Java SE 8 Date and Time API

Time is a scalar quantity, so it would only need a number to express a time value.
Since you want to relate time to calendars, which are made for humans, things get more complicated.

Java date and time classes' history:

• the java.util.Date class was introduced in Java 1.0
• the java.util.Calendar class was introduced in Java 1.1
• third-party date and time libraries, such as Joda-Time, were used by developer to overcome Java standard API's issues
• the java.time API was introduced in Java 1.8 to fix the flaws in the previous versions of the platforms

1 The Time Line

The unit of time is the second, which is derived from the Earth's rotation around its axis.

Universal Time

• Earth rotation is not uniform: in 1967, a more precise definition of second was derived from the property of atoms of caesium-133 and atomic clocks were introduced to keep the official time.
• Since rotation rate of Earth varies with climate events, official time keepers synchronize absolute time with solar mean time
  • official time keepers add or remove a second to keep the Universal Time close to the mean solar time
• In UTC, a day has 24 * 60 * 60 = 86400 seconds.
  • the number of seconds in a minute is usually 60, but with an occasional leap second it may be 61.

Universal Time and Computer Systems

• Computer system keeps 86400 seconds per day and do not respect leap seconds.
• when a leap second is officially introduced, computer systems slow down or speed up before the leap second.

Java Date and Time API specification for the time scale:

• A day has 86,400 seconds
• Time scale matches the official time at noon each day

The Time Line in Java and the Instant class

• the Instant class represents a point in the time line
• The static method Instant.now() returns the current instant
• time is measured on time scale with origin set at midnigth of January 1, 1970 at Greenwich meridian
• from that origin, time is measured in 86,400 seconds per day
• Instant.MIN and Instant.MAX are one billion behind and ahead the origin

The Duration class represents the amount of time between two instants

• the static method Duration.between gives the difference between two instants

Example:

 Instant start = Instant.now();
 callMethod();
 Instant stop = Instant.now();
 Duration timeElapsed = Duration.between(start, stop);
 long millis = timeElapsed.toMillis(); 

Serializing Java Objects with JAXB

Java Model

The model class Team

package j2s.team;

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

/* 
 * As the class Team is annotated with @XmlRootElement, JAXB transforms a Team 
 * instance into an XML document whose root element is <team> 
 */
@XmlRootElement(name="team")
@XmlType(propOrder={"name","description","playerList"})
@XmlAccessorType(XmlAccessType.FIELD)
public class Team {

 // fields
 private String name;
 private String description;

 @XmlElement(name = "player")
 @XmlElementWrapper(name = "players")
 private List<Player> playerList;

 public Team(String name, String description) {
  this.name = name;
  this.description = description;
  this.playerList = new ArrayList<Player>();
 }

 // default constructor required by JAXB
 public Team() {
  this("team name", "team descr");
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 public List<Player> getPlayers() {
  return playerList;
 }

 public void setPlayers(List<Player> playerList) {
  this.playerList = playerList;
 }

 public void addPlayer(Player dev) {
  playerList.add(dev);
 }

 @Override
 public String toString() {
  return "Team [name=" + name + ", description=" + description + ", players=" + playerList + "]";
 }

}

How to Install Java In Ubuntu

Which Version of Java to Use?

• JDK 7 supports:
• eclipse version 4.4 - 4.5
• JBoss AS 7 with Java EE 6 features
• Tomcat 7-8 with WebSockets
• JDK 8 (LTS) supports:
• eclipse from eclipse Neon (4.6) to eclipse 2020-06 (4.16)
• WildFly from 8 to 13 with Java EE 7 features
• WildFly from 14 to 25 with Jakarta EE 8 features
• Tomcat 9 with Java EE 8 features
• Eclipse GlassFish 5.1.0 with Java EE 8 and Jakarta EE 8
• Eclipse Glassfish 6.0.0 with Jakarta EE 9
• JDK 11 (LTS) supports:
• eclipse IDE from Eclipse 4.17 (2020-09) to Eclipse 4.24 (2022-06)
• Eclipse Glassfish 6.1 with Jakarta EE 9.1
• JDK 17 (LTS) supports:
• eclipse IDE from eclipse 4.25 (2022-09) to eclipse 4.28 (2023-06)
• Eclipse Glassfish 6.2.2 with Jakarta EE 9.1

By march 2019, Java SE 8 will go through the End of Public Updates process, because it is a legacy release.

The java programming language is an open source project. The OpenJDK community works for a free open-source implementation of the Java SE standard and Oracle contribute to the OpenJDK project. Beginning with Java SE 11 (September 2018, LTS), Oracle provide free releases and commercially supported releases for use with Oracle products: OpenJDK (free) and Oracle JDK (commercial) builds from Oracle. The OpenJDK project is the basis for both OpenJDK and Oracle JDK builds: applications will run interchangeably on Oracle JDK-11 and OpenJDK JDK-11.