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我試圖製作一個簡單的應用程序，可以將用戶輸入的信息從android手機存儲到數據庫中。我的代碼如下：空指針從應用程序創建SQLite數據庫時出現異常
import android.database.sqlite.SQLiteOpenHelper;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import java.io.IOException;
public class MainActivity extends AppCompatActivity {
EditText userentry;
TextView seeuserdata;
DataBaseHelper DataBaseHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
userentry = (EditText) findViewById(R.id.userentry);
seeuserdata = (TextView) findViewById(R.id.seeUserData);
DataBaseHelper db = new DataBaseHelper(this, null, null, 1);
printDatabase();
}
public void addButtonClicked(View view){
Products product = new Products(userentry.getText().toString());
}
public void deleteButtonClicked(View view){
String inputText = userentry.getText().toString();
DataBaseHelper.deleteProduct(inputText);
printDatabase();
}
public void printDatabase(){
String dbString = DataBaseHelper.databaseToString();
seeuserdata.setText(dbString);
userentry.setText("");
}
};
產品類
public class Products {
private int _id;
private String _productname;
public Products(String _productname){
this._productname=_productname;
}
public void set_id(int _id) {
this._id = _id;
}
public void set_productname(String _productname) {
this._productname = _productname;
}
public int get_id() {
return _id;
}
public String get_productname() {
return _productname;
}
}
和類DataBaseHelper
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DataBaseHelper extends SQLiteOpenHelper {
private static String DB_PATH = "/data/data/your.package/databases/";
private static String DB_NAME = "products.db";
private static int DB_VERSION = 1;
private static final String TABLE_PRODUCTS = "products";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_PRODUCTNAME = "productname";
private static short WRITE_BUFFER_SIZE = 8192;
private Context context;
public DataBaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
super(context, name, factory, version);
}
public void addProduct(Products product){
ContentValues values = new ContentValues();
values.put(COLUMN_PRODUCTNAME, product.get_productname());
SQLiteDatabase db = getWritableDatabase();
db.insert(TABLE_PRODUCTS, null, values);
db.close();
}
public void deleteProduct(String productName){
SQLiteDatabase db = getWritableDatabase();
db.execSQL("DELETE FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + "=\"" + productName + "\";");
}
public String databaseToString(){
String dbString = "";
SQLiteDatabase db = getWritableDatabase();
String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";
Cursor c = db.rawQuery(query, null);
c.moveToFirst();
while(!c.isAfterLast()){
if(c.getString(c.getColumnIndex("productname"))!=null){
dbString += c.getString(c.getColumnIndex("productname"));
dbString += "\n";
}
}
db.close();
return dbString;
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP_TABLE_IF_EXISTS" + TABLE_PRODUCTS);
onCreate(db);
}
@Override
public void onCreate(SQLiteDatabase db) {
String query = "CREATE TABLE" + TABLE_PRODUCTS + "(" + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT "+ COLUMN_PRODUCTNAME + " TEXT " + ");";
db.execSQL(query);
}
}
的錯誤是：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.barry.recordtracker, PID: 11938
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.barry.recordtracker/com.example.barry.recordtracker.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.barry.recordtracker.DataBaseHelper.databaseToString()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3253)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3349)
at android.app.ActivityThread.access$1100(ActivityThread.java:221)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7224)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.barry.recordtracker.DataBaseHelper.databaseToString()' on a null object reference
at com.example.barry.recordtracker.MainActivity.printDatabase(MainActivity.java:40)
at com.example.barry.recordtracker.MainActivity.onCreate(MainActivity.java:26)
at android.app.Activity.performCreate(Activity.java:6876)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1135)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3206)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3349)
at android.app.ActivityThread.access$1100(ActivityThread.java:221)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7224)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
我的理解是，空對象的日誌是重新ferring to是在onCreate方法中初始化的數據庫幫助器。它在我看來，數據庫幫助器創建！它不是指向數據庫幫助器嗎？我用（this，null，null，1）初始化DB助手是否有問題？ （這意味着它應該用別的東西來初始化？）
作品，現在到其他錯誤... – bdpolinsky
@bdpolinsky很高興我可以幫助和祝你好運快樂編碼:) –