getFilesDir（）方法中的NullPointerException錯誤

Question

我嘗試在android上的內部存儲器中保存一些數據，但我不斷收到NullPointerException我認爲它與我使用的getFilesDir（）有關，但我不確定。可以一些請幫助澄清，如果是這種情況，並幫助我寫這個文件到設備。這裏的錯誤消息IM正在逐漸

01-20 22:11:59.020: E/AndroidRuntime(329): FATAL EXCEPTION: main 
01-20 22:11:59.020: E/AndroidRuntime(329): java.lang.NullPointerException 

01-20 22:11:59.020: E/AndroidRuntime(329): at android.content.ContextWrapper.getFilesDir(ContextWrapper.java:178) 

01-20 22:11:59.020: E/AndroidRuntime(329): at yantz.imageapp4.main.writeElement(main.java:89) 

01-20 22:11:59.020: E/AndroidRuntime(329): at yantz.imageapp4.Panel.onTouchEvent(Panel.java:102) 

---

這裏是主類的OnCreate

public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    FrameLayout sv = new FrameLayout(this); 
    LinearLayout ll = new LinearLayout(this); 
    test = new Panel(this); 
    test.settest(1) ; 
    ll.setOrientation(LinearLayout.VERTICAL); 
    sv.addView(test); 
    sv.addView(ll); 
    setContentView(sv);} 

---

在這裏，在面板類

public boolean onTouchEvent(MotionEvent event) { 
    mainactivity=new main();  
    mainactivity.writeElement(new Element(getResources(),(int) event.getX(),(int) event.getY())); 
     Log.v("Gesture", "is 1 "); 
     return super.onTouchEvent(event); 
} 

---

我OnTouch方法

這裏是我的主類

public void writeElement(Element obj){ 
    Log.v("main", "made it to method writeElement"); 
    File f = new File(getFilesDir()+FILENAME); 
    try { 
fos = new FileOutputStream(f); 
    ObjectOutputStream objectwrite = new ObjectOutputStream(fos); 
    objectwrite.writeObject(obj); 
fos.close(); 
Log.v("main", "file was made File "); 

}catch (FileNotFoundException e){ 
    e.printStackTrace(); 
    Log.v("main", "file was not made File not found "); 
} catch (IOException e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
    Log.v("main", "file was not made File IOException "); 
} 
} 

---

清單裏我writeObject方法

<uses-sdk android:minSdkVersion="10" /> 

<application 
    android:icon="@drawable/ic_launcher" 
    android:label="@string/app_name" > 
    <activity 
     android:label="@string/app_name" 
     android:name="yantz.imageapp4.main" > 
     <intent-filter > 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 

     <activity android:name=".charactors" android:label="@string/app_name" 
     android:theme="@android:style/Theme.Black"> 
     <intent-filter> 
    <action android:name="yantz.imageapp4.charactors" /> 
    <category android:name="android.intent.category.DEFAULT" /> 
     </intent-filter> 
    </activity> 
<activity android:name=".main2" android:label="@string/app_name" 
     android:theme="@android:style/Theme.Black"> 
     <intent-filter> 
<action android:name="yantz.imageapp4.main2" /> 
<category android:name="android.intent.category.DEFAULT" /> 
     </intent-filter> 
    </activity> 
</application> 

</manifest> 

Answer 1

我知道什麼是錯的。你不能只是寫

mainactivity=new main(); 

getFilesDir應該得到正確的上下文實例，但現在沒有。 試試這樣的：

public boolean onTouchEvent(MotionEvent event) { 
    main.writeElement(new Element(...), this.getContext); 
    return super.onTouchEvent(event); 
} 

public static void writeElement(Element obj, Context context){ 
    ... 
    File f = new File(context.getFilesDir(), FILENAME); 
    ... 
} 

Answer 2

請嘗試，

打開你的AndroidManifest.xml中，寫後以下行標籤的關閉。

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission> 
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"></uses-permission>