面对我练习的练习应用程序的问题。我面临与toString方法相关的NullPointerException问题。作为Android应用程序开发的新手，即使在我对此进行研究之后，我也不确定其确切原因。因此，我要求更熟悉堆栈跟踪的人帮助我。
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。然而，它似乎根本没有进入编辑页面。
在您的下方，我会找到它发生的活动代码和堆栈跟踪。
活动代码：
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);
}
}
这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)
对于解决方案的任何帮助将不胜感激。
修改
因此，在从数据库与其交互的活动之间的代码中来回盯着之后，我设法让它再次运行。以下是我按照确切顺序执行的操作：
老实说，我不知道哪一步真正解决了它。我猜测数据库中的日期字段在没有收到任何数据的情况下为我搞砸了。
答案 0 :(得分：31)
ArrayAdapter中的数组至少包含一个null条目。那里必须没有空值。
数组填充在getDiaryDBDataList()中，因此也存在问题。
答案 1 :(得分：2)
我想而不是
String selectedEntry = someTextView.getText().toString();
你必须使用
String selectedEntry = listAdapter.getItem(position);
由于您使用的是阵列适配器，因此无法从View
中获取所选项目希望这会帮助你
答案 2 :(得分：1)
在displayItems（）
中初始化“db_data_list”ArrayList db_data_list = new ArrayList();
答案 3 :(得分：1)
view因为你在这里命名它似乎是未初始化的，并没有绑定任何东西
Textview someTextView = (TextView) data_list.findViewById(R.id.mytextview);
^假设textview在data_list视图中，否则丢弃该位
然后，在您的代码中，您还需要添加
的长度检查 // Selected item store
String selectedEntry = someTextView.getText().toString();
//Check length before trying to split anything
if (selectedEntry.length() > 0){
// Test for regular expression
String[] listViewItemSplit = selectedEntry.split(" - ");
String listViewItempt1 = listViewItemSplit[0]; // For date and time
这将有助于解决问题。
答案 4 :(得分：0)
最新答案，但就我而言，我将资源标签名称从integer-array更改为string-array，并且效果很好。