Java 当使用游标从SQLITE数据库读取数据时，应用程序崩溃

我正在尝试制作一个简单的应用程序，在这个应用程序中，我可以使用SQLite数据库保存注释并使用listview查看它们。 问题是我看不到listview的内容。应用程序立即崩溃。我发现cursor database.query有问题，但我没有找到解决方法

Mainactivity.java：

package com.example.notemaker;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.content.Intent;
import android.view.Window;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.widget.ArrayAdapter;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {  

private ListView listView;
     private ArrayList<String> listItem;
     private ArrayAdapter adapter;
     private DBOpenHelper helper;
     private SQLiteDatabase database;
     private TextView noNotesView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, Edit_notes.class);
                startActivity(intent);
            }
        });

        noNotesView = findViewById(R.id.empty_notes);
        listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
        ViewData();
    }

   private void ViewData(){

       helper = new DBOpenHelper(this, "notes.db", null, 1); // db
       database = helper.getWritableDatabase();

       String table_name = "note_table";
       String[] columns = {"ID", "NOTE_TEXT"};
       String where = null;
       String where_args[] = null;
       String group_by = null;
       String having = null;
       String order_by = null;
       Cursor result = database.query(table_name, columns, where, where_args, group_by, having, order_by);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onResume()
    {
        super.onResume();
        ViewData();
    }
}

Editnotes.java：

package com.example.notemaker;

import android.content.ContentValues;
import android.content.DialogInterface;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;
import java.util.Date;

public class Edit_notes extends AppCompatActivity {

    private DBOpenHelper dbop;
    private SQLiteDatabase sdb;

  //  private TestDBOpenHelper tdb;
  //  private SQLiteDatabase sdb;

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.note_edit);

        getSupportActionBar().setTitle("");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.cancel_button_menu, menu);
        getMenuInflater().inflate(R.menu.save_button_menu, menu);
        return super.onCreateOptionsMenu(menu);
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        // Create confirmation dialog when click on CANCEL button
        if (id == R.id.cancel_note) {
            final AlertDialog.Builder alert = new AlertDialog.Builder(this);
            alert.setTitle("NoteMaker");
            alert.setMessage("Cancel this note ?");
            alert.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(alert.getContext(), "Note cancelled", Toast.LENGTH_SHORT).show();
                    finish();
                }

            });

            alert.setNegativeButton("No", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                }
            });
            alert.create().show();
            return true;
        }

        // Save note title and note text to the database
        if (id == R.id.save_note)
        {

         //   tdb = new TestDBOpenHelper(this, "test.db", null, 1);
         //   sdb = tdb.getWritableDatabase();

            String note_title_string = findViewById(R.id.input_note_title).toString();
            String note_text_string = findViewById(R.id.input_note).toString();

            //if (!note_title_string.isEmpty()){
              //  long date = new Date().getTime(); // Get date

                AddData(note_title_string); // Add title to the database
                Toast.makeText(this, "Note saved", Toast.LENGTH_SHORT).show();
                finish();
           // }
            //else{
              //  Toast.makeText(this, "Title cannot be empty", Toast.LENGTH_SHORT).show();
           // }

            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    public void AddData (String newEntry){
        dbop = new DBOpenHelper(this, "notes.db", null, 1);
        sdb = dbop.getWritableDatabase();

        ContentValues cv = new ContentValues();
        cv.put("note", newEntry);

        sdb.insert("note_table", null, cv);
        //long insertData = dbop.insertNote(newEntry);
    }
}

---

导致崩溃的问题是，您可能在DBOpenHelper的onCreate方法创建表之前运行应用程序

DBOpenHelper的onCreate方法在数据库的整个生命周期内只自动运行一次，因此没有应用为创建表所做的更改。卸载应用程序会导致应用程序的数据以及数据库被删除，从而允许运行onCreate方法

但是，我仍然无法查看我的listview的内容-

要使ListView显示数据，您需要实例化一个合适的适配器，然后使用ListView的setAdapter方法设置ListView要使用的适配器

工作示例 以下是一个简化版本（无晶圆厂），以方便使用SimpleCorsorAdapter并基于您的代码

请注意，对于游标适配器，游标必须有一个名为\u id的列，因此列名在整个过程中都发生了更改

大多数更改已被注释（除已删除的晶圆厂代码外），因此请阅读注释

应用上述内容时，您需要卸载应用程序，以便应用更改的列名。

DBOpenHelper.java

public类DBOpenHelper扩展了SQLiteOpenHelper{
//类的构造函数在这里我们只是映射到
//超级级
public DBOpenHelper（上下文、字符串名称、SQLiteDatabase.CursorFactory
工厂，int版本）{
超级（上下文、名称、工厂、版本）；
}
//在创建数据库时调用的重写方法
public void onCreate（SQLiteDatabase db）{
//创建数据库
db.execSQL（创建_表）；
}
//要升级数据库时调用的重写方法
//请注意，在本例中，我们只是重建数据库，而不考虑
//数据丢失理想情况下，您应该有一种在
//我们正在重建数据库
public void onUpgrade（SQLiteDatabase db，int version\u old，int version\u new）
{
//删除这些表并重新创建它们
db.execSQL（drop_table）；
db.execSQL（创建_表）；
}
//创建和删除所需的一组常量字符串
//数据库
私有静态最终字符串create\u table=“create table note\u table（”+
BaseColumns.\u ID+“integer主键自动增量”，+//尝试卸载应用程序并重新运行。我刚卸载了，应用程序不再崩溃。但是我仍然无法查看我的ListView的内容。不要添加错误的屏幕截图：将其粘贴到问题中。我感谢您的帮助。我正在阅读您的代码。但是应用程序崩溃了（我卸载并重新编译了它），但没有成功尝试在空对象引用上调用虚拟方法“void android.widget.ListView.setAdapter（android.widget.ListAdapter）”，这将指示行
ListView=（ListView）findViewById（R.id.ListView）；
已被忽略，或者id为ListView的ListView不在活动_主布局中。很抱歉，无法使其工作：（创建一个新项目，复制上述代码，使其工作，然后应用缺少的代码，例如FAB。并在高兴时应用于原始项目。
package com.example.notemaker;

import android.content.ContentValues;
import android.content.DialogInterface;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;
import java.util.Date;

public class Edit_notes extends AppCompatActivity {

    private DBOpenHelper dbop;
    private SQLiteDatabase sdb;

  //  private TestDBOpenHelper tdb;
  //  private SQLiteDatabase sdb;

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.note_edit);

        getSupportActionBar().setTitle("");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.cancel_button_menu, menu);
        getMenuInflater().inflate(R.menu.save_button_menu, menu);
        return super.onCreateOptionsMenu(menu);
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        // Create confirmation dialog when click on CANCEL button
        if (id == R.id.cancel_note) {
            final AlertDialog.Builder alert = new AlertDialog.Builder(this);
            alert.setTitle("NoteMaker");
            alert.setMessage("Cancel this note ?");
            alert.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(alert.getContext(), "Note cancelled", Toast.LENGTH_SHORT).show();
                    finish();
                }

            });

            alert.setNegativeButton("No", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                }
            });
            alert.create().show();
            return true;
        }

        // Save note title and note text to the database
        if (id == R.id.save_note)
        {

         //   tdb = new TestDBOpenHelper(this, "test.db", null, 1);
         //   sdb = tdb.getWritableDatabase();

            String note_title_string = findViewById(R.id.input_note_title).toString();
            String note_text_string = findViewById(R.id.input_note).toString();

            //if (!note_title_string.isEmpty()){
              //  long date = new Date().getTime(); // Get date

                AddData(note_title_string); // Add title to the database
                Toast.makeText(this, "Note saved", Toast.LENGTH_SHORT).show();
                finish();
           // }
            //else{
              //  Toast.makeText(this, "Title cannot be empty", Toast.LENGTH_SHORT).show();
           // }

            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    public void AddData (String newEntry){
        dbop = new DBOpenHelper(this, "notes.db", null, 1);
        sdb = dbop.getWritableDatabase();

        ContentValues cv = new ContentValues();
        cv.put("note", newEntry);

        sdb.insert("note_table", null, cv);
        //long insertData = dbop.insertNote(newEntry);
    }
}

public class DBOpenHelper extends SQLiteOpenHelper {
    // constructor for the class here we just map onto the constructor of the
    // super class
    public DBOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory
            factory, int version) {
        super(context, name, factory, version);
    }
    // overridden method that is called when the database is to be created
    public void onCreate(SQLiteDatabase db) {
        // create the database
        db.execSQL(create_table);
    }
    // overridden method that is called when the database is to be upgraded
    // note in this example we simply reconstruct the database not caring for
    // data loss ideally you should have a method for storing the data while
    // are reconstructing the database
    public void onUpgrade(SQLiteDatabase db, int version_old, int version_new)
    {
        // drop the tables and recreate them
        db.execSQL(drop_table);
        db.execSQL(create_table);
    }
    // a bunch of constant strings that will be needed to create and drop
    // databases
    private static final String create_table = "create table note_table(" +
            BaseColumns._ID + " integer primary key autoincrement, " + //<<<<<<<<<< changed column name to _id for CursorAdapter
            "NOTE_TEXT string" +
            ")";
    private static final String drop_table = "drop table note_table";

    /**
     * ADDED to demonstrate typical inclusion of DB access within the helper
     * @param note
     * @return the id of the added note or -1 if not added.
     */
    public long addNote(String note) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put("NOTE_TEXT",note);
        return db.insert("note_table",null,cv);
    }
}

public class MainActivity extends AppCompatActivity {

    private ListView listView;
    private ArrayList<String> listItem;
    private ArrayAdapter adapter; //<<<<<<<<<< Not used
    private SimpleCursorAdapter sca; //<<<<<<<<<< ADDED
    private Cursor csr; //<<<<<<<<<< ADDED
    private DBOpenHelper helper;
    private SQLiteDatabase database;
    private TextView noNotesView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        helper = new DBOpenHelper(this,"notes.db", null, 1); //<<<<<< moved to here
        database = helper.getWritableDatabase(); //<<<<<<<<<< moved to here so has scope throughout the class

        //noNotesView = findViewById(R.id.empty_notes); //<<<<<<<<<< commented out for simplicity
        listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
        addSomeTestData(); //Adds a row every time the App is run (for testing)
        ViewData();
    }

    private void ViewData(){

        String table_name = "note_table";
        String[] columns = {"_id", "NOTE_TEXT"}; //<<<<<<<<<< ID column is now _id
        String where = null;
        String where_args[] = null;
        String group_by = null;
        String having = null;
        String order_by = null;
        csr = database.query(table_name, columns, where, where_args, group_by, having, order_by);
        //<<<<<<<<<< ADDED following code to method to instantiate the adapter and set the adapter
        //                 and to alternatively swap the cursor to refresh the listview
        if (sca == null) {
            sca = new SimpleCursorAdapter(
                    this,
                    android.R.layout.simple_list_item_1,
                    csr,
                    new String[]{"NOTE_TEXT"},
                    new int[]{android.R.id.text1},
                    0);
            listView.setAdapter(sca);
        } else {
            sca.swapCursor(csr);
        }
    }

    private void addSomeTestData() {
        helper.addNote("My Note"); //<<<<<<<<<< Uses the addNote method in the DB helper
    }

    /**
     * ADDED for correct cursor handling i.e. close
     * although not really required in mainactivity
     */
    @Override
    protected void onDestroy() {
        super.onDestroy();
        csr.close();
    }

    @Override
    public void onResume()
    {
        super.onResume();
        ViewData();
    }
}