Android 尝试在sqlite中的空对象引用上调用虚拟方法
您好,我是android新手,我正在尝试从sqlite检索数据并获取Android 尝试在sqlite中的空对象引用上调用虚拟方法,android,sqlite,nullpointerexception,Android,Sqlite,Nullpointerexception,您好,我是android新手,我正在尝试从sqlite检索数据并获取nullpointerreference..请帮助我 这是我的密码:- Databasehelper.java public class DatabaseHelper extends SQLiteOpenHelper{ private final static String DATABASE_NAME="Reminder_details.db"; private final static String TABLE
nullpointer
reference..请帮助我
这是我的密码:-
Databasehelper.java
public class DatabaseHelper extends SQLiteOpenHelper{
private final static String DATABASE_NAME="Reminder_details.db";
private final static String TABLE_NAME="Reminder_detail_insert_tbl";
private final static String CAL_NO1="Category_img";
private final static String CAL_NO2="Category_Type";
private final static String CAL_NO3="Title";
private final static String CAL_NO4="Description";
private final static String CAL_NO5="Repeat_type";
private final static String CAL_NO6="Repeat_value";
private final static String CAL_NO7="sun_Remind";
private final static String CAL_NO8="mon_remind";
private final static String CAL_NO9="tue_remind";
private final static String CAL_NO10="wed_remind";
private final static String CAL_NO11="thu_remind";
private final static String CAL_NO12="fri_remind";
private final static String CAL_NO13="sat_remind";
private final static String CAL_NO14="active_date";
private final static String CAL_NO15="active_end_date";
private final static String CAL_NO16="creation_date";
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("Create Table " +TABLE_NAME +"(ID INTEGER PRIMARY KEY AUTOINCREMENT,Category_img BLOB,Category_Type TEXT,Title TEXT,Description TEXT,Repeat_type TEXT,Repeat_value INTEGER,sun_Remind TEXT,mon_remind TEXT,tue_remind TEXT,wed_remind TEXT,thu_remind TEXT,fri_remind TEXT,sat_remind TEXT,active_date TEXT,active_end_date TEXT,creation_date DATETIME DEFAULT CURRENT_TIMESTAMP)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
onCreate(db);
}
public boolean DataInsert(
byte[] Category_img,
String Category_Type,
String Title,
String Description,
String Repeat_type,
int Repeat_value,
String sun_Remind,
String mon_Remind,
String tue_remind,
String wed_remind,
String thu_remind,
String fri_remind,
String sat_remind,
String active_date,
String active_end_date
)
{
SQLiteDatabase db= getWritableDatabase();
ContentValues cv=new ContentValues();
cv.put(CAL_NO1,Category_img);
cv.put(CAL_NO2,Category_Type);
cv.put(CAL_NO3,Title);
cv.put(CAL_NO4,Description);
cv.put(CAL_NO5,Repeat_type);
cv.put(CAL_NO6,Repeat_value);
cv.put(CAL_NO7,sun_Remind);
cv.put(CAL_NO8,mon_Remind);
cv.put(CAL_NO9,tue_remind);
cv.put(CAL_NO10,wed_remind);
cv.put(CAL_NO11,thu_remind);
cv.put(CAL_NO12,fri_remind);
cv.put(CAL_NO13,sat_remind);
cv.put(CAL_NO14,active_date);
cv.put(CAL_NO15,active_end_date);
long result=db.insert(TABLE_NAME,null,cv);
if(result==-1)
return false;
else
return true;
}
public Cursor getdata()
{
SQLiteDatabase db= getWritableDatabase();
Cursor res=db.rawQuery("select * from "+TABLE_NAME,null);
return res;
}
}
public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.ViewHolder>
{
DatabaseHelper mydb;
List<String> title_list,description_list;
private String[] title ={"Timetable","Subjects","Faculty","Resources"};
private String[] description={"description1","description2","description3","description4"};
private int[] images={R.drawable.table,R.drawable.books,R.drawable.faculty,R.drawable.res};
//
// public void getdatadb()
// {
//
//
// title_list=new ArrayList<String>();
// description_list=new ArrayList<String>();
//
//
// Cursor result=mydb.getdata();
//
// if(result.getCount()==0)
// {
// return;
// }else
// {
// while (result.moveToNext()){
// title_list.add(result.getString(2));
// description_list.add(result.getString(3));
//
//
// }
// }
//
//
//
// }
public class ViewHolder extends RecyclerView.ViewHolder{
TextView mTextTile,mTextDescription;
ImageView mImageview;
public ViewHolder(View itemView)
{
super(itemView);
mTextTile=(TextView)itemView.findViewById(R.id.tvmain);
mTextDescription=(TextView)itemView.findViewById(R.id.tvdescription);
mImageview=(ImageView)itemView.findViewById(R.id.ivmain);
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position=getAdapterPosition();
String text=mTextTile.getText().toString();
Snackbar.make(v,"Click Detected onItem "+position+" Item :"+text,Snackbar.LENGTH_LONG).setAction("Action",null).show();
}
});
}
}
@Override
public RecyclerAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
View view= LayoutInflater.from(parent.getContext()).inflate(R.layout.cardview_layout,parent,false);
ViewHolder viewHolder=new ViewHolder(view);
title_list=new ArrayList<String>();
description_list=new ArrayList<String>();
Cursor result=mydb.getdata();
while (result.moveToNext()){
title_list.add(result.getString(2));
description_list.add(result.getString(3));
}
return viewHolder;
}
@Override
public void onBindViewHolder(RecyclerAdapter.ViewHolder holder, int position)
{
holder.mTextTile.setText(title_list.get(position));
holder.mTextDescription.setText(description_list.get(position));
holder.mImageview.setImageResource(images[position]);
}
@Override
public int getItemCount() {
return title.length;
}
}
这是RecyclerAdaptor,用于在运行时从表中获取数据。请在此帮助我
在这个recycleradaptor中,我从表中获取数据时出错
cursorresult=mydb.getdata()代码>在这一行中,我遇到了错误
RecyclerAdaptor.java
public class DatabaseHelper extends SQLiteOpenHelper{
private final static String DATABASE_NAME="Reminder_details.db";
private final static String TABLE_NAME="Reminder_detail_insert_tbl";
private final static String CAL_NO1="Category_img";
private final static String CAL_NO2="Category_Type";
private final static String CAL_NO3="Title";
private final static String CAL_NO4="Description";
private final static String CAL_NO5="Repeat_type";
private final static String CAL_NO6="Repeat_value";
private final static String CAL_NO7="sun_Remind";
private final static String CAL_NO8="mon_remind";
private final static String CAL_NO9="tue_remind";
private final static String CAL_NO10="wed_remind";
private final static String CAL_NO11="thu_remind";
private final static String CAL_NO12="fri_remind";
private final static String CAL_NO13="sat_remind";
private final static String CAL_NO14="active_date";
private final static String CAL_NO15="active_end_date";
private final static String CAL_NO16="creation_date";
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("Create Table " +TABLE_NAME +"(ID INTEGER PRIMARY KEY AUTOINCREMENT,Category_img BLOB,Category_Type TEXT,Title TEXT,Description TEXT,Repeat_type TEXT,Repeat_value INTEGER,sun_Remind TEXT,mon_remind TEXT,tue_remind TEXT,wed_remind TEXT,thu_remind TEXT,fri_remind TEXT,sat_remind TEXT,active_date TEXT,active_end_date TEXT,creation_date DATETIME DEFAULT CURRENT_TIMESTAMP)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
onCreate(db);
}
public boolean DataInsert(
byte[] Category_img,
String Category_Type,
String Title,
String Description,
String Repeat_type,
int Repeat_value,
String sun_Remind,
String mon_Remind,
String tue_remind,
String wed_remind,
String thu_remind,
String fri_remind,
String sat_remind,
String active_date,
String active_end_date
)
{
SQLiteDatabase db= getWritableDatabase();
ContentValues cv=new ContentValues();
cv.put(CAL_NO1,Category_img);
cv.put(CAL_NO2,Category_Type);
cv.put(CAL_NO3,Title);
cv.put(CAL_NO4,Description);
cv.put(CAL_NO5,Repeat_type);
cv.put(CAL_NO6,Repeat_value);
cv.put(CAL_NO7,sun_Remind);
cv.put(CAL_NO8,mon_Remind);
cv.put(CAL_NO9,tue_remind);
cv.put(CAL_NO10,wed_remind);
cv.put(CAL_NO11,thu_remind);
cv.put(CAL_NO12,fri_remind);
cv.put(CAL_NO13,sat_remind);
cv.put(CAL_NO14,active_date);
cv.put(CAL_NO15,active_end_date);
long result=db.insert(TABLE_NAME,null,cv);
if(result==-1)
return false;
else
return true;
}
public Cursor getdata()
{
SQLiteDatabase db= getWritableDatabase();
Cursor res=db.rawQuery("select * from "+TABLE_NAME,null);
return res;
}
}
public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.ViewHolder>
{
DatabaseHelper mydb;
List<String> title_list,description_list;
private String[] title ={"Timetable","Subjects","Faculty","Resources"};
private String[] description={"description1","description2","description3","description4"};
private int[] images={R.drawable.table,R.drawable.books,R.drawable.faculty,R.drawable.res};
//
// public void getdatadb()
// {
//
//
// title_list=new ArrayList<String>();
// description_list=new ArrayList<String>();
//
//
// Cursor result=mydb.getdata();
//
// if(result.getCount()==0)
// {
// return;
// }else
// {
// while (result.moveToNext()){
// title_list.add(result.getString(2));
// description_list.add(result.getString(3));
//
//
// }
// }
//
//
//
// }
public class ViewHolder extends RecyclerView.ViewHolder{
TextView mTextTile,mTextDescription;
ImageView mImageview;
public ViewHolder(View itemView)
{
super(itemView);
mTextTile=(TextView)itemView.findViewById(R.id.tvmain);
mTextDescription=(TextView)itemView.findViewById(R.id.tvdescription);
mImageview=(ImageView)itemView.findViewById(R.id.ivmain);
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position=getAdapterPosition();
String text=mTextTile.getText().toString();
Snackbar.make(v,"Click Detected onItem "+position+" Item :"+text,Snackbar.LENGTH_LONG).setAction("Action",null).show();
}
});
}
}
@Override
public RecyclerAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
View view= LayoutInflater.from(parent.getContext()).inflate(R.layout.cardview_layout,parent,false);
ViewHolder viewHolder=new ViewHolder(view);
title_list=new ArrayList<String>();
description_list=new ArrayList<String>();
Cursor result=mydb.getdata();
while (result.moveToNext()){
title_list.add(result.getString(2));
description_list.add(result.getString(3));
}
return viewHolder;
}
@Override
public void onBindViewHolder(RecyclerAdapter.ViewHolder holder, int position)
{
holder.mTextTile.setText(title_list.get(position));
holder.mTextDescription.setText(description_list.get(position));
holder.mImageview.setImageResource(images[position]);
}
@Override
public int getItemCount() {
return title.length;
}
}
公共类RecyclerAdapter扩展了RecyclerView.Adapter
{
数据库助手mydb;
清单标题清单、说明清单;
私有字符串[]title={“时间表”、“科目”、“教员”、“资源”};
私有字符串[]description={“description1”、“description2”、“description3”、“description4”};
private int[]images={R.drawable.table,R.drawable.books,R.drawable.faculty,R.drawable.res};
//
//public void getdatadb()
// {
//
//
//title_list=new ArrayList();
//description_list=new ArrayList();
//
//
//游标结果=mydb.getdata();
//
//if(result.getCount()==0)
// {
//返回;
//}其他
// {
//while(result.moveToNext()){
//title_list.add(result.getString(2));
//description_list.add(result.getString(3));
//
//
// }
// }
//
//
//
// }
公共类ViewHolder扩展了RecyclerView.ViewHolder{
text查看mTextTile,mTextDescription;
图像视图;
公共视图持有者(视图项视图)
{
超级(项目视图);
mTextTile=(TextView)itemView.findviewbyd(R.id.tvmain);
mTextDescription=(TextView)itemView.findViewById(R.id.tvdescription);
mImageview=(ImageView)itemView.findViewById(R.id.ivmain);
itemView.setOnClickListener(新视图.OnClickListener(){
@凌驾
公共void onClick(视图v){
int position=getAdapterPosition();
字符串text=mTextTile.getText().toString();
Snackbar.make(v,“点击按钮”+position+”项:“+text,Snackbar.LENGTH_LONG”).setAction(“Action”,null.show();
}
});
}
}
@凌驾
public RecyclerAdapter.ViewHolder onCreateViewHolder(视图组父级,int-viewType)
{
View-View=LayoutFlater.from(parent.getContext()).flate(R.layout.cardview\u布局,parent,false);
ViewHolder ViewHolder=新的ViewHolder(视图);
title_list=new ArrayList();
description_list=new ArrayList();
游标结果=mydb.getdata();
while(result.moveToNext()){
title_list.add(result.getString(2));
description_list.add(result.getString(3));
}
返回视图持有者;
}
@凌驾
BindViewHolder上的公共无效(RecyclerAdapter.ViewHolder,int位置)
{
holder.mTextTile.setText(title_list.get(position));
holder.mTextDescription.setText(description_list.get(position));
holder.mImageview.setImageResource(图像[位置]);
}
@凌驾
public int getItemCount(){
返回title.length;
}
}
在的“onCreateViewHolder()”
开头添加此行:
因此,您的方法如下:
@Override
public CustomAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
mydb = new DatabaseHelper(parent.getContext());
View view= LayoutInflater.from(parent.getContext()).inflate(R.layout.cardview_layout,parent,false);
ViewHolder viewHolder=new ViewHolder(view);
title_list=new ArrayList<String>();
description_list=new ArrayList<String>();
Cursor result=mydb.getdata();
while (result.moveToNext()){
title_list.add(result.getString(2));
description_list.add(result.getString(3));
}
return viewHolder;
}
@覆盖
public CustomAdapter.ViewHolder onCreateViewHolder(视图组父级,int-viewType)
{
mydb=newDatabaseHelper(parent.getContext());
View-View=LayoutFlater.from(parent.getContext()).flate(R.layout.cardview\u布局,parent,false);
ViewHolder ViewHolder=新的ViewHolder(视图);
title_list=new ArrayList();
description_list=new ArrayList();
游标结果=mydb.getdata();
while(result.moveToNext()){
title_list.add(result.getString(2));
description_list.add(result.getString(3));
}
返回视图持有者;
}
在此发布您的错误MyDB未初始化谢谢您解决了我的问题…我可以知道为什么我们需要添加这一行吗?请您解释一下。因此,将来我可能会遇到此类问题。您需要初始化“Database Helper”以访问数据库方法。谢谢,如果您需要更多信息,请告诉我。