java.lang.runtimeException : unable to start activity componentinfo{}: java. lang.nullpointerException

Question

我创建了一个类和 Activity ，只是让用户填写表单并将其存储在数据库中，但我遇到了空指针异常。我查了很多类似的帖子，但似乎没有一个能解决我的问题。请让我知道我在这里做错了什么。

非常感谢您的时间和考虑。

这是我的 Activity :

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Splash" 
><!--  android:background="@drawable/erc"-->

<EditText
    android:id="@+id/prayerRequestEditText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:layout_marginLeft="30dp"
    android:layout_marginTop="10dp"
    android:ems="10"
    android:hint="@string/prayer_request"
    android:inputType="textMultiLine" >
    <requestFocus />
</EditText>

<EditText
    android:id="@+id/prayForEditText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/prayerRequestEditText"
    android:layout_below="@+id/prayerRequestEditText"
    android:layout_marginTop="30dp"
    android:ems="10"
    android:hint="@string/pray_for" />

<TextView
    android:id="@+id/confidentialTextView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/prayForEditText"
    android:layout_below="@+id/prayForEditText"
    android:layout_marginTop="30dp"
    android:text="@string/confidential"
    android:textAppearance="?android:attr/textAppearanceSmall" />

<Switch
    android:id="@+id/confidentialSwitch"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/confidentialTextView"
    android:layout_alignRight="@+id/prayForEditText"
    android:textOff="@string/no"
    android:textOn="@string/yes" />

<TextView
    android:id="@+id/anonymousTextView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignRight="@+id/confidentialTextView"
    android:layout_below="@+id/confidentialTextView"
    android:layout_marginTop="30dp"
    android:text="@string/anonymous"
    android:textAppearance="?android:attr/textAppearanceSmall" />

<Switch
    android:id="@+id/anonymousSwitch"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/anonymousTextView"
    android:layout_alignBottom="@+id/anonymousTextView"
    android:layout_alignRight="@+id/confidentialSwitch"
    android:textOff="@string/no"
    android:textOn="@string/yes" />

<Button
    android:id="@+id/sendRequestButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/anonymousTextView"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="30dp"
    android:onClick="sendRequest"
    android:text="@string/send_request" />

这是我的类(class):

    package com.eastridgetoday.prayerrequest;
    import android.app.Activity;
    import android.content.ContentValues;
    import android.database.sqlite.SQLiteDatabase;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.EditText;

    public class PrayerRequest extends Activity{
SQLiteDatabase db;
DbHelper dbHelper;

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);         
        setContentView(R.layout.activity_prayerrequest);
        dbHelper = new DbHelper(this);
        db = dbHelper.getWritableDatabase();        
        }

 public void sendRequest(View view){
     String prayers_request, pray_for, confidential, anonymous;

     EditText prayers_requestET = (EditText) view.findViewById(R.id.prayerRequestEditText);
     EditText pray_forET = (EditText) view.findViewById(R.id.prayForEditText);
     EditText confidentialET = (EditText) view.findViewById(R.id.confidentialSwitch);
     EditText anonymousET = (EditText) view.findViewById(R.id.anonymousSwitch);

     prayers_request = prayers_requestET.getText().toString();
     pray_for = pray_forET.getText().toString();
     confidential = confidentialET.getText().toString();
     anonymous = anonymousET.getText().toString();

     ContentValues cv = new ContentValues();
     cv.put(DbHelper.PRAYER_REQUEST, prayers_request);
     cv.put(DbHelper.PRAY_FOR, pray_for);
     cv.put(DbHelper.CONFIDENTIAL, confidential);
     cv.put(DbHelper.ANONYMOUS, anonymous);

     db.insert(DbHelper.TABLE_NAME, null, cv);
 }

}

我的启动 Activity 有一个启动 Activity ，并且启动 Activity 显示 3 秒后应该会弹出祈祷请求表。这是我的 list :

    <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.eastridgetoday.prayerrequest"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="14"
    android:targetSdkVersion="17" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.eastridgetoday.prayerrequest.Splash"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name="com.eastridgetoday.prayerrequest.PrayerRequest"
        android:label="@string/app_name" >

    </activity>
</application>

这是我的控制台:

    [2013-05-31 12:04:07 - EastridgePrayerRequest] ------------------------------
[2013-05-31 12:04:07 - EastridgePrayerRequest] Android Launch!
[2013-05-31 12:04:07 - EastridgePrayerRequest] adb is running normally.
[2013-05-31 12:04:07 - EastridgePrayerRequest] Performing     com.eastridgetoday.prayerrequest.Splash activity launch
[2013-05-31 12:04:07 - EastridgePrayerRequest] Automatic Target Mode: using existing emulator 'emulator-5554' running compatible AVD 'AVD_for_Nexus_S_by_Google'
[2013-05-31 12:04:07 - EastridgePrayerRequest] Uploading EastridgePrayerRequest.apk    onto device 'emulator-5554'
[2013-05-31 12:04:20 - EastridgePrayerRequest] Installing EastridgePrayerRequest.apk...
[2013-05-31 12:04:39 - EastridgePrayerRequest] Success!
[2013-05-31 12:04:39 - EastridgePrayerRequest] Starting activity     com.eastridgetoday.prayerrequest.Splash on device emulator-5554
[2013-05-31 12:04:43 - EastridgePrayerRequest] ActivityManager: Starting: Intent {       act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER]     cmp=com.eastridgetoday.prayerrequest/.Splash }

这是我的日志:

    05-31 01:16:45.353: E/Trace(3070): error opening trace file: No such file or directory (2)
   05-31 01:16:52.834: E/AndroidRuntime(3070): FATAL EXCEPTION: main
   05-31 01:16:52.834: E/AndroidRuntime(3070): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.eastridgetoday.prayerrequest/com.eastridgetoday.prayerrequest.PrayerRequest}: java.lang.NullPointerException
   05-31 01:16:52.834: E/AndroidRuntime(3070):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
    05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
   05-31 01:16:52.834: E/AndroidRuntime(3070):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
    05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.os.Looper.loop(Looper.java:137)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at java.lang.reflect.Method.invokeNative(Native Method)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at java.lang.reflect.Method.invoke(Method.java:511)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at dalvik.system.NativeStart.main(Native Method)
05-31 01:16:52.834: E/AndroidRuntime(3070): Caused by: java.lang.NullPointerException
05-31 01:16:52.834: E/AndroidRuntime(3070):     at com.eastridgetoday.prayerrequest.DbHelper.onCreate(DbHelper.java:35)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:252)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at com.eastridgetoday.prayerrequest.PrayerRequest.onCreate(PrayerRequest.java:20)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.Activity.performCreate(Activity.java:5104)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
05-31 01:16:52.834: E/AndroidRuntime(3070):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
05-31 01:16:52.834: E/AndroidRuntime(3070):     ... 11 more

这是我的 DbHelper 类:

    package com.eastridgetoday.prayerrequest;
    import android.content.Context;
    import android.database.sqlite.SQLiteDatabase;
    import android.database.sqlite.SQLiteOpenHelper;

    public class DbHelper extends SQLiteOpenHelper {

SQLiteDatabase db;

public static final String DATABASE_NAME = "prayersdb";
public static final String TABLE_NAME = "prayers_table";
public static final String P_ID = "_id";
public static final String PRAYER_REQUEST = "prayers";
public static final String PRAY_FOR = "pray_for";
public static final String CONFIDENTIAL = "confidential";
public static final String ANONYMOUS = "anonymous";
public static final int VERSION = 1;

public final String createTable = 
        "create table if not exists " + TABLE_NAME + " ( "
        + P_ID + " integer primary key autoincrement "
        + PRAYER_REQUEST + " text "
        + PRAY_FOR + " text "
        + CONFIDENTIAL +" text "
        + ANONYMOUS + " text);";

public DbHelper (Context context){
    super(context, DATABASE_NAME, null, VERSION);

}

@Override
public void onCreate(SQLiteDatabase arg0) {
    db.execSQL(createTable);

}

@Override
public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
    db.execSQL("drop table " + TABLE_NAME);

}

}

Answer 1

问题出在 DbHelper 类中:

@Override
public void onCreate(SQLiteDatabase arg0) {
    db.execSQL(createTable);
}

db为空，你必须初始化它。