logcat中的空指針異常

Question

上午面臨空指針異常。在Xml中，我添加了兩個圖像和drawView在layout.By單擊圖像我想繪製該表面上的顏色，但它不工作。 請參閱此代碼並解決錯誤PLZ。

**MainActivity:** 

     public class `MainActivity` extends Activity { 



      private Paint mPaint; 
       View content; 
      @Override 
      protected void onCreate(Bundle savedInstanceState) { 
       super.onCreate(savedInstanceState); 
       setContentView(R.layout.activity_main); 

       /*layout.addView(mView, new LayoutParams( 
       LinearLayout.LayoutParams.MATCH_PARENT, 
       LinearLayout.LayoutParams.MATCH_PARENT)); */ 
       init(); 
       ImageView next = (ImageView) findViewById(R.id.imageView1); 
       next.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View view) { 
         mPaint = new Paint(); 
         mPaint.setDither(true); 

         mPaint.setStyle(Paint.Style.STROKE); 
         mPaint.setStrokeJoin(Paint.Join.ROUND); 
         mPaint.setStrokeCap(Paint.Cap.ROUND); 
         mPaint.setStrokeWidth(3); 
         mPaint.setColor(Color.GREEN); 
        } 
       }); 

       DrawView drawingSurface = (DrawView) findViewById(R.id.drawingSurface); 

       content.setDrawingCacheEnabled(true); 

      } 

      private void init() { 
       mPaint = new Paint(); 
       mPaint.setDither(true); 
       mPaint.setColor(0xFFFFFF00); 
       mPaint.setStyle(Paint.Style.STROKE); 
       mPaint.setStrokeJoin(Paint.Join.ROUND); 
       mPaint.setStrokeCap(Paint.Cap.ROUND); 
       mPaint.setStrokeWidth(3); 
      } 
     } 



**DrawView.java** 


    public class DrawView extends View { 
      private Path path; 
      private Bitmap mBitmap; 
      private Canvas mCanvas; 
      private Paint mPaint; 

      public DrawView(Context context, AttributeSet attrs) { 
      super(context,attrs); 
      path = new Path(); 
      mBitmap = Bitmap.createBitmap(820, 480, Bitmap.Config.ARGB_8888); 
      mCanvas = new Canvas(mBitmap); 
      this.setBackgroundColor(Color.BLACK); 
      } 

      private ArrayList<PathWithPaint> _graphics1 = new ArrayList<PathWithPaint>(); 

      @Override 
      public boolean onTouchEvent(MotionEvent event) { 
      PathWithPaint pp = new PathWithPaint(); 
      mCanvas.drawPath(path, mPaint); 
      if (event.getAction() == MotionEvent.ACTION_DOWN) { 
      path.moveTo(event.getX(), event.getY()); 
      path.lineTo(event.getX(), event.getY()); 
      } else if (event.getAction() == MotionEvent.ACTION_MOVE) { 
      path.lineTo(event.getX(), event.getY()); 
      pp.setPath(path); 
      pp.setmPaint(mPaint); 
      _graphics1.add(pp); 
      } 
      invalidate(); 
      return true; 
      } 

      @Override 
      protected void onDraw(Canvas canvas) { 
      super.onDraw(canvas); 
      if (_graphics1.size() > 0) { 
      canvas.drawPath( 
       _graphics1.get(_graphics1.size() - 1).getPath(), 
       _graphics1.get(_graphics1.size() - 1).getmPaint()); 
      } 
      } 
     } 

**pathwithpaint.java** 


    public class PathWithPaint { 


     private Path path; 

     public Path getPath() { 
      return path; 
     } 

     public void setPath(Path path) { 
      this.path = path; 
     } 

     private Paint mPaint; 

     public Paint getmPaint() { 
      return mPaint; 
     } 

     public void setmPaint(Paint mPaint) { 
      this.mPaint = mPaint; 
     } 

    } 


Logcat 

    Errors 

    03-19 19:08:36.997: E/AndroidRuntime(478): FATAL EXCEPTION: main 
    03-19 19:08:36.997: E/AndroidRuntime(478): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ex/com.example.ex.MainActivity}: java.lang.NullPointerException 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread.access$2300(ActivityThread.java:125) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.os.Handler.dispatchMessage(Handler.java:99) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.os.Looper.loop(Looper.java:123) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread.main(ActivityThread.java:4627) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at java.lang.reflect.Method.invokeNative(Native Method) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at java.lang.reflect.Method.invoke(Method.java:521) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at dalvik.system.NativeStart.main(Native Method) 
    03-19 19:08:36.997: E/AndroidRuntime(478): Caused by: java.lang.NullPointerException 
    03-19 19:08:36.997: E/AndroidRuntime(478): at com.example.ex.MainActivity.onCreate(MainActivity.java:48) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
    03-19 19:08:36.997: E/AndroidRuntime(478): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627) 
    03-19 19:08:36.997: E/AndroidRuntime(478): ... 11 more 


Xml File 


<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:id="@+id/myDrawing" 
    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" > 

    <com.example.ex.DrawView 
      android:id="@+id/drawingSurface" 
      android:layout_height="fill_parent" 
      android:layout_width="fill_parent" 
      android:layout_marginTop="11dp" 
      android:layout_marginLeft="12dp" 
      android:layout_marginBottom="67dp" 
      android:layout_marginRight="16dp"/> 


    <ImageView 
     android:id="@+id/imageView1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:src="@drawable/ic_launcher" /> 

     <ImageView 
     android:id="@+id/imageView2" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:src="@drawable/ic_launcher" /> 

</LinearLayout> 

Answer 1

查看content活動未初始化。所以當您嘗試訪問時

content.setDrawingCacheEnabled(true); 

它將爲空。