Android： java.lang.NullPointerException：尝试在 null 对象引用上调用虚拟方法 'java.lang.String java.lang.Object.toString（）'
2022-09-03 08:27:03
面对我正在开发的练习应用程序的问题。我正面临着与toString方法相关的NullPointerException问题。作为Android应用程序开发的新手，即使在我对此进行了研究之后，我也不确定确切的原因。因此，我要求更熟悉堆栈跟踪的人来帮我。
注意：当我单击列表视图条目以访问日记条目的编辑页面时，会发生错误。但是，它似乎根本没有转到编辑页面。
在下面，您将找到它发生的我的活动代码和堆栈跟踪。
作业代码：
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.content.Intent;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
public class ViewDiaryEntries extends AppCompatActivity {
// Database Helper
MyDBHandler db;
// Listview
ListView data_list;
// Test var
public final static String KEY_EXTRA_DATA_ID = "KEY_EXTRA_DATA_ID";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_diary_entries);
db = new MyDBHandler(this);
// Displays the database items.
displayItems();
}
// To display items in the listview.
public void displayItems(){
// To display items in a listview.
ArrayList db_data_list = db.getDiaryDBDataList();
ArrayAdapter listAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, db_data_list);
// Set the adapter for the listview
data_list = (ListView) findViewById(R.id.dataListView);
data_list.setAdapter(listAdapter);
/* Experiment -------------------------------------------------------------*/
data_list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
// Selected item store
String selectedEntry = ((TextView) view).getText().toString();
// Test for regular expression
String[] listViewItemSplit = selectedEntry.split(" - ");
String listViewItempt1 = listViewItemSplit[0]; // For date and time
//String listViewItempt2 = listViewItemSplit[1]; // For save file name
//Toast.makeText(ViewDiaryEntries.this, listViewItempt1, Toast.LENGTH_LONG).show();
if(listViewItempt1.equals("")){
Toast.makeText(ViewDiaryEntries.this, "Error. Unable to detect entry ID.", Toast.LENGTH_LONG).show();
}
else{
// Pass on the data:
Intent editEntry = new Intent(ViewDiaryEntries.this, editdiaryentry.class);
editEntry.putExtra(KEY_EXTRA_DATA_ID, listViewItempt1);
startActivity(editEntry);
}
}
});
}
// For the go back button.
public void viewdiarytoinitialdiary_backbutt(View v){
// Create and start new intent going back ot main page.
Intent main_page = new Intent(ViewDiaryEntries.this, User_Main_Menu_Options.class);
main_page.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(main_page);
}
// For the about button.
public void viewdiarypage_directionabout_butt(View v){
// Create an alert dialog
final AlertDialog.Builder about_page_dialog = new AlertDialog.Builder(ViewDiaryEntries.this);
about_page_dialog.setTitle("About This Page:");
// Inputs values for the dialog message.
final String dialog_message = "This page will show you any saved diary entries you've.\n\n To edit an entry, do the following: \n\n- Take note of the Entry ID# (first value on entry display) \n- Type it in the number box at the bottom. \n- Press Edit Record icon next to number box, and wait for it to load.";
about_page_dialog.setMessage(dialog_message);
about_page_dialog.setPositiveButton("Got it!", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
// Closes the dialog.
dialog.cancel();
}
});
// Shows the dialog.
about_page_dialog.show();
}
// Main menu button.
public void viewDiaryEntriesMainMenushortcut_butt(View v){
// Creates main menu alert dialog.
AlertDialog.Builder mainMenu_Dialog = new AlertDialog.Builder(this);
mainMenu_Dialog.setIcon(R.drawable.main_menu_symbol);
mainMenu_Dialog.setTitle("Main Menu");
// Creates array adapter with items to fill the menu with.
final ArrayAdapter<String> menuItemsAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1);
menuItemsAdapter.add("Home Screen");
menuItemsAdapter.add("Diary");
menuItemsAdapter.add("Tests");
menuItemsAdapter.add("Activity");
menuItemsAdapter.add("Media");
menuItemsAdapter.add("Thought of the Day");
menuItemsAdapter.add("Inspirational Quotes");
menuItemsAdapter.add("Resources");
menuItemsAdapter.add("Settings");
// To close menu.
mainMenu_Dialog.setPositiveButton("Cancel", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.cancel();
}
});
// To go to appropriate page upon selection.
mainMenu_Dialog.setAdapter(menuItemsAdapter, new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
String selectedItem = menuItemsAdapter.getItem(which);
if(selectedItem.equals("Home Screen")){
// Goes to main menu.
Intent mainMenu = new Intent(ViewDiaryEntries.this, User_Main_Menu_Options.class);
mainMenu.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(mainMenu);
}
else if(selectedItem.equals("Diary")){
// Goes to diary page.
Intent diaryPage = new Intent(ViewDiaryEntries.this, ViewDiaryEntries.class);
diaryPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(diaryPage);
}
else if(selectedItem.equals("Tests")){
// Goes to tests page.
Intent testsPage = new Intent(ViewDiaryEntries.this, TestChoices.class);
testsPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(testsPage);
}
else if(selectedItem.equals("Media")){
// Goes to media page.
Intent mediaPage = new Intent(ViewDiaryEntries.this, initialMediaPage.class);
mediaPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(mediaPage);
}
else if(selectedItem.equals("Thought of the Day")){
// Goes to thought of the day page.
Intent thoughtofthedayPage = new Intent(ViewDiaryEntries.this, thoughtQuotes.class);
thoughtofthedayPage.putExtra("quote_or_thought", 2);
thoughtofthedayPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(thoughtofthedayPage);
}
else if(selectedItem.equals("Inspirational Quotes")){
// Goes to inspirational quotes page.
Intent inspirationalquotesPage = new Intent(ViewDiaryEntries.this, thoughtQuotes.class);
inspirationalquotesPage.putExtra("quote_or_thought", 1);
inspirationalquotesPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(inspirationalquotesPage);
}
else if(selectedItem.equals("Settings")){
// Goes to settings page.
Intent settingsPage = new Intent(ViewDiaryEntries.this, settings.class);
settingsPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(settingsPage);
}
}
});
mainMenu_Dialog.show();
}
// For the settings button.
public void viewdiarypagelisttoSettings_butt(View v){
// Goes to settings page.
Intent settingsPage = new Intent(ViewDiaryEntries.this, settings.class);
settingsPage.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(settingsPage);
}
// For new entry.
public void viewdiarypageaddEntry_butt(View v){
// Opening up the diary add intent.
Intent newdiaryEntry = new Intent(ViewDiaryEntries.this, newdiaryentry.class);
startActivity(newdiaryEntry);
}
}
以下是我看到的堆栈跟踪：
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:401)
at android.widget.ArrayAdapter.getView(ArrayAdapter.java:369)
at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:194)
at android.widget.Spinner.onMeasure(Spinner.java:580)
at android.support.v7.widget.AppCompatSpinner.onMeasure(AppCompatSpinner.java:407)
at android.view.View.measure(View.java:18794)
at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
at android.view.View.measure(View.java:18794)
at android.widget.ScrollView.measureChildWithMargins(ScrollView.java:1283)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.widget.ScrollView.onMeasure(ScrollView.java:340)
at android.view.View.measure(View.java:18794)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
at android.view.View.measure(View.java:18794)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18794)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:18794)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18794)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
at android.view.View.measure(View.java:18794)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
at android.view.Choreographer.doCallbacks(Choreographer.java:670)
at android.view.Choreographer.doFrame(Choreographer.java:606)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
任何帮助解决方案将不胜感激。
编辑：
因此，在来回盯着数据库和它所交互的活动之间的代码之后，我设法让它再次工作。以下是我按确切顺序执行的操作：
- 我意识到我有一个日期字段没有收到任何数据，并纠正了这一点。
- 已清理项目。
- 重新启动Android Studio（基本上停止了开发环境的所有操作）。
- 已从我的开发手机中卸载该应用。
- 重新启动安卓工作室并重新安装该应用程序。
- 我以某种方式工作=_=，是的，这是魔术。
老实说，我不知道哪一步实际上解决了它。我猜是数据库中的日期字段在没有接收数据的情况下搞砸了我。
