogC1_idLayoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT); dialogC1_id.setLayoutParams(dialogC1_idLayoutParams); dialogC1_id.setText(item_name); LinearLayout layout = new LinearLayout(ListActivity.this); layout.setOrientation(LinearLayout.VERTICAL); layout.addView(dialogTxt_id); layout.addView(dialogC1_id); myDialog.setView(layout); myDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() { // do something when the button is clicked public void onClick(DialogInterface arg0, int arg1) { databaseHelper.delete_byID(item_id); updateList(); } }); myDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() { // do something when the button is clicked public void onClick(DialogInterface arg0, int arg1) { } }); myDialog.show(); } }; @Override protected void onDestroy() { // TODO Auto-generated method stub super.onDestroy(); databaseHelper.close(); } private void updateList() { customAdapter.changeCursor(databaseHelper.getAllData()); }}
根据我所看到的，错误就在这里，在changeCursor：
databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener);我还注意到，当我删除changeCursor时，错误将显示在setOnItemClickListener上，说明此虚方法也在空对象引用上被调用。
我能做些什么吗？ 我在许多网站上进行了广泛的搜索，但无法在任何地方找到解决方案。 也许我是瞎子。
更新：感谢您的帮助，我意识到我没有首先初始化customAdapter。 但是，现在我做了，我继续得到两个与customAdapater完全相同的NPE错误，除了第一个在setOnItemClickListener，另一个在listView.setAdapter。 两者显然也在null对象上调用。 但是这一次，与customAdapter不同，我真的无法发现错误。 有帮助吗？
sorry for the stupid and repetitive question. I am new to android programming and in progress of making an app which can scan QR Codes and save the data to an SQLite database and show it in a ListView of another activity. Also, the ListView has a "search" function which enables the user to type text which will bring up a similar result (if found in the listview) from the items in the listview. Lastly, the delete function deletes the item from the ListView when the user taps on the item.
The application worked fine and opened the ListView Activity showing the ListView with the contents until I integrated the search and delete function. The application states that I am trying to invoke a virtual method on a null object reference. I have looked up on how to solve it, but couldn't find anything. Now it shows this error:
07-31 18:52:31.688 2900-2900/app.num.barcodescannerproject E/AndroidRuntime: FATAL EXCEPTION: main Process: app.num.barcodescannerproject, PID: 2900 java.lang.RuntimeException: Unable to start activity ComponentInfo{app.num.barcodescannerproject/app.num.barcodescannerproject.ListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void app.num.barcodescannerproject.CustomCursorAdapter.changeCursor(android.database.Cursor)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442) at android.app.ActivityThread.access$800(ActivityThread.java:156) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:211) at android.app.ActivityThread.main(ActivityThread.java:5389) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void app.num.barcodescannerproject.CustomCursorAdapter.changeCursor(android.database.Cursor)' on a null object reference at app.num.barcodescannerproject.ListActivity.onCreate(ListActivity.java:41) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2332) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442) at android.app.ActivityThread.access$800(ActivityThread.java:156) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:211) at android.app.ActivityThread.main(ActivityThread.java:5389) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815) 07-31 18:52:32.968 2900-2900/app.num.barcodescannerproject I/Process: Sending signal. PID: 2900 SIG: 9Here is the code for the Activity which sends the Intent with the data to the ListView Activity:
public void saveOnClick (View saveButton){ String prescriptionName = scanResult.getText().toString(); if (prescriptionName.length() != 0) { Intent newIntent = new Intent (this, ListActivity.class); newIntent.putExtra("tag_person_name", prescriptionName); startActivity(newIntent); finish(); } }And this is the code for the ListActivity with the ListView, search and delete functions:
@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); databaseHelper = new PersonDatabaseHelper(this); listView = (ListView) findViewById(R.id.list_data); databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener); customAdapter = new CustomCursorAdapter(ListActivity.this, databaseHelper.getAllData()); listView.setAdapter(customAdapter); Cursor cursor = databaseHelper.getAllData(); EditText myFilter = (EditText) findViewById(R.id.editText); myFilter.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) { customAdapter.getFilter().filter(s.toString()); } }); customAdapter.setFilterQueryProvider(new FilterQueryProvider() { public Cursor runQuery(CharSequence constraint) { return databaseHelper.fetchDataByName(constraint.toString()); } }); } //public void onClickEnterData(View btnAdd) { // startActivityForResult(new Intent(this, ResultActivity.class), ENTER_DATA_REQUEST_CODE); //} // @Override //protected void onActivityResult(int requestCode, int resultCode, Intent data) { // super.onActivityResult(requestCode, resultCode, data); // if (requestCode == ENTER_DATA_REQUEST_CODE && resultCode == RESULT_OK) { // databaseHelper.insertData(data.getExtras().getString("tag_person_name")); // customAdapter.changeCursor(databaseHelper.getAllData()); // } // } private OnItemClickListener listContentOnItemClickListener = new OnItemClickListener() { @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { // TODO Auto-generated method stub Cursor cursor = (Cursor) parent.getItemAtPosition(position); final int item_id = cursor.getInt(cursor.getColumnIndex(PersonDatabaseHelper.PERSON_TABLE_COLUMN_ID)); String item_name = cursor.getString(cursor.getColumnIndex(PersonDatabaseHelper.PERSON_TABLE_COLUMN_NAME)); AlertDialog.Builder myDialog = new AlertDialog.Builder(ListActivity.this); myDialog.setTitle("Delete?"); TextView dialogTxt_id = new TextView(ListActivity.this); ViewGroup.LayoutParams dialogTxt_idLayoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT); dialogTxt_id.setLayoutParams(dialogTxt_idLayoutParams); dialogTxt_id.setText(String.valueOf(item_id)); TextView dialogC1_id = new TextView(ListActivity.this); ViewGroup.LayoutParams dialogC1_idLayoutParams = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT); dialogC1_id.setLayoutParams(dialogC1_idLayoutParams); dialogC1_id.setText(item_name); LinearLayout layout = new LinearLayout(ListActivity.this); layout.setOrientation(LinearLayout.VERTICAL); layout.addView(dialogTxt_id); layout.addView(dialogC1_id); myDialog.setView(layout); myDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() { // do something when the button is clicked public void onClick(DialogInterface arg0, int arg1) { databaseHelper.delete_byID(item_id); updateList(); } }); myDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() { // do something when the button is clicked public void onClick(DialogInterface arg0, int arg1) { } }); myDialog.show(); } }; @Override protected void onDestroy() { // TODO Auto-generated method stub super.onDestroy(); databaseHelper.close(); } private void updateList() { customAdapter.changeCursor(databaseHelper.getAllData()); }}
According to what I see, the error is here, at the changeCursor:
databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener);I have also noticed that when I remove the changeCursor, the error will be shown at the setOnItemClickListener, stating that this virtual method is also being invoked on a null object reference.
Is there anything I can do about this? I have searched far and wide across many sites but cannot find the solution anywhere. Perhaps I'm blind.
UPDATE: Thank you for the help, I have realised that I did not initialise the customAdapter first. However, now that I did, I keep getting two exactly same NPE errors as the one with the customAdapater, except the first one is at the setOnItemClickListener and the other at the listView.setAdapter. Both are apparently also invoked on a null object. However this time, unlike with the customAdapter, I really can't spot the mistake. Any help?
最满意答案
您在第一次使用CursorAdapter后正在初始化它。 这是错误的，因为您在使用之前首先需要创建CursorAdapter对象。
... databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); /* This line is moved from below to this place i.e before you use the changeCursor method on it*/ customAdapter = new CustomCursorAdapter(ListActivity.this, databaseHelper.getAllData()); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener); ...您正在获取空指针异常（ NPE ），因为您正在使用尚未使用new运算符创建的对象
You are initializing the CursorAdapter after its first use. This is wrong as you first need to create the CursorAdapter object before using it.
... databaseHelper.insertData(getIntent().getExtras().getString("tag_person_name")); /* This line is moved from below to this place i.e before you use the changeCursor method on it*/ customAdapter = new CustomCursorAdapter(ListActivity.this, databaseHelper.getAllData()); customAdapter.changeCursor(databaseHelper.getAllData()); listView.setOnItemClickListener(listContentOnItemClickListener); ...You are getting a Null Pointer Exception (NPE) because you are using an object that has not been created yet using the new operator
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