ViewModel을 사용하여 데이터를 저장하기
LiveData와 Factory 패턴으로 실시간 데이터 변경

1. MyViewModel.java

public class MyViewModel extends ViewModel {

    private MutableLiveData<Reserves> reserves;
    public MutableLiveData<Reserves> getReserves() {
        if (reserves == null) {
            reserves = new MutableLiveData<Reserves>();

        }
        return reserves;
    }

}

2. MyViewModelFactory.java

public class MyViewModelFactory implements ViewModelProvider.Factory {
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        try {
            return modelClass.newInstance();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
            throw new RuntimeException("팩토리 런타임 에러");
        } catch (InstantiationException e) {
            e.printStackTrace();
            throw new RuntimeException("팩토리 런타임 에러2");
        }
    }
}

Reserves.java

3. Reserves.java

public class Reserves {
    private static Integer nowYear;
    private static Integer nowMonth;
    private static Integer nowDay;
    private static Integer nowHour;
    private static Integer nowMinute;

    private static Boolean timePicker = false;
    private static Boolean calendarPicker = false;

    public Reserves(Integer nowYear, Integer nowMonth, Integer nowDay){
        this.calendarPicker = true;
        this.nowYear = nowYear;
        this.nowMonth = nowMonth;
        this.nowDay = nowDay;
    }

    public Reserves(Integer nowHour, Integer nowMinute) {
        this.timePicker = true;
        this.nowHour = nowHour;
        this.nowMinute = nowMinute;
    }

    public void setNowHour(Integer hour){nowHour = hour;}
    public Integer getNowHour(){return nowHour;}

    public void setNowMinute(Integer minute){nowMinute = minute;}
    public Integer getNowMinute(){return nowMinute;}

    public void setNowYear(Integer year){nowYear = year;}
    public Integer getNowYear(){return nowYear;}

    public void setNowMonth(Integer month){nowMonth= month;}
    public Integer getNowMonth(){return nowMonth;}

    public void setNowDay(Integer day){nowDay = day;}
    public Integer getNowDay(){return nowDay;}

    public Boolean getCalendarPicker(){return calendarPicker;}
    public Boolean getTimePicker(){return timePicker;}


}

4. MainActivity.java

reserves = new Reserves(changeYear, changeMonth, changeDay);

            //뷰모델 객체 생성
            viewModel = new ViewModelProvider(this, new MyViewModelFactory())
                    .get(MyViewModel.class);

            //옵저버 정의 - 데이터가 변하는 이벤트 발생시 처리할 핸들러(람다)
            Observer<Reserves> PickerObserver = PickerData -> reserves = PickerData;

            //뷰모델에 옵저버 등록
            viewModel.getReserves().observe(this, PickerObserver);
            viewModel.getReserves().setValue(reserves);

데이터의 사용
ex)

viewModel.getReserves().getValue().getNowYear()
viewModel.getReserves().getValue().getNowMonth()
viewModel.getReserves().getValue().getNowDay()

참고
https://developer.android.com/topic/libraries/architecture/viewmodel?hl=ko#java
https://javachoi.tistory.com/138
https://readystory.tistory.com/173