setContentView这个方法一定要在findViewById方法之前，不然就会出错：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

@Override
protected void onCreate(Bundle savedInstanceState) {

   super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);//this method should be in front of the findViewById

    startSensor=(Button)findViewById(R.id.startSensorButton);

    

    startSensor.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {

            
        }

    });

}