我试图在一个方法中存储特定的列数据，然后在另一个类String []中调用该方法，微调器将访问该数组以显示列行，以供用户选择，然后将其存储在另一个数据库中。
我得到nulpointerexception，当我尝试打开数据库时，数据库被锁定。对不起，听起来像是一个完整的业余爱好者，相对于android来说还比较新。预先感谢您的任何帮助。
这是我在主类中调用getInstance（）和getCPnames（）时的代码
String[] carParks = CarParkDb.getInstance().getCpnames();
这是我的数据库代码：
package com.example.parkangel;
import java.util.ArrayList;
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;
public class CarParkDb {
public static final String KEY_ID = "_id";
public static final String KEY_CPNAME = "cpname";
public static final String KEY_COST = "cost";
private static final String DATABASE_NAME = "CPDB";
private static final String DATABASE_TABLE = "CPTable";
private static final int DATABASE_VERSION = 1;
private CPDbHelper cpdbHelper;
private Context ourContext;
private SQLiteDatabase ourDatabase;
private static CarParkDb instance;
private static class CPDbHelper extends SQLiteOpenHelper{
public CPDbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
// TODO Auto-generated constructor stub
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
KEY_CPNAME + " TEXT NOT NULL, " + KEY_COST + " TEXTNOT NULL);");
db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('1','LearningResource Center','2');");
db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values ('2','ParkandRide','1');");
db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values ('3','deHavilland Campus','2');");
db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('4','MultiStorey Building','2');");
db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('5','Reception','2');");
}
public void onOpen(final SQLiteDatabase db) {
super.onOpen(db);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
onCreate(db);
}
}
public CarParkDb (final Context c)
{
this.cpdbHelper= new CPDbHelper(c);
establishDb();
//ourContext = c;
}
public void establishDb()
{
if (this.ourDatabase == null)
{
this.ourDatabase = this.cpdbHelper.getWritableDatabase();
}
}
public CarParkDb() {
// TODO Auto-generated constructor stub
}
public CarParkDb open() throws SQLException
{
System.out.println ("running open");
cpdbHelper = new CPDbHelper(ourContext);
ourDatabase = cpdbHelper.getWritableDatabase();
return this;
}
public void close()
{
ourDatabase.close();
}
/*public long createEntry(String cpname, String cost){
ContentValues cv = new ContentValues();
cv.put(KEY_CPNAME, cpname);
cv.put(KEY_COST, cost);
return ourDatabase.insert(DATABASE_TABLE, null, cv);
}*/
public String getData() {
// TODO Auto-generated method stub
//open();
String[] columns = new String[] {KEY_ID, KEY_CPNAME, KEY_COST};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null,null,null, null, null);
String result = " ";
int iRow = c.getColumnIndexOrThrow(KEY_ID);
int iCpname = c.getColumnIndexOrThrow(KEY_CPNAME);
int iCost = c.getColumnIndexOrThrow(KEY_COST);
for(c.moveToFirst(); !c.isAfterLast(); c.moveToNext()){
result = result + c.getString(iRow) + " " +c.getString(iCpname) + " " + c.getString(iCost) + " " + "\n";
c.close();
ourDatabase.close();
}
return result;
}
public static CarParkDb getInstance()
{
synchronized(CarParkDb.class)
{
if (instance == null)
{
instance = new CarParkDb();
}
return instance;
}
}
public String[] getCpnames()
{
//open();
if (ourDatabase == null) System.out.println ("is null");
Cursor c = null;
ArrayList<String> list = new ArrayList<String>();
ourDatabase = cpdbHelper.getReadableDatabase();
//SQLiteDatabase db = cpdbHelper.getReadableDatabase();
String query = "SELECT " + KEY_CPNAME + " FROM " + DATABASE_TABLE;
{
c = this.ourDatabase.rawQuery(query, null);
int iCpname = c.getColumnIndexOrThrow(KEY_CPNAME);
if (c.moveToFirst())
{
do
{
list.add(c.getString(iCpname));;
}
while (c.moveToNext());
}
if (c != null && !c.isClosed())
{
c.close();
ourDatabase.close();
}
return list.toArray(new String[]{});
}
}
}
**LogCat**
03-12 01:32:39.759: E/AndroidRuntime(4176): Caused by:java.lang.NullPointerException
03-12 01:32:39.759: E/AndroidRuntime(4176):
at com.example.parkangel.CarParkDb.getCpnames(CarParkDb.java:191)
03-12 01:32:39.759: E/AndroidRuntime(4176):
at com.example.parkangel.BookTicket.<init>(BookTicket.java:22)
03-12 01:32:39.759: E/AndroidRuntime(4176):
at java.lang.Class.newInstanceImpl(Native Method)
03-12 01:32:39.759: E/AndroidRuntime(4176):
at java.lang.Class.newInstance(Class.java:1208)
最佳答案
尝试使用以下方法更新您的getInstance()代码：
public static CarParkDb getInstance(Context c) // <-- added context as parameter
{
synchronized(CarParkDb.class)
{
if (instance == null)
{
instance = new CarParkDb(c); // <-- used context in constructor
}
return instance;
}
}
问题似乎是您正在使用什么都不做的构造函数（用于
CarParkDb）。您有另一个构造函数，该构造函数采用Context作为参数，并初始化稍后使用的某些对象。