Java SQLite和Android，移动到数据库中的下一行，检索并显示结果
我正在尝试让一个电话簿在Android上工作，它使用SQLite数据库 我正在努力解决的问题是，当我单击“下一步”按钮时，我希望调出数据库中的下一行数据。然而，当我运行应用程序时，我不断地得到这个错误 03-10 12:05:22.455:E/AndroidRuntime（772）：致命异常：主 03-10 12:05:22.455:E/AndroidRuntime（772）：java.lang.RuntimeException:无法实例化活动组件信息{com.example.addressBooktake2/com.example.addressBooktake2.MainActivity}:java.lang.NullPointerException 这是我正在使用的代码 MainActivity.JavaJava SQLite和Android，移动到数据库中的下一行，检索并显示结果,java,android,database,sqlite,Java,Android,Database,Sqlite,我正在尝试让一个电话簿在Android上工作，它使用SQLite数据库 我正在努力解决的问题是，当我单击“下一步”按钮时，我希望调出数据库中的下一行数据。然而，当我运行应用程序时，我不断地得到这个错误 03-10 12:05:22.455:E/AndroidRuntime（772）：致命异常：主 03-10 12:05:22.455:E/AndroidRuntime（772）：java.lang.RuntimeException:无法实例化活动组件信息{com.example.addressBo
public class MainActivity extends Activity {
DBAdaptor db = new DBAdaptor(this);
int count = 1;
Cursor cursor = db.getallRecord();
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DisplayRecord(cursor);
}
public void Nextdata (View view){
if (cursor.moveToNext())
{
DisplayRecord(cursor);
}
}
public void Previousdata (View view){
if (cursor.moveToPrevious())
{
DisplayRecord(cursor);
}
}
public void DisplayRecord(Cursor c) {
EditText nameTxt = (EditText)findViewById(R.id.Name);
EditText phoneTxt = (EditText)findViewById(R.id.Phone);
EditText emailTxt = (EditText)findViewById(R.id.Email);
nameTxt.setText(c.getString(1));
phoneTxt.setText(c.getString(2));
emailTxt.setText(c.getString(3));//
}
package com.example.adressbooktake2;
import android.annotation.SuppressLint;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class DBAdaptor {
public static final String KEY_ROWID = "id";
public static final String KEY_NAME = "name";
public static final String KEY_PHONENUMBER = "phone";
public static final String KEY_EMAIL = "email";
private static final String TAG = "DBAdaptor";
public static final String DATABASE_NAME = "AddressBookDataBase1";
public static final String DATABASE_TABLE = "AddressBook";
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_CREATE = "create table if not exists AddressBook (id integer primary key autoincrement, "
+ "name VARCHAR, phone VARCHAR, email VARCHAR);";
private final Context context;
private DatabaseHelper DBHelper;
private SQLiteDatabase db;
public DBAdaptor(Context ctx)
{
this.context = ctx;
DBHelper = new DatabaseHelper(context);
}
private static class DatabaseHelper extends SQLiteOpenHelper
{
DatabaseHelper(Context context)
{
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db)
{
try
{
db.execSQL(DATABASE_CREATE);
}
catch (SQLException e)
{
e.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
{
Log.w(TAG, "Upgrading database from version " + oldVersion + " to" + newVersion +
", which will destroy all old data");
db.execSQL("Drop TABLE IF EXISTS AddressBook");
onCreate(db);
}
}
//--counts rows in database--
public int rowCounter()
{
Cursor mCount= db.rawQuery("select count(*) from AddressBook where id=" + KEY_ROWID, null);
mCount.moveToFirst();
int rowCount= mCount.getInt(0);
return rowCount;
}
//--opens the database---
public DBAdaptor open() throws SQLException
{
db = DBHelper.getWritableDatabase(); //Line 88
return this;
}
//--close the dabatabase
public void close()
{
DBHelper.close();
}
//--insert a record into database--
public long insertRecord(String name, String email, String phone)
{
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_NAME, name);
initialValues.put(KEY_EMAIL, email);
initialValues.put(KEY_PHONENUMBER, phone);
return db.insert(DATABASE_TABLE, null, initialValues);
}
//--deletes a record
public void deleteRecord(long rowId)
{
db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null);
}
//--retrieves a particular record--
public Cursor getRecord(long rowId) throws SQLException
{
Cursor mCursor =
db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID, KEY_NAME,
KEY_EMAIL, KEY_PHONENUMBER}, KEY_ROWID + "=" + rowId, null, null, null, null, null);
if (mCursor !=null) {
mCursor.moveToFirst();
}
return mCursor;
}
//--Retrieves all data--
public Cursor getallRecord() throws SQLException
{
{
Cursor gaCursor = db.rawQuery("SELECT* FROM " + DATABASE_TABLE, null);
if (gaCursor !=null)
{
gaCursor.moveToFirst();
}
return gaCursor;
}
}
}
EditText nameTxt = (EditText)findViewById(R.id.Name);
EditText phoneTxt = (EditText)findViewById(R.id.Phone);
EditText emailTxt = (EditText)findViewById(R.id.Email);
setContentView(R.layout.activity_main);
EditText nameTxt = (EditText)findViewById(R.id.Name);
EditText phoneTxt = (EditText)findViewById(R.id.Phone);
EditText emailTxt = (EditText)findViewById(R.id.Email);
DisplayRecord(cursor);
gaCursorelseDisplayRecordc.toString（）c另外，您不必在DBAdaptor.Java文件中实例化
dbopen（）onCreatethis.db=dbopengetallRecord（）游标对象为空。。。
而空指针异常是指在DisplayRecord（）
中访问的空光标
对象
在显示记录之前执行检查，如下所示：
if(c != null)
//Display Records
else
Log.e(TAG, "Cursor is Null");
而且，从数据库中检索联系人的方式似乎不正确。。。
创建某些ContentProvider
，以使数据库中的数据可供应用程序使用。。。可能会有一些帮助…在调用onCreate（）
之前，执行逻辑通常是bug的来源。就你而言：
DBAdaptor db = new DBAdaptor(this);
int count = 1;
Cursor cursor = db.getallRecord();
尝试仅在onCreate
中声明变量并初始化它们：
public class MainActivity extends Activity {
DBAdaptor db;
int count = 1;
Cursor cursor;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
db = new DBAdaptor(this);
cursor = db.getallRecord();
DisplayRecord(cursor);
}
[...]
}
再看一看我删除了它，同样的异常仍然发生：（它又在同一个位置了吗？比较两个StackTrace。NPE被抛出在哪一行？getallRecord行是上面的第二个代码窗口。这意味着光标可以获取所有数据库数据。NPE仍然在同一个位置，即使我删除了ELSE部分。对于类似的问题，你应该指出e所在的行异常出现在带有注释的代码示例中，因为我们不能按行号。还要确定该语句中涉及的各种变量/子表达式的值。（即，什么等于null
）这对于日志记录或调试器来说比通过查看代码容易得多。
DBAdaptor db = new DBAdaptor(this);
int count = 1;
Cursor cursor = db.getallRecord();
public class MainActivity extends Activity {
DBAdaptor db;
int count = 1;
Cursor cursor;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
db = new DBAdaptor(this);
cursor = db.getallRecord();
DisplayRecord(cursor);
}
[...]
}