使用SQLite数据库，并将其强制关闭用于创建数据库

Question

类：

import android.app.Activity; 
import android.database.Cursor; 
import android.database.sqlite.SQLiteDatabase; 

public class sqlite extends Activity { 
    public void sqlitetry(final SQLiteDatabase sqLiteDB) { 
     String createSql = "CREATE TABLE" + 
       "(_id integer primary key autoincrement, " + 
       "name text not null);"; 
     sqLiteDB.execSQL(createSql); 
    } 

    public void sqlinit() { 
     SQLiteDatabase db = openOrCreateDatabase("MyDatabase",MODE_PRIVATE,null); 
     db.execSQL("CREATE TABLE id note exist MyTable(username Varchar, password Varchar PRIMARY KEY);"); 
     Cursor c = db.rawQuery("Select * from MyTable", null); 
     c.moveToFirst(); 
     while(c.isAfterLast() == false) { 

     }  
    } 
} 

当我将此代码添加到我的登录和注册确认的班级并启动活动，它强制关闭：

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.login); 

    sqlite as = new sqlite(); 
    SQLiteDatabase sqLiteDB = null; 
    as.sqlitetry(sqLiteDB); 
    as.sqlinit(); 

在我的设备上安装apk后，这显示在logcat中：

07-18 09:51:39.875: D/AndroidRuntime(25677): Shutting down VM 
07-18 09:51:39.875: W/dalvikvm(25677): threadid=1: thread exiting with uncaught exception (group=0x40018578) 
07-18 09:51:39.882: E/AndroidRuntime(25677): FATAL EXCEPTION: main 
07-18 09:51:39.882: E/AndroidRuntime(25677): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.login1/com.example.login1.Login}: java.lang.NullPointerException 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.os.Looper.loop(Looper.java:130) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread.main(ActivityThread.java:3687) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at java.lang.reflect.Method.invokeNative(Native Method) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at java.lang.reflect.Method.invoke(Method.java:507) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at dalvik.system.NativeStart.main(Native Method) 
07-18 09:51:39.882: E/AndroidRuntime(25677): Caused by: java.lang.NullPointerException 
07-18 09:51:39.882: E/AndroidRuntime(25677): at com.example.login1.sqlite.sqlitetry(sqlite.java:12) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at com.example.login1.Login.onCreate(Login.java:25) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615) 
07-18 09:51:39.882: E/AndroidRuntime(25677): ... 11 more 

我需要把用户在数据库中注册后的信息并将其提取出来以便用户登录，但强制关闭，我不知道该怎么办。谁能帮我？

Answer 1

您的sqlite类扩展了Activity。你也不用在sqlite的

onCreate方法然后你有这个

sqlite as = new sqlite(); 

你需要使用startActivity(new Intent(myintent))开始活动。

建议你学习活动。

http://developer.android.com/guide/components/activities.html

还你，如果它只是一个辅助类，这

sqlite as = new sqlite(); //sqlite class extends activity 
SQLiteDatabase sqLiteDB = null; // null value 
as.sqlitetry(sqLiteDB); 
as.sqlinit(); 

你的sqlite不需要延伸Activity。

还要检查这个教程

http://www.androidhive.info/2011/11/android-sqlite-database-tutorial/

退房教程@的Youtube特拉维斯

http://www.youtube.com/watch?v=gEg9OdufXmM（系列112到119我猜）

Answer 2

基础上logcat的，你可以看到这行报告实际的错误：

07-18 09:51:39.882: E/AndroidRuntime(25677): Caused by: java.lang.NullPointerException 

下面那行，你会看到2行是执行代码的生成实际的错误堆栈：

07-18 09:51:39.882: E/AndroidRuntime(25677): at com.example.login1.sqlite.sqlitetry(sqlite.java:12) 
07-18 09:51:39.882: E/AndroidRuntime(25677): at com.example.login1.Login.onCreate(Login.java:25) 

在此基础上，你的问题只是监守空值sqLiteDB的：

SQLiteDatabase sqLiteDB = null; 
as.sqlitetry(sqLiteDB);