In Java 8 added some new methods with Legacy java.util Date and Time classes to convert object of java.time classes to get advantage new java 8 Date and Time classes with minimal changes.

Points To Remember:

• java.util.Date and java.time.Instance classes are similar. The Date.from(Instant) and Date.toInstant() methods allow conversion between these classes. It represents instantaneous point of time in UTC which is independent of TimeZone  as epoch-seconds (since 1970-01-01T00:00:00Z) plus nanoseconds.
• The java.time.ZonedDateTime class is the replacement for java.util.GregorianCalendar. The GregorianCalendar.from(ZonedDateTime) and GregorianCalendar.to(ZonedDateTime) methods faciliate conversions between these classes. It present time as follows:
  • LocalDate : year, month and day
  • LocalTime: hour, minute, second, nanoseconds
  • ZoneId: time zone
  • ZoneOffset: current offset from GMT
• The ZoneOffset class specifies only an offset from Greenwich/UTC. The ZoneId class specifies a time zone identifier and has access to the rules used each time zone.
• GeregorianCalendar with date as 1970-01-01 will use date component can be replaced with an instance of LocalDate.
• GeregorianCalendar with time as 00:00 will use time component can be replaced with an instance of LocalTime.

java.util.Date to an java.time.Instant

Date date=new Date();
Instant instant=date.toInstant();

An java.time.Instant to java.util.Date

Date date=Date.from(instant);

java.util.Calendar to an java.time.Instant

Calendar cal=Calendar.getInstance();
Instant instant=cal.toInstant();

java.util.GregorianCalendar instance to an java.time.ZoneDateTime

Calendar gCal=GregorianCalendar.getInstance();
ZoneDateTime zoneDateTime=ZoneDateTime.ofInstant(gCal.toInstant(),ZoneId.systemDefault());

java.time.ZoneDateTime instance to java.util.GregorianCalendar

GregorianCalendar gCal=GregorianCalendar.from(zoneDateTime);

java.util.TimeZone object to java.time.ZoneId

Calendar gCal1=GregorianCalendar.getInstance();
TimeZone tz = cal.getTimeZone();
ZoneId zoneID=tz.toZoneId();
int tzoffset = cal.get(Calendar.ZONE_OFFSET);

Example

In below example considered all the cases of Legacy to Java 8 object classes.

package com.date;

import java.text.SimpleDateFormat;
import java.time.Instant;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.TimeZone;

public class Legacy2Java8DateTimeInteroprability {

public static void main(String[] args) {
SimpleDateFormat utilDateTimeFormat=new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
DateTimeFormatter java8DateTimeFormat=DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss").withZone(ZoneId.systemDefault());;

//Date to Instant
Date date=new Date();
Instant inst = date.toInstant();
System.out.println("Initial Date :"+utilDateTimeFormat.format(date));
System.out.println("Converted Instant :"+java8DateTimeFormat.format(inst));

//Instant To Date
Date newDate = Date.from(inst);
System.out.println("Converted Date :"+utilDateTimeFormat.format(date));

//Calendar to ZoneDateTime
Calendar cal = Calendar.getInstance();
ZonedDateTime zdt = ZonedDateTime.ofInstant(cal.toInstant(), ZoneId.systemDefault());
System.out.println("Calendar to ZoneDateTime :"+java8DateTimeFormat.format(zdt));

//Calendar to Instant
Instant inst1 = cal.toInstant();
System.out.println("Calendar to Instant :"+java8DateTimeFormat.format(inst1));

//Geregorian calendar from Zone Date Time
GregorianCalendar gcal = GregorianCalendar.from(zdt);
System.out.println("Gregorian Calendar Date Time:"+utilDateTimeFormat.format(gcal.getTime()));
ZonedDateTime zdt1 = gcal.toZonedDateTime();
System.out.println("Calendar to ZoneDateTime Date :"+java8DateTimeFormat.format(zdt1));

LocalDateTime ldt = zdt1.toLocalDateTime();
System.out.println("Local Date Time :"+java8DateTimeFormat.format(ldt));
LocalDate localDate = zdt1.toLocalDate();
System.out.println("Local Date :"+java8DateTimeFormat.format(ldt));
LocalTime localTime = zdt1.toLocalTime();
System.out.println("Local Time :"+java8DateTimeFormat.format(ldt));

//TimeZone to Zone Id
Calendar gCal1=GregorianCalendar.getInstance();
TimeZone tz = cal.getTimeZone();
System.out.println("Time Zone :"+tz.getDisplayName());
ZoneId zoneID=tz.toZoneId();
System.out.println("Zone ID:"+zoneID.getId());
int tzoffset = cal.get(Calendar.ZONE_OFFSET);
<span 				data-mce-type="bookmark" 				id="mce_SELREST_start" 				data-mce-style="overflow:hidden;line-height:0" 				style="overflow:hidden;line-height:0" 			>&#65279;</span>System.out.println("Zone Offset:"+tzoffset);
	}
}

Output

Initial Date :2018/08/04 07:48:45
Converted Instant :2018/08/04 07:48:45
Converted Date :2018/08/04 07:48:45
Calendar to ZoneDateTime :2018/08/04 07:48:45
Calendar to Instant :2018/08/04 07:48:45
Gregorian Calendar Date Time:2018/08/04 07:48:45
Calendar to ZoneDateTime Date :2018/08/04 07:48:45
Local Date Time :2018/08/04 07:48:45
Local Date :2018/08/04 07:48:45
Local Time :2018/08/04 07:48:45
Time Zone :Pacific Standard Time
Zone ID:America/Los_Angeles
Zone Offset:-28800000