Android-日付類の使用

package me.chunyu.Pedometer.manager;

import android.util.Pair;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;

import me.chunyu.Pedometer.database.counter.DailyData;

/**
 *      ,        "yyyy-MM-dd"      ,      .
 *     ,              ,         0.
 *    ,          .
 * 
 * Created by wangchenlong on 15/8/17.
 */
public class DateManager {

    private static DateManager sInstance; //   
    private final RecordManager mRecordManager; //      
    private final SimpleDateFormat sFormatter; //    

    private DateManager() {
        mRecordManager = RecordManager.getInstance();
        sFormatter = new SimpleDateFormat("yyyy-MM-dd", Locale.ENGLISH);
    }

    public static DateManager getInstance() {
        if (sInstance == null) {
            sInstance = new DateManager();
        }
        return sInstance;
    }

    /**
     *  RecordManager    ,        ,             .
     *
     * @return     [    ,  ,     ]
     */
    public ArrayList> getDataDates() {
        String[] days = mRecordManager.getSortedFilenames();
        if (days.length <= 0) {
            return null;
        }

        String firstDay = days[0];
        String lastDay = days[days.length - 1];

        if (isAfterToday(lastDay)) {
            lastDay = sFormatter.format(new Date());
        }

        long n = daysBetweenTwoDates(firstDay, lastDay) + 1;

        ArrayList allSteps = new ArrayList<>();
        ArrayList allDays = new ArrayList<>();

        for (int i = 0; i < n; ++i) {
            String perDay = getBeforeDay(lastDay, i);
            DailyData dailyData = mRecordManager.getDailySteps(perDay);
            if (dailyData != null) {
                int[] steps = dailyData.steps;
                int maxStep = steps[steps.length - 1];
                allSteps.add(maxStep);
            } else {
                allSteps.add(0);
            }
            allDays.add(perDay);
        }

        ArrayList> totalData = new ArrayList<>();
        for (int i = 0; i < allSteps.size(); ++i) {
            Pair dateWithStep = Pair.create(allDays.get(i), allSteps.get(i));
            totalData.add(dateWithStep);
        }

        return totalData;
    }

    /**
     *            
     *
     * @param earlyDay      
     * @param lateDay       
     * @return     
     */
    public long daysBetweenTwoDates(String earlyDay, String lateDay) {
        Date earlyDate = getDateFromStr(earlyDay);
        Date lateDate = getDateFromStr(lateDay);

        Calendar startCalendar = Calendar.getInstance();
        Calendar lateCalendar = Calendar.getInstance();
        startCalendar.setTime(earlyDate);
        lateCalendar.setTime(lateDate);

        long diff = lateCalendar.getTimeInMillis() - startCalendar.getTimeInMillis();

        return diff / (24 * 60 * 60 * 1000);
    }

    /**
     *          Date 
     *
     * @param day      
     * @return   Data 
     */
    public Date getDateFromStr(String day) {
        Date date;
        try {
            date = sFormatter.parse(day);
        } catch (ParseException e) {
            e.printStackTrace();
            return null;
        }
        return date;
    }

    /**
     *          
     *
     * @param day     
     * @param i       
     * @return       
     */
    public String getBeforeDay(String day, int i) {
        Date date = getDateFromStr(day);
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.DAY_OF_YEAR, i * (-1));
        return sFormatter.format(calendar.getTime());
    }

    /**
     *           
     *
     * @param day   
     * @return   
     */
    public boolean isAfterToday(String day) {
        Date lateDate = getDateFromStr(day);
        Date todayDate = new Date();
        return lateDate.getTime() > todayDate.getTime();
    }
}