我正在尝试查看我在外部创建并放入我的应用程序的SQLite数据库中的所有数据。我的代码是一些由几个教程和我自己的代码组成的集合。该应用运行正常，直到我点击＆＃34;查看全部＆＃34;按钮。然后我在LogCat中收到以下错误：
java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor com.example.fourth.DBHelper.getAllData()' on a null object reference at com.example.fourth.MainActivity$1.onClick
我目前有2个班级：
DBHelper
package com.example.fourth;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
public class DBHelper extends SQLiteOpenHelper {
// The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.example.fourth/databases/";
private static String DB_NAME = "bar_info.db";
private SQLiteDatabase myDataBase;
private final Context myContext;
public static final String TABLE_NAME = "bar_info";
/**
* Constructor Takes and keeps a reference of the passed context in order to
* access to the application assets and resources.
*
* @param context
*/
public DBHelper(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 {
this.close();
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[1024];
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;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
@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) {
}
public Cursor getAllData() {
SQLiteDatabase db = this.getWritableDatabase();
Cursor res = db.rawQuery("select * from " + TABLE_NAME, null);
return res;
}
// 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.
}
和MainActivity：
package com.example.fourth;
import android.support.v7.app.ActionBarActivity;
import java.io.IOException;
import android.app.AlertDialog;
import android.database.Cursor;
import android.database.SQLException;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class MainActivity extends ActionBarActivity {
DBHelper myDB;
EditText editBarName, editBarCity, editDrinkName;
Button btnAddData, btnViewAll;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DBHelper myDbHelper = new DBHelper(this);
myDbHelper = new DBHelper(this);
try {
myDbHelper.createDataBase();
} catch (IOException ioe) {
throw new Error("Unable to create database");
}
try {
myDbHelper.openDataBase();
} catch (SQLException sqle) {
throw sqle;
}
/*
* editBarName = (EditText) findViewById(R.id.editBarName); editBarCity
* = (EditText) findViewById(R.id.editBarCity); editDrinkName =
* (EditText) findViewById(R.id.editDrinkName); btnAddData = (Button)
* findViewById(R.id.btnAddData);
*/
btnViewAll = (Button) findViewById(R.id.btnViewAll);
// AddData();
ViewAll();
}
/*
* public void AddData() { btnAddData.setOnClickListener(new
* View.OnClickListener() {
*
* @Override public void onClick(View v) { boolean isInsterted =
* myDB.insertData(editBarName.getText().toString(),
* editBarCity.getText().toString(), editDrinkName.getText().toString()); if
* (isInsterted == true) Toast.makeText(MainActivity.this, "Data Inserted",
* Toast.LENGTH_LONG).show(); else Toast.makeText(MainActivity.this,
* "Data Not Inserted", Toast.LENGTH_LONG).show(); } }); }
*/
public void ViewAll() {
btnViewAll.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Cursor res = myDB.getAllData();
if (res.getCount() == 0) {
// show message
showMessage("Error", "No Data Found");
return;
}
StringBuffer buffer = new StringBuffer();
while (res.moveToNext()) {
buffer.append("ID :" + res.getString(0) + "\n");
buffer.append("Bar Name :" + res.getString(1) + "\n");
buffer.append("Bar City :" + res.getString(2) + "\n");
buffer.append("Drink Name :" + res.getString(3) + "\n\n");
}
// Show All Data
showMessage("Data", buffer.toString());
}
});
}
public void showMessage(String title, String message) {
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setCancelable(true);
builder.setTitle(title);
builder.setMessage(message);
builder.show();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
昨天我能够使用大部分相同的代码（已经有一些变化）从虚拟设备一次创建一个DB一个条目。今天我试图使用外部创建的数据库并查看我在其中的3个结果。我知道数据库成功转移到模拟器，没有错误。只有当我点击＆＃34;查看全部＆＃34; （昨天工作正常）我收到错误
答案 0 :(得分：0)
myDB为空，您尚未将其设置为主要活动 只需替换
DBHelper myDbHelper = new DBHelper(this);
到
myDB = new DBHelper(this);
始终使用myDb对象并删除myDbHelper