在SQLite的的onCreate错误 [英] SQLite error in onCreate
问题描述
IEM以下YouTube上的教程关于Android和code使用SQLite崩溃的开始code中的应用
iem following a tutorial on youtube about using SQLite in android and the code crashes the app on start code
MainActivity.java
MainActivity.java
package com.example.asd;
import android.app.Activity;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
public class MainActivity extends Activity {
VivzHelper vivzHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
vivzHelper = new VivzHelper(this);
SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase();
}
@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);
}
}
Message.java
Message.java
package com.example.asd;
import android.content.Context;
import android.widget.Toast;
public class Message {
public static void message(Context context, String message) {
Toast.makeText(context, message, Toast.LENGTH_LONG).show();
}
}
VivzHelper.java
VivzHelper.java
package com.example.asd;
import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class VivzHelper extends SQLiteOpenHelper {
private static final String DATABASE_NAME = "vivzdatabase";
private static final String TABLE_NAME = "VIVZTABLE";
private static final int DATABASE_VERSION = 1;
private static final String UID = "_id";
private static final String NAME = "Name";
private static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " (" + UID
+ " INTEGER PRIMARY KEY AUTOINCREMENT, " + NAME + " VARCHAR(255));";
private static final String DROP_TABLE = "DROP TABLE IF EXISTS" + TABLE_NAME;
private Context context;
public VivzHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
Message.message(context, "constructor called");
}
@Override
public void onCreate(SQLiteDatabase db) {
try {
db.execSQL(CREATE_TABLE);
Message.message(context, "onCreate called");
} catch (SQLException e) {
Message.message(context, "" + e);
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
try {
Message.message(context, "onUpgrade called");
db.execSQL(DROP_TABLE);
onCreate(db);
} catch (SQLException e) {
Message.message(context, "" + e);
}
}
}
在此应用崩溃
SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase();
错误日志
12-11 19:49:16.459: E/AndroidRuntime(6043): FATAL EXCEPTION: main
12-11 19:49:16.459: E/AndroidRuntime(6043): Process: com.example.asd, PID: 6043
12-11 19:49:16.459: E/AndroidRuntime(6043): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.asd/com.example.asd.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2695)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2769)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread.access$900(ActivityThread.java:177)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1430)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.os.Handler.dispatchMessage(Handler.java:102)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.os.Looper.loop(Looper.java:135)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread.main(ActivityThread.java:5910)
12-11 19:49:16.459: E/AndroidRuntime(6043): at java.lang.reflect.Method.invoke(Native Method)
12-11 19:49:16.459: E/AndroidRuntime(6043): at java.lang.reflect.Method.invoke(Method.java:372)
12-11 19:49:16.459: E/AndroidRuntime(6043): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
12-11 19:49:16.459: E/AndroidRuntime(6043): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)
12-11 19:49:16.459: E/AndroidRuntime(6043): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.widget.Toast.<init>(Toast.java:130)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.widget.Toast.makeText(Toast.java:427)
12-11 19:49:16.459: E/AndroidRuntime(6043): at com.example.asd.Message.message(Message.java:9)
12-11 19:49:16.459: E/AndroidRuntime(6043): at com.example.asd.VivzHelper.onCreate(VivzHelper.java:29)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:251)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
12-11 19:49:16.459: E/AndroidRuntime(6043): at com.example.asd.MainActivity.onCreate(MainActivity.java:18)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.Activity.performCreate(Activity.java:6178)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
12-11 19:49:16.459: E/AndroidRuntime(6043): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2648)
12-11 19:49:16.459: E/AndroidRuntime(6043): ... 10 more
我不知道什么是错的code，它看起来完全一样的视频???
I do not know whats wrong with the code it looks exactly the same as in video ???
推荐答案
这是什么问题？
<一个href=\"http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html#getWritableDatabase()\"相对=nofollow> getWritableDatabase 调用你的的onCreate（）
第一次这样叫，数据库将被打开，的onCreate（SQLiteDatabase），onUpgrade（SQLiteDatabase，INT，INT）和/或的OnOpen（SQLiteDatabase）将会被调用。
The first time this is called, the database will be opened and onCreate(SQLiteDatabase), onUpgrade(SQLiteDatabase, int, int) and/or onOpen(SQLiteDatabase) will be called.
在的onCreate（）你
Message.message(context, "onCreate called");
背景，在这一点上，是空，因为它被声明为在顶部作为成员变量
context, at this point, is null because it is declared as a member variable at the top as
private Context context;
，但它从未初始化。你通过上下文从活动的分贝辅助类的构造函数。初始化成员变量，背景，也给了，当你把它传递给你的变量的值信息（）功能，因此，摆脱了 NPE 。
but it is never initialized. You pass the Context from your Activity to the db helper class constructor. Initializing the member variable, context, there gives that variable a value when you pass it to your message() function, hence getting rid of the NPE.
我们怎么会在这里？
您堆栈跟踪显示
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method
'android.content.res.Resources android.content.Context.getResources()'
on a null object reference
所以，你试图用一个上下文对象做点什么，在这种情况下，是创建和显示吐司对象。 因为如果我们在堆栈跟踪到它引用您的项目中，我们发现的第一行看起来只是一个小进一步向上
So you are trying to use a Context object to "do something" which, in this case, is to create and show a Toast object. Because if we look just a little further up in the stacktrace to the first line which references your project we find
at com.example.asd.Message.message(Message.java:9)
这是此行
Toast.makeText(context, message, Toast.LENGTH_LONG).show();
这可能在这里出现问题了给定的堆栈跟踪？是的，背景是空。
这把我们带回到
在 SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase（）的应用程序崩溃;
如您在您的文章说。它崩溃那里，而不是因为一些在该行明确是错误的，但由于一些在该行调用了其他的呼救声。
as you stated in your post. It crashes there, not because something in that line is explicitly wrong, but because something in that line calls something else that is crying for help.
这篇关于在SQLite的的onCreate错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
