我正在制作一个机器人应用程序,用户可以在其中定义机器人稍后从设备中获取的打孔组合。为了允许用户存储这些培训,我定义了一个类“培训”,其中包含培训的ID,名称和冲床组合。此培训后来保存在一个数据库中,为此我写了一个DatabaseHandler类。添加和显示数据正常工作,但每当我想删除下述方法的条目:Nullpointer从SQL数据库中删除条目后出现异常
public void deleteTraining(Training training) {
SQLiteDatabase db = this.getWritableDatabase();
db.delete(TABLE_TRAININGS, KEY_ID + " = ?",
new String[] { String.valueOf(training.getID()) });
db.close();
}
,并稍后尝试重新填充我的GridView(由GridAdapter类处理),我得到一个空指针例外
显示java.lang.NullPointerException:尝试从字段 'java.lang.String中com.noeth.tobi.mcrobektrainingsplaner.Training._name' 上读取空对象引用
在com.noeth.tobi。 mcrobektrainingsplaner.GridAdapter.getView(GridAdapter.java:50)
的GridAdapter的getView方法:
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
// if it's not recycled, initialize some attributes
btn = new Button(context);
btn.setLayoutParams(new GridView.LayoutParams(370, 350));
btn.setPadding(2,100,2,100);
btn.setOnClickListener(new CustomOnClickListener(position, context));
btn.setOnLongClickListener(new CustomOnLongClickListener(position, context, btn));
}
else {
btn = (Button) convertView;
}
btn.setText(db.getTraining(position)._name); //Here the programm throws a Nullpointer Exception AFTER deleting an entry from the database
btn.setTextColor(Color.WHITE);
btn.setBackgroundResource(R.drawable.button_border);
btn.setTag("not_activated");
btn.setId(position);
return btn;
}
我计算过,它必须是与删除训练的ID,作为循环简单地遍历所有的ID,所以我写了一个方法recalcIDs其重新计算每个项目的删除训练后传来的ID:
recalcIDs
public void recalcIDs(){
int k = 1;
int subtract = 1;
int id;
Training training;
for(int i = deleted.get(0)+1; i < db.getTrainingCount(); i++){
if(deleted.size() > 1){
if(i < deleted.get(k)){
training = db.getTraining(i);
id = training.getID();
training.setID(id-subtract);
}
else{
k+=1;
subtract+=1;
}
}
else{
training = db.getTraining(i);
id = training.getID();
training.setID(id-subtract);
}
}
}
尽管如此,这并不修理它。
当重新安装应用程序并从一个全新的数据库开始时,所有东西都可以再次使用。
有没有人知道我做错了什么?
P.S:这里的getTraining方法在那里找不到名称:
Training getTraining(int id) {
SQLiteDatabase db = this.getReadableDatabase();
Training training;
Cursor cursor = db.query(TABLE_TRAININGS, new String[] { KEY_ID,
KEY_NAME, KEY_SK}, KEY_ID + "=?",
new String[] { String.valueOf(id) }, null, null, null, null);
if (cursor != null && cursor.moveToFirst()){
training = new Training(Integer.parseInt(cursor.getString(0)),
cursor.getString(1), cursor.getLong(2));
cursor.close();
}
else{
training = null;
Toast.makeText(con,"Couldn't find any training sessions!", Toast.LENGTH_LONG).show();
}
// return training
return training;
}
首先从db中查询所有有效的ID,然后将它们映射到位置。 –