这是类 Db.java
package calc.three;
import java.io.IOException;
import android.app.Activity;
import android.app.ListActivity;
import android.content.ContentValues;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.database.Cursor;
import android.database.SQLException;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.TextView;
import android.widget.Toast;
import android.database.sqlite.SQLiteDatabase;
import android.widget.AdapterView;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
public class Db extends ListActivity {
private SQLiteDatabase newDb;
DataBaseHelper myDbHelper = new DataBaseHelper(this);
public ListAdapter adapter;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
openDatabase();
Cursor c = newDb.rawQuery("SELECT _id, organisation FROM shoplist", null);
adapter = new SimpleCursorAdapter(this, R.layout.main, c, new String[] { "organisation" }, new int[] { android.R.id.list });
setListAdapter(adapter);
//inserttodata();
myDbHelper.close();
}
private long inserttodata() {
// TODO Auto-generated method stub
ContentValues cv = new ContentValues();
cv.put("id", "112");
cv.put("organisation","test" );
return newDb.insert("shoplist", null, cv);
}
private void openDatabase() {
try {
myDbHelper.createDataBase();
} catch (IOException ioe) {
throw new Error("Unable to create database");
}
try {
myDbHelper.openDataBase();
} catch (SQLException sqle) {
System.out.println("failed to open");
throw sqle;
}
//String[] columns= new String[]{"_id", "Organisation"};
//cursor = newDb.query("Shop", columns, null, null, null, null, null);
}
}
这是数据库助手类
package calc.three;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
public class DataBaseHelper extends SQLiteOpenHelper{
//The Android's default system path of your application database.
private static String DB_PATH = "/data/data/calc.three/databases/";
private static String DB_NAME = "charity.db";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
public DataBaseHelper(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
/**
* Check if the database already exist to avoid re-copying the file each time you open the application.
* @return true if it exists, false if it doesn't
*/
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DB_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[2048];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
public void openDataBase() throws SQLException{
//Open the database
String myPath = DB_PATH + DB_NAME;
SQLiteDatabase.CursorFactory c = null;
myDataBase = SQLiteDatabase.openDatabase(myPath, c , SQLiteDatabase.OPEN_READWRITE);
}
@Override
public synchronized void close() {
if(myDataBase != null)
myDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
// Add your public helper methods to access and get content from the database.
// You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
// to you to create adapters for your views.
}
XMl 布局
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ListView
android:id="@android:id/list"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="3dip" >
</ListView>
</LinearLayout>
我不想在这里做太多事情。我在资产文件夹中发送了一个.db文件，复制它成功了......创建副本后，我在SQLite浏览器中从data/data/calc.three/database打开了.db文件，我在那里有表格。
但我似乎无法运行它强制关闭的应用程序，并在将 rawQuery 传递给光标的行中给我一个 nullPointerException，我在没有该行的情况下运行程序并且没有错误，但我无法在列表视图中显示数据。请帮助我过去 3 天一直被困在这个上面。
谢谢
您尚未设置newDb... 您声明了它，但当您尝试使用它时它仍然是 null，从而给您带来错误。