FindViewbyID空指针问题

Question

先看类：

public class MainActivity extends Activity {

    Canvas canvas;
    public int barometerRotationAngle=0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,  WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.activity_main);
        Resources res = getResources();   
         canvas=new Canvas();

        final Altimeter altimeter=(Altimeter) this.findViewById(R.id.altimeter);
        final Button buttonup=(Button) this.findViewById(R.id.buttonup);

        buttonup.setOnClickListener(new OnClickListener()
        {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                barometerRotationAngle+=10;
                altimeter.setBarometerRotationAngle(barometerRotationAngle);

            }

        });

    }

        }

    }

程序在 altimeter.setBarometerRotationAngle(barometerRotationAngle);这里出现了空指针错误提示，但是alimeter类已经成功显示了图片，也就是说，他的onDraw（）方法已被调用了。为什么在Onclick里面会变成了空指针？

Answer 1

不知道你的图片怎么显示的，你先确认
final Altimeter altimeter=(Altimeter) this.findViewById(R.id.altimeter);是否找到了对象