位图创建空指针异常

Question

Here is my xml file : Image view im trying to access

<ImageView
    android:layout_width="350dip"
    android:layout_height="400dip"
    android:id="@+id/imgview"
    android:background="@drawable/pattern"
    android:layout_centerVertical="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />

Here is my code.. kindly review the code and let me know where i am doin things wrong i have searched all over the internet just cant try to figure it out how to make bitmap object and draw canvas on image view

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setContentView(new SampleView(this));
}

private static class SampleView extends View {

    private ImageView img;
    // CONSTRUCTOR
    public SampleView(Context context) {
        super(context);
        setFocusable(true);
       img = (ImageView)findViewById(R.id.imgview);
    }
    @Override
    protected void onDraw(Canvas canvas) {
        Paint paint = new Paint();

        canvas.drawColor(Color.GREEN);
  //I keep on getting null pointer exception here
        Bitmap b = Bitmap.createBitmap(img.getMeasuredWidth(),img.getMeasuredHeight(), Bitmap.Config.ARGB_8888);
        Canvas c = new Canvas(b);
        c.drawRect(0, 0, 200, 200, paint);

        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
        paint.setTextSize(40);
        paint.setTextScaleX(1.f);
        paint.setAlpha(0);
        paint.setAntiAlias(true);
        c.drawText("Your text", 30, 40, paint);
        paint.setColor(Color.RED);

        canvas.drawBitmap(b, 10,10, paint);
    }

   }
}

Answer 1

If you're getting a NullPointerException on that line, it means img is null, which either means findViewById(R.id.imgview); returned null, or onDraw is being executed as part of super(context); (not sure if that's the case).

Answer 2

When width or height you want to have in your bitmap exceeds the size of device then you may get this error. You should use createScaledBitmap . This will help you.