在TripList.java
类中,我有一个listView,其中填充了我的旅行目的地和日期到这些单独的数组中。现在在listView中的每个项目上我都有一个X,当点击它时我希望它从数据库中删除该项目。
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.nikscodingmachine.truckersexpence.DatabaseHelper.deleteData(java.lang.String)' on a null object reference
at com.example.nikscodingmachine.truckersexpence.TripList.getBack(TripList.java:185)
我尝试了很多不同的东西,但似乎没有任何效果。如果我试图进入已存储在那里的东西的数据库,我不明白为什么有一个空指针。看起来,一旦我退出TripList和TruckAdapter类中的onCreate循环,从TripList.java
回到TruckAdapter.java
就是问题所在。我尝试过创建另一个类并在那里传递数据,但是因为其中一些类扩展了其他类,所以当我尝试这样做时,它会给我带来错误。如果有人请告诉我如何做对。非常感谢,我的课程如下。
import android.app.DatePickerDialog;
import android.content.Intent;
import android.database.Cursor;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Adapter;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.Toast;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;
public class TripList extends ActionBarActivity {
//Instance of my created database
public DatabaseHelper myDb;
private TruckAdapter adapter;
//my variables
private String destination;
private int day;
private int month;
private int year;
private ListView listView;
private String da;
private String mo;
private String ye;
//ArrayLists you populate from database so we can later use them in the list
private List destinationArray = new ArrayList();
private List monthArray = new ArrayList();
private List dayArray = new ArrayList();
private List yearArray = new ArrayList();
int[] deleteArray = {R.drawable.delete_button};
//Just an empty constructor
public TripList(){
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_trip_list);
//Creating myDb class
myDb = new DatabaseHelper(this);
//receiving information from InitialEntry
destination = getIntent().getStringExtra("names");
day = getIntent().getIntExtra("toDay", -1);
month = getIntent().getIntExtra("toMonth", -1);
year = getIntent().getIntExtra("toYear", -1);
//Setting my dates toString
mo = Integer.toString(month);
da = Integer.toString(day);
ye = Integer.toString(year);
//Sending to method below
addData(destination, mo, da, ye);
}
//called from onCreate above
public void addData(String destination, String mo, String da, String ye) {
//passing three arguments into the insertData
boolean isInserted = myDb.insertData(destination.toString(), mo.toString(), da.toString(), ye.toString());
//It would return true if data was inserted
if (isInserted == true) {
Toast.makeText(TripList.this, "Data Inserted", Toast.LENGTH_LONG).show();
} else {
Toast.makeText(TripList.this, "Data not Inserted", Toast.LENGTH_LONG).show();
}
//calling these methods to upload from database to local ArrayList
destinationList();
monthList();
dayList();
yearList();
listView = (ListView) findViewById(R.id.list_view);
//custom class
adapter = new TruckAdapter(getApplication(), R.layout.list_custom_layout);
listView.setAdapter(adapter);
for(int i = 0; i < destinationArray.size(); i++){
//passing destination, month, day, and year that we collected from our database to ArrayLists
TruckDataProvider dataProvider = new TruckDataProvider(destinationArray.get(i).toString(), monthArray.get(i).toString(), dayArray.get(i).toString(), yearArray.get(i).toString(), deleteArray[0]);
//adding the objects from TruckDataProvider to TruckAdapter
adapter.add(dataProvider);
}
//when one of the destinations in the list is clicked
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
//We need to increment the position by one because position starts at zero but our table starts at one
position++;
Intent in = new Intent("com.example.nikscodingmachine.truckersexpence.MainActivity");
//Setting position toString
String pO = String.valueOf(position);
//sending position to MainActivity before we go there
in.putExtra("toPosition", pO);
startActivity(in);
}
});
}
//Adding from the database to list so my destinations will be on the list array
public void destinationList() {
//reaching into DatabaseHelper class
Cursor res = myDb.getAllData();
//Moves down the rows
while (res.moveToNext()) {
//From the row that it moved we are getting it's colume in position 1
//and adding it to the list array
destinationArray.add(res.getString(1));
}
}
}
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteOpenHelper;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.view.View;
import android.widget.Toast;
//needed for database methods below
private SQLiteDatabase db = this.getWritableDatabase();
private ContentValues contentValues = new ContentValues();
//our constructor
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
//(ID is Primary Key which also auto increments) (... ...)
db.execSQL("create table " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, DESTINATION TEXT, MONTH INTEGER, DAY INTEGER, YEAR INTEGER, FUEL INTEGER, MEAL INTEGER, MAINTENANCE INTEGER, LICENCING INTEGER, TRAILER INTEGER, INSURANCE INTEGER, EXTRA INTEGER, MISCELLANEOUS INTEGER)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
onCreate(db);
}
//set and checking data method
public boolean insertData(String destination, String month, String day, String year) {
contentValues.put(COL_2, destination);
contentValues.put(COL_3, month);
contentValues.put(COL_4,day);
contentValues.put(COL_5,year);
long result = db.insert(TABLE_NAME,null ,contentValues);
//returns -1 if nothing is in the data base
if(result == -1)
return false;
else
return true;
}
//inserting the amount of fuel
public void insertFuelData(String addedResult, String Id) {
contentValues.put(COL_1, Id);
contentValues.put(COL_6, addedResult);
//sendoff to method below
ourUpdate(Id);
}
//inserting the meal amount
public void insertMealData(String addedResult, String Id) {
contentValues.put(COL_1, Id);
contentValues.put(COL_7, addedResult);
//sendoff to method below
ourUpdate(Id);
}
//getting data method
//which is used in a TripList to find and add to the list array the destinations
public Cursor getAllData(){
//db is instance of a data base, a built in class
Cursor res = db.rawQuery("select * from " + TABLE_NAME, null);
return res;
}
public void deleteData(String id){
db.delete(TABLE_NAME, "ID = ?", new String[] {id});
}
//part of update more convenient this way
public void ourUpdate(String Id){
//built in update Id is where
db.update(TABLE_NAME, contentValues, "ID = ?", new String[]{Id});
}
}
import android.content.Context;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
public class TruckAdapter extends ArrayAdapter {
TripList tList;
List list = new ArrayList();
private int pos;
public TruckAdapter(Context context, int resource) {
super(context, resource);
tList = new TripList();
}
//made a class
static class DataHandler{
TextView title;
TextView month;
TextView day;
TextView year;
ImageView delete;
}
DataHandler handler = new DataHandler();
@Override
public void add(Object object) {
super.add(object);
list.add(object);
}
@Override
public int getCount() {
return this.list.size();
}
@Override
public Object getItem(int position) {
return this.list.get(position);
}
public View getView(final int position, View convertView, ViewGroup parent) {
}
public class TruckDataProvider {
private String des;
private String month;
private String day;
private String year;
private int delete;
//Constructor
public TruckDataProvider(String des, String month, String day, String year, int delete){
this.des = des;
this.month = month;
this.day = day;
this.year = year;
this.delete = delete;
}
public String getDes() {
return des;
}
public String getMonth() {
return month;
}
public String getDay() {
return day;
}
public String getYear() {
return year;
}
public int getDelete() {
return delete;
}
}
答案 0 :(得分:1)
我认为我发现了问题但仍然应该根据好的例子重写代码
我更改了以下内容
Context context;
public TruckAdapter(Context context, int resource) {
super(context, resource);
tList = new TripList();
this.context = context;
}
调用getback的问题
handler.delete.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
tList.getBack(position, context.getApplicationContext());
}
});
并在TripList中
public void getBack(int del , Context ctx){
String delete = Integer.toString(del);
new DatabaseHelper(ctx).deleteData(delete);
}
======================
在你的方法中:
public void getBack(int del){
String delete = Integer.toString(del);
myDb.deleteData(delete);
}
变量myDB
尚未初始化,因此为空。在尝试使用它之前,应该将变量设置为数据处理类的有效实例