Question

在创建新的Intent对象时，我总是会遇到Nullpointer异常。我的目的是创建一个新的Activity，它以“onCreate”方法打开一个新的View。

这是我的主要活动：

public class MainActivity extends Activity {
    public void activateTimer(){
        // some code ....           
        MyTimer timer = new MyTimer(MainActivity.this);
    }
}

这是我的Timer类：

public class MyTimer extends MainActivity{
    private Context contextMain;
    public MyTimer(Context context){
        this.contextMain = context;
        // some more code ...
        openMyActivity(contextMain); 
}
public void openMyActivity(Context contextMain){
    // some code ...
    // The next line throws the NPE!
Intent intent = new Intent(contextMain, MyNotificationActivity.class);
this.startActivity(intent);
    }
}

应该创建我的活动：

public class MyNotificationActivity extends Activity {  
    @Override
    public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.notification);
 } 
}

以下是错误日志：

03-21 12:47:51.949: W/dalvikvm(2442): threadid=11: thread exiting with uncaught exception (group=0x41465700)
03-21 12:47:51.959: E/AndroidRuntime(2442): FATAL EXCEPTION: Timer-0
03-21 12:47:51.959: E/AndroidRuntime(2442): java.lang.NullPointerException
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivityForResult(Activity.java:3390)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivityForResult(Activity.java:3351)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivity(Activity.java:3587)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivity(Activity.java:3555)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer.openNotificationInMain(MyTimer.java:135)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer.prompt(MyTimer.java:80)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer$1.run(MyTimer.java:38)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at java.util.Timer$TimerImpl.run(Timer.java:284)

这是我的Manifest.xml文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.myApp"
    android:versionCode="1"
    android:versionName="1.0" >
<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="18" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.myApp.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="com.myApp.MyNotifikationActivity"
              android:label="@string/app_name"
              android:parentActivityName="com.myApp.MainActivity"  >
        <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
        <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="com.myApp.MainActivity" />            
    </activity>         
</application>
</manifest>

Answer 1

您的MyTimer延伸Activity，但在此您自己实例化了这些内容：

MyTimer timer = new MyTimer(MainActivity.this)

现在，当您执行此操作时，活动未正确初始化以用作活动或上下文，并且可能出现各种类型的故障。

似乎您应该完全从extends Activity中删除MyTimer，并将Activity或Context参数传递给需要它的方法。例如，

public void openMyActivity(Activity act) {
  // ...
  act.startActivityForResult(...);

Answer 2

为什么要将Mytimer扩展为MainActivity？我认为不需要那个

将其更改为....

public class MyTimer{
private Context contextMain;
   public MyTimer(Context context){
    this.contextMain = context;
    // some more code ...
    openMyActivity(contextMain); 
  }
 public void openMyActivity(Context contextMain){
// some code ...
// The next line throws the NPE!
Intent intent = new Intent(contextMain, MyNotificationActivity.class);
this.startActivity(intent);
}

}

Android：NullPointerException创建新的Intent

2 个答案: