android双击返回键退出程序

public void onBackPressed() {
Calendar cal = Calendar.getInstance();
if(Math.abs(date-cal.getTimeInMillis())>500){
date  = cal.getTimeInMillis();
Toast.makeText(this, "双击返回键退出", Toast.LENGTH_SHORT).show();
//return;
}else {
super.onBackPressed();
}
}