我已经创建了 sdcard 并在其中保存了数据库文件 UPData.db。在这个数据库中，我创建了一个表名 Customer，但没有得到结果。正在显示 DBList 中的 NullPointerException。请帮助我按照相同的结构执行此操作？
activity_up.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".UP"
android:orientation="vertical">
<LinearLayout android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="UserName" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:hint="UserName"
android:id="@+id/un"/>
</LinearLayout>
<LinearLayout android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="PassWord" />
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:hint="PassWord"
android:id="@+id/pw" />
</LinearLayout>
<Button android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:text="Submit"
android:id="@+id/submit"/>
</LinearLayout>
res/layout/list.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".UP"
>
<ListView
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:id="@+id/listview"></ListView>
</LinearLayout>
viewholder.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".UP"
android:orientation="vertical">
<TextView
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:id="@+id/tvid"/>
<TextView
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:id="@+id/tvname"/>
<TextView
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:id="@+id/tvaddress"/>
<TextView
android:layout_height="wrap_content"
android:layout_width="wrap_content"
android:id="@+id/tvphone"/>
</LinearLayout>
**manifest.xml**
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.up"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme"
android:name="Main">
<activity
android:name="com.example.up.UP"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name="ShowList"></activity>
</application>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
</manifest>
**UP.java**
package com.example.up;
public class UP extends Activity {
EditText un,pw;
Button submit;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_up);
un=(EditText)findViewById(R.id.un);
pw=(EditText)findViewById(R.id.pw);
submit=(Button)findViewById(R.id.submit);
submit.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String user=un.getText().toString();
String pwd=pw.getText().toString();
System.out.println("user--"+user +"&&"+"Password is--"+pwd);
Intent showlist=new Intent(UP.this,ShowList.class);
startActivity(showlist);
}
});
}
}
**ShowList.java**
package com.example.up;
public class ShowList extends Activity {
ListView list;
ArrayList<ListData> alist;
ArrayList<ListData> achildlist;
ArrayAdapter<String> adapter;
int id=-1;
Cursor cur=null;
ListData listdata;
DBHelper help;
DBList dblist;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.list);
list=(ListView)findViewById(R.id.listview);
loadData();
}
public void loadData(){
try{
alist=new ArrayList<ShowList.ListData>();
dblist=new DBList(this);
cur=dblist.getListData(this);
if(cur!=null){
if(cur.moveToFirst()){
do{
listdata=new ListData();
listdata.setCustomerId(cur.getInt(cur.getColumnIndex("customerId")));
listdata.setCustomerName(cur.getString(cur.getColumnIndex("customerName")));
listdata.setCustomerName(cur.getString(cur.getColumnIndex("customerAddress")));
listdata.setCustomerId(cur.getInt(cur.getColumnIndex("phone")));
String id=Integer.toString(cur.getInt(cur.getColumnIndex("customerId")));
System.out.println(id);
alist.add(listdata);
}while(cur.moveToNext());
}
cur.close();
}
list.setAdapter(new ListDataAdapter(this,alist));
}catch(Exception e){
e.printStackTrace();
}
}
class ListDataAdapter extends BaseAdapter{
Context con;
ListDataAdapter(Context c,ArrayList<ListData> listadapter){
this.con=c;
achildlist=listadapter;
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return achildlist.size();
}
@Override
public Object getItem(int arg0) {
// TODO Auto-generated method stub
return achildlist.get(arg0);
}
@Override
public long getItemId(int arg0) {
// TODO Auto-generated method stub
return arg0;
}
@Override
public View getView(int arg0, View arg1, ViewGroup arg2) {
// TODO Auto-generated method stub
ViewHolder obj=null;
try{
if(arg1==null){
obj=new ViewHolder();
LayoutInflater vi=(LayoutInflater)getSystemService(Context.LOCATION_SERVICE);
arg1=vi.inflate(R.layout.viewholder, null);
obj.tvid=(TextView)arg1.findViewById(R.id.tvid);
obj.tvname=(TextView)arg1.findViewById(R.id.tvname);
obj.tvaddress=(TextView)arg1.findViewById(R.id.tvaddress);
obj.tvphone=(TextView)arg1.findViewById(R.id.tvphone);
arg1.setTag(obj);
}else{
obj=(ViewHolder)arg1.getTag();
}
if(obj!=null){
ListData listdata=achildlist.get(arg0);
obj.tvid.setText(listdata.getCustomerId());
obj.tvname.setText(listdata.getCustomerName());
obj.tvaddress.setText(listdata.getCustomerAddress());
obj.tvphone.setText(listdata.getPhone());
}
}catch(Exception e){
e.printStackTrace();
}
return arg1;
}
}
class ViewHolder {
TextView tvid;
TextView tvname;
TextView tvaddress;
TextView tvphone;
}
class ListData{
int customerId;
String customerName;
String customerAddress;
int phone;
public int getCustomerId() {
return customerId;
}
public void setCustomerId(int customerId) {
this.customerId = customerId;
}
public String getCustomerName() {
return customerName;
}
public void setCustomerName(String customerName) {
this.customerName = customerName;
}
public String getCustomerAddress() {
return customerAddress;
}
public void setCustomerAddress(String customerAddress) {
this.customerAddress = customerAddress;
}
public int getPhone() {
return phone;
}
public void setPhone(int phone) {
this.phone = phone;
}
}
}
**Main.java**
package com.example.up;
public class Main extends Application {
public DBHelper adapter;
}
**DBHelper.java**
package com.example.up;
public class DBHelper{
public static final int version = 1;
public String DBName = "/sdcard/UPData.db";
private SQLiteDatabase dbSqlite;
private Context c;
public SQHelper openhelper;
//public static final String PATH_APPLICATION = "/Android/Data/com.example.up/files/";
//public static final String FOLDER_LOGS = "Logs/";
//public static final String LOG_FILE_NAME = "ErrorLog.txt";
public DBHelper(Context context){
c=context;
openhelper=new SQHelper(c, DBName, null, version);
}
public class SQHelper extends SQLiteOpenHelper{
public SQHelper(Context context, String name,
CursorFactory factory, int version) {
super(context, name, factory, version);
// TODO Auto-generated constructor stub
}
@Override
public void onCreate(SQLiteDatabase db) {
try{
System.out.println("inside oncreate of dbhelper+++++++++");
File sdcard = Environment.getExternalStorageDirectory();
File f = new File(sdcard,"UPData.db");
if(!f.exists()){
f.createNewFile();
}
BufferedReader buf = new BufferedReader(new FileReader(f));
String readString = new String();
while((readString = buf.readLine())!= null){
Log.d("oncreate of DBHelper in UP..........", readString);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e){
e.printStackTrace();
}
// TODO Auto-generated method stub
/*try {
if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
String strDirPath = Environment.getExternalStorageDirectory().getPath() + PATH_APPLICATION + FOLDER_LOGS;
File dirLog = new File(strDirPath);
if (!dirLog.isDirectory() || !dirLog.exists()) {
dirLog.mkdirs();
}
//String strLogFile = "Logs_" + CommonMethods.getCurrentDateTime(Values.DATE_FORMAT) + ".txt";
String strLogFile = LOG_FILE_NAME;
File fileLog = new File(dirLog, strLogFile);
if (!fileLog.exists()) {
fileLog.createNewFile();
}
FileWriter fileWrite = new FileWriter(fileLog, true);
BufferedWriter bufWriter = new BufferedWriter(fileWrite);
//String strLog = CommonMethods.getCurrentDateTime(Values.DATE_TIME_2_FORMAT) + " ERROR : " + strModule + "\t" + strLogData + " ; " + "\r\n";
//bufWriter.write(strLog);
bufWriter.close();
fileWrite.close();
}
} catch (Exception ex) {
ex.printStackTrace();
}*/
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
}
}
public void close() {
try {
dbSqlite.close();
} catch (Exception e) {
e.printStackTrace();
}
}
public void open() throws SQLiteException {
try {
dbSqlite = openhelper.getWritableDatabase();
} catch (SQLiteException e) {
dbSqlite = openhelper.getReadableDatabase();
e.printStackTrace();
}
}
public Cursor getRecordsWithRawQuery(String strSql, String[] sArrSelectionArgs) {
Cursor cursor = null;
try {
cursor = dbSqlite.rawQuery(strSql, sArrSelectionArgs);
} catch (SQLiteException e) {
e.printStackTrace();
}
return cursor;
}
public void executeNonQuery(String strQuery) {
try {
dbSqlite.execSQL(strQuery);
} catch (SQLException e) {
e.printStackTrace();
}
}
}
//这个java类（DBList）我得到空指针异常。我应该在哪一部分编写代码以从 sdcard 读取表。或者这是否需要！
DBList.java
package com.example.up;
public class DBList {
private int customerId;
private String customerName;
private String customerAddress;
private int phone;
Context context;
Main maincall;
SQLiteDatabase db;
public DBList(Context con){
this.context=con;
maincall=(Main)context.getApplicationContext();
}
public Cursor getListData(Context context){
Cursor cur=null;
try{
//String query="Select customerId,customerName,customerAddress,phone from Customer where customerId="+id;
String query="Select customerId,customerName,customerAddress,phone from Customer";
Main maincall=(Main)context.getApplicationContext();
cur=maincall.adapter.getRecordsWithRawQuery(query, null);
}catch(Exception e){
e.printStackTrace();
}
return cur;
}
public void insert(){
try{
String query="Insert into Customer(customerId,customerName,customerAddress,phone) Values" +"("+customerId+","+customerName+","+customerAddress+","+phone+")";
maincall.adapter.executeNonQuery(query);
}catch(Exception e){
e.printStackTrace();
}
}
public int getCustomerId() {
return customerId;
}
public void setCustomerId(int customerId) {
this.customerId = customerId;
}
public String getCustomerName() {
return customerName;
}
public void setCustomerName(String customerName) {
this.customerName = customerName;
}
public String getCustomerAddress() {
return customerAddress;
}
public void setCustomerAddress(String customerAddress) {
this.customerAddress = customerAddress;
}
public int getPhone() {
return phone;
}
public void setPhone(int phone) {
this.phone = phone;
}
}