java - 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()'

Question

我正在处理的练习应用程序遇到问题。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帮助我。

注意:当我单击 ListView 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。

您将在下面找到它发生的我的 Activity 代码和堆栈跟踪。

Activity 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)

任何对解决方案的帮助将不胜感激。

编辑:

因此，在数据库和与之交互的 Activity 之间的代码来回查看之后，我设法让它再次工作。以下是我按确切顺序执行的操作:

1. 我意识到我有一个日期字段未收到任何数据，已纠正该问题。
2. 清理了项目。
3. 重新启动 Android Studio(基本上停止了开发环境的所有操作)。
4. 从我的开发手机上卸载了该应用。
5. 重启 android studio 并重新安装应用。
6. 我以某种方式工作 =_=，是的，这很神奇。

老实说，我不知道哪个步骤真正解决了它。我猜是数据库中的日期字段在没有接收到数据时把事情搞砸了。

Answer 1

ArrayAdapter 中的数组至少包含一个 null 条目。那里不能有空值。

数组是在 getDiaryDBDataList() 中填充的，所以问题也在那里。