添加button和onclicklistener后Att…

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 'voidandroid.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) {
            
        }
    });

}