Android：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“java.lang.String java.lang.Object.toString()”
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时间：2020-08-11 16:08:29 来源:igfitidea点击:
Android: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
提问by Mohit G
Facing a problem with a practice app I'm working on. I'm facing a NullPointerException problem relating to the toString method. Being new to android app development, I'm unsure of the exact cause even after my research into this. Hence I ask for someone who is more familiar with the stack trace to kindly help me out.
面对我正在开发的练习应用程序的问题。我正面临与 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，我也不确定确切的原因。因此，我请求更熟悉堆栈跟踪的人来帮助我。
Note: The error occurs when I click on the listview entry to access an edit page for the diary entry. However it doesn't seem to go to the edit page at all.
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进入编辑页面。
Below you'll find my activity code it occurs on and the stack trace.
您将在下面找到它发生的我的活动代码和堆栈跟踪。
Activity code:
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);
}
}
Here is my stack trace that I see:
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)
Any help towards a solution will be appreciated.
对解决方案的任何帮助将不胜感激。
EDIT:
编辑：
So after staring back and forth from the code between the database and the activity it interacts with, I managed to get it working again. Below is what I did in the exact order:
因此，在从数据库和它与之交互的活动之间来回查看代码之后，我设法让它再次工作。以下是我按确切顺序执行的操作：
- I realized that I had a date field that was receiving no data, rectified that.
- Cleaned the project.
- Restarted Android Studio (basically stopping all the operations of the development environment).
- Uninstalled the app from my dev phone.
- Restarted android studio and re-installed the app.
- I somehow works =_=, yes it is magic.
- 我意识到我有一个没有接收数据的日期字段，纠正了这个问题。
- 清理了项目。
- 重启Android Studio（基本停止开发环境的所有操作）。
- 从我的开发手机上卸载了该应用程序。
- 重新启动android studio并重新安装该应用程序。
- 我以某种方式工作 =_=，是的，这很神奇。
Honestly I've no idea which step actually solved it. I'm guessing it was the date field in the database that was messing things up for me while it was receiving no data.
老实说，我不知道哪一步真正解决了它。我猜是数据库中的日期字段在它没有接收到任何数据时把事情搞砸了。
采纳答案by laalto
The array in your ArrayAdaptercontains at least one entry that is null. There must be no nulls there.
您的数组中ArrayAdapter至少包含一个条目，即null. 那里必须没有空值。
The array is populated in getDiaryDBDataList()so the problem is also there.
该数组已填充，getDiaryDBDataList()因此问题也存在。
回答by Nikunj
Initialize "db_data_list" in displayItems()
在 displayItems() 中初始化“db_data_list”
ArrayList db_data_list = new ArrayList();
回答by childofthehorn
viewas you have so named it here appears to be uninitialized and not tied to anything
view正如您在此处命名的那样，它似乎未初始化且与任何内容无关
Textview someTextView = (TextView) data_list.findViewById(R.id.mytextview);
^ assuming the textview is inside of the data_list view, otherwise discard that bit
^ 假设 textview 在 data_list 视图内，否则丢弃该位
Then, in your code, you will want to also add length checks for
然后，在您的代码中，您还需要添加长度检查
// 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
And this will help with issues down the road.
这将有助于解决未来的问题。
回答by Muralikrishna G S
I guess instead of
我想而不是
String selectedEntry = someTextView.getText().toString();
you have to use
你必须使用
String selectedEntry = listAdapter.getItem(position);
Because you are using array adapter you can't get selected item from View
因为您使用的是数组适配器，所以无法从 View 中获取所选项目
Hope this will help you out
希望这会帮助你
回答by raed
Late answer but, n my case, I changed the resources tag name from integer-arrayto string-arrayand it worked fine.
迟到的答案但是，在我的情况下，我将资源标签名称从 更改integer-array为string-array并且它工作正常。
