我是Android新手，在这里我希望在点击按钮时向数据库添加一些数据，但是当我这样做时，应用程序崩溃并且日志cat返回“无法实例化活动，空指针异常” 。
我出了什么问题？
MainActivity.java
package com.example.lockerbox;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
public class MainActivity extends Activity {
final Context context=this;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@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;
}
public void faq(View view)
{
Intent intent2 = new Intent(context,faq.class);
intent2.setAction("com.example.lockerbox.faq");
startActivity(intent2);
}
public void question(View arg0)
{
Intent intent1 = new Intent(context,questions.class);
intent1.setAction("com.example.lockerbox.questions");
startActivity(intent1);
}
}
questions.java
import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
public class questions extends Activity {
String questiontext=findViewById(R.id.qtext).toString();
String answertext=findViewById(R.id.atext).toString();
Button button;
final Context context=this;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.questionplace);
}
public void Done(View view) //when i remove this the app works by showing me the
//graphic layout defined in questionplace.xml
{
DatabaseClass db=new DatabaseClass(this);
// Inserting Questions and Answers
Log.d("Insert: ", "Inserting ..");
db.addQuestionAnswer(questiontext,answertext);
}
}
DatabaseClass.java
package com.example.lockerbox;
import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DatabaseClass extends SQLiteOpenHelper{
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "LockerBox_DB";
// Contacts table name
private static final String TABLE_LOCKERBOX="LockerBox_T";
// Contacts Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_Q = "question";
private static final String KEY_A = "answer";
public DatabaseClass(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
//Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
String CREATE_QUERY = "CREATE TABLE " + TABLE_LOCKERBOX + "("
+ KEY_ID + " INTEGER PRIMARY KEY," + KEY_Q + " TEXT,"
+ KEY_A + " TEXT" + ")";
db.execSQL(CREATE_QUERY);
}
// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older table if existed
db.execSQL("DROP TABLE IF EXISTS " + TABLE_LOCKERBOX);
// Create tables again
onCreate(db);
}
// Adding new question and answer
public void addQuestionAnswer(String question,String answer){
SQLiteDatabase db = this.getWritableDatabase();
ContentValues value=new ContentValues();
value.put(KEY_Q,question);
value.put(KEY_A,answer);
db.insert(TABLE_LOCKERBOX,null,value);
db.close();
}
}
questionplace.xml
<LinearLayout 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:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin" >
<EditText
android:id="@+id/qtext"
android:layout_width="285dp"
android:layout_height="wrap_content"
android:ems="10"
android:hint="@string/Question_Here"
android:inputType="textMultiLine" />
<EditText
android:id="@+id/atext"
android:layout_width="285dp"
android:layout_height="wrap_content"
android:ems="10"
android:hint="@string/Answer_Here"
android:inputType="textPassword"
/>
<Button
android:id="@+id/Done_Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/QFinish"
android:onClick="Done"
/>
<Button
android:id="@+id/AnotherQ"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/Another_Question"
android:onClick="AnotherQ" />
</LinearLayout>
的AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.lockerbox"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="19" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.lockerbox.MainActivity"
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=".questions"
android:label="@string/app_name"
/>
<activity
android:name=".faq"
android:label="@string/app_name"
/>
</application>
</manifest>
创建日志
02-11 10:55:19.861: I/Choreographer(1326): Skipped 72 frames! The application may be doing too much work on its main thread.
02-11 10:55:36.451: D/gralloc_goldfish(1369): Emulator without GPU emulation detected.
02-11 10:55:38.281: I/Choreographer(1369): Skipped 52 frames! The application may be doing too much work on its main thread.
02-11 10:55:38.591: D/AndroidRuntime(1369): Shutting down VM
02-11 10:55:38.591: W/dalvikvm(1369): threadid=1: thread exiting with uncaught exception (group=0xaf683b90)
02-11 10:55:38.711: E/AndroidRuntime(1369): FATAL EXCEPTION: main
02-11 10:55:38.711: E/AndroidRuntime(1369): Process: com.example.lockerbox, PID: 1369
02-11 10:55:38.711: E/AndroidRuntime(1369): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.lockerbox/com.example.lockerbox.questions}: java.lang.NullPointerException
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2102)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread.access$700(ActivityThread.java:135)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.os.Handler.dispatchMessage(Handler.java:102)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.os.Looper.loop(Looper.java:137)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread.main(ActivityThread.java:4998)
02-11 10:55:38.711: E/AndroidRuntime(1369): at java.lang.reflect.Method.invokeNative(Native Method)
02-11 10:55:38.711: E/AndroidRuntime(1369): at java.lang.reflect.Method.invoke(Method.java:515)
02-11 10:55:38.711: E/AndroidRuntime(1369): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
02-11 10:55:38.711: E/AndroidRuntime(1369): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
02-11 10:55:38.711: E/AndroidRuntime(1369): at dalvik.system.NativeStart.main(Native Method)
02-11 10:55:38.711: E/AndroidRuntime(1369): Caused by: java.lang.NullPointerException
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.Activity.findViewById(Activity.java:1883)
02-11 10:55:38.711: E/AndroidRuntime(1369): at com.example.lockerbox.questions.<init>(questions.java:13)
02-11 10:55:38.711: E/AndroidRuntime(1369): at java.lang.Class.newInstanceImpl(Native Method)
02-11 10:55:38.711: E/AndroidRuntime(1369): at java.lang.Class.newInstance(Class.java:1208)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
02-11 10:55:38.711: E/AndroidRuntime(1369): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2093)
02-11 10:55:38.711: E/AndroidRuntime(1369): ... 11 more
02-11 10:55:42.551: I/Process(1369): Sending signal. PID: 1369 SIG: 9
02-11 10:55:46.261: D/gralloc_goldfish(1403): Emulator without GPU emulation detected.
02-11 11:00:48.971: I/Choreographer(1403): Skipped 113 frames! The application may be doing too much work on its main thread.
我想要的是当我点击/点击按钮“完成”时应该在表格中输入一个条目，但我相信我在questions.java中处理按钮“完成”以执行查询的方式是错误的。
答案 0 :(得分：2)
初始化上下文如下
final Context context;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
context =this;
以下内容应位于onCreate之后的setContentView。
String questiontext=findViewById(R.id.qtext).toString();
String answertext=findViewById(R.id.atext).toString();