当我尝试使用视图数据方法从数据库中获取数据时,即使我的数据库中有数据,它也会给我空指针异常,即视图数据方法:
public String viewdata3(){
SQLiteDatabase sqLiteDatabase4 = dataBase.getWritableDatabase();
String [] classdetail = {DataBase.classname,DataBase.studentsnumber};
@SuppressLint("Recycle") Cursor cursor = sqLiteDatabase4.query(DataBase.tablename1,classdetail,null,
null,null,null,null,null);
StringBuilder stringBuilder = new StringBuilder();
while (cursor.moveToNext()){
String classnamee = cursor.getString(0);
int stdntnmbr = cursor.getInt(1);
stringBuilder.append(classnamee+" "+stdntnmbr+" "+"/n");
}
return viewdata3();
每当我调用此方法时,应用程序就会崩溃,并且与其他任何此类方法一样,即数据恢复方法:
DataBaseConnection db;
ArrayAdapter<String> adapter;
String [] data = {Objects.requireNonNull(db).viewdata3()};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main3);
ListView classes = findViewById(R.id.ListView);
adapter= new ArrayAdapter<>(this, android.R.layout.simple_list_item_1,data);
classes.setAdapter(adapter);
答案 0 :(得分:0)
答案 1 :(得分:0)
我认为问题在这里
import React, { Component } from "react";
export default WrappedComponent => {
class Form extends Component {
state = {
buttonFields: [
{ id: "Apples", quantity: 1 },
{ id: "Strawberries", quantity: 1 },
{ id: "Grapes", quantity: 1 },
{ id: "Apricots", quantity: 1 }
]
};
handleButtonClick = id => {
this.setState(prevState => ({
buttonFields: prevState.buttonFields.map(
item =>
id === item.id ? { id, quantity: item.quantity + 1 } : { ...item }
)
}));
};
render = () => (
<WrappedComponent
{...this.state}
onButtonClick={this.handleButtonClick}
title="Wrapper"
/>
);
}
return Form;
};
您应该在调用上述行之前初始化数据库。
String [] data = {Objects.requireNonNull(db).viewdata3()};