Android：Intent setClass導致NullPointer異常

Question

我想從TcpClient使用Intent轉到PreCamera。它會導致java.lang.NullPointer異常。

Intent intent = new Intent(); 
Log.v("Pic", "context"); 
intent.setClass(TcpClient.this, PreCamera.class); 
Log.v("Pic", "ok"); 
this.mContext.startActivity(intent); 

這是TcpClient的構建體：

public TcpClient(Context context) { 
    mContext = context; 
} 

這是異常信息：

12-08 10:56:53.515: V/Pic(2579): context 
12-08 10:56:53.519: W/dalvikvm(2579): threadid=13: thread exiting with uncaught exception (group=0x40020ac0) 
12-08 10:56:53.519: E/AndroidRuntime(2579): FATAL EXCEPTION: Thread-14 
12-08 10:56:53.519: E/AndroidRuntime(2579): java.lang.NullPointerException 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at android.content.ContextWrapper.getPackageName(ContextWrapper.java:120) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at android.content.ComponentName.<init>(ComponentName.java:75) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at android.content.Intent.setClass(Intent.java:4920) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at com.Image.Collect.TcpClient.takePic(TcpClient.java:330) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at com.Image.Collect.TcpClient.commandExecute(TcpClient.java:467) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at com.Image.Collect.TcpClient.receive(TcpClient.java:278) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at com.Image.Collect.TcpClient.talk(TcpClient.java:179) 
12-08 10:56:53.519: E/AndroidRuntime(2579):  at com.Image.Collect.InfoCollect$6.run(InfoCollect.java:303) 
12-08 10:56:53.539: E/InfoCollect(2579): onPause 

這是一些AndroidMainfest.xml信息：

<activity android:name=".TcpClient" 
       ... (other info) 
     </activity> 
<activity android:name=".PreCamera" 
       ... (other info) 
     </activity> 

API是級別8.

當我試試這個：

Intent intent = new Intent(TcpClient.this, PreCamera.class); 

它會導致：

12-08 11:31:45.221: W/dalvikvm(2805): threadid=13: thread exiting with uncaught exception (group=0x40020ac0) 
12-08 11:31:45.229: E/AndroidRuntime(2805): FATAL EXCEPTION: Thread-14 
12-08 11:31:45.229: E/AndroidRuntime(2805): java.lang.NullPointerException 
12-08 11:31:45.229: E/AndroidRuntime(2805):  at android.content.ContextWrapper.getPackageName(ContextWrapper.java:120) 
12-08 11:31:45.229: E/AndroidRuntime(2805):  at android.content.ComponentName.<init>(ComponentName.java:75) 

我看到有人說這是因爲我的init mContext是太早了，應該以後onCreate（）方法來初始化或後來。但是如何在這個方法之後初始化？我是Android Dev的初學者。

TcpClient是extends Activity。我用：

mContext = this; 
Intent intent = new Intent(TcpClient.this, PreCamera.class); 

它不是很好。

Answer 1

你爲什麼不嘗試從TcpClient的活動這個代替...

Intent intent = new Intent(TcpClient.this, PreCamera.class); 


this.mContext.startActivity(intent); 

Answer 2

如果您正在啓動PreCamera活動： 試試這個

TcpClient.this.startActivity(intent); 

Answer 3

編輯：改變，因爲TcpClient的延伸活動：

簡單地說：

Intent intent = new Intent(this, PreCamera.class); 
startActivity(intent); 

這樣做會工作的你其他的方式，它只是需要更多的代碼：

Intent intent = new Intent(); 
intent.setClass(this, PreCamera.class); 
startActivity(intent);