JDK Tutorials - Herong's Tutorial Examples

∟Java Date-Time API

∟Converting java.util.Date to java.time.Instant

This section provides a tutorial example how to migrate Java code from the old java.util.Date class to the new java.time.Instant class.

If you are currently using the old java.util.Date class to manipulate time measured as Epoch milliseconds, you should migrate your code to use the new java.time.Instant class.

If you are using both classes in your code, you can use two new methods provided on java.util.Date class to convert objects between classes:

The following sample program provides some comparison between java.util.Date and java.time.Instant that might be helpful to you in migrating your code: 

/* DateClassMigration.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.Date;
import java.time.Instant;
class DateClassMigration {
   public static void main(String[] a) {
      java.io.PrintStream out = System.out;
      Date oDate = new Date();
      Instant oInstant = Instant.now();

      out.println();
      out.println("Epoch milliseconds:");
      out.println("Date class: " + oDate.getTime());
      out.println("Instant class: " + oInstant.toEpochMilli());

      out.println();
      out.println("Epoch seconds:");
      out.println("Date class: " + (long) oDate.getTime()/1000);
      out.println("Instant class: " + oInstant.getEpochSecond());

      out.println();
      out.println("Nanoseconds-of-Second:");
      out.println("Date class: Not supported");
      out.println("Instant class: " + oInstant.getNano());

      out.println();
      out.println("String presentation:");
      out.println("Date class: " + oDate.toString());
      out.println("Instant class: " + oInstant.toString());

      out.println();
      out.println("Converting Date to Instant:");
      Instant cInstant = oDate.toInstant();
      out.println("Date class: " + oDate.toString());
      out.println("Instant class: " + cInstant.toString());

      out.println();
      out.println("Converting Date from Instant:");
      Date cDate = Date.from(oInstant);
      out.println("Date class: " + cDate.toString());
      out.println("Instant class: " + oInstant.toString());
   }
}

When running DateClassMigration.java, I got the following output: 

herong> java DateClassMigration.java

Epoch milliseconds:
Date class: 1708372209690
Instant class: 1708372209690

Epoch seconds:
Date class: 1708372209
Instant class: 1708372209

Nanoseconds-of-Second:
Date class: Not supported
Instant class: 690177000

String presentation:
Date class: Mon Feb 19 16:50:09 ART 2024
Instant class: 2024-02-19T19:50:09.690177Z

Converting Date to Instant:
Date class: Mon Feb 19 16:50:09 ART 2024
Instant class: 2024-02-19T19:50:09.690Z

Converting Date from Instant:
Date class: Mon Feb 19 16:50:09 ART 2024
Instant class: 2024-02-19T19:50:09.690177Z

Table of Contents

 About This JDK Tutorial Book

 JDK (Java Development Kit)

►Java Date-Time API

 What Is Date-Time API

 java.time.Instant - Representing a Moment of Time

 java.time.Instant Usage Examples

 java.time.Instant - get(INSTANT_SECONDS) Error

►Converting java.util.Date to java.time.Instant

 java.time.ZonedDateTime - Calendar and Timezone

 java.time.ZonedDateTime Usage Examples

 Converting java.util.GregorianCalendar to java.time.ZonedDateTime

 java.time.OffsetDateTime - Calendar and UTC Offset

 java.time.OffsetDateTime Usage Examples

 java.time.LocalDateTime - Local Date and time without Timezone

 Partial Date and Time Objects and Classes

 java.time.format.DateTimeFormatter - Date-Time Strings

 java.time.Duration - Time Durations

 java.time.Duration Usage Examples

 java.time.Period - Periods in Days and Months

 java.time.Period Usage Examples

 Date, Time and Calendar Classes

 Date and Time Object and String Conversion

 Number Object and Numeric String Conversion

 Locales, Localization Methods and Resource Bundles

 Calling and Importing Classes Defined in Unnamed Packages

 HashSet, Vector, HashMap and Collection Classes

 Character Set Encoding Classes and Methods

 Character Set Encoding Maps

 Encoding Conversion Programs for Encoded Text Files

 Java Logging

 Socket Network Communication

 Datagram Network Communication

 DOM (Document Object Model) - API for XML Files

 SAX (Simple API for XML)

 DTD (Document Type Definition) - XML Validation

 XSD (XML Schema Definition) - XML Validation

 XSL (Extensible Stylesheet Language)

 Message Digest Algorithm Implementations in JDK

 Private key and Public Key Pair Generation

 PKCS#8/X.509 Private/Public Encoding Standards

 Digital Signature Algorithm and Sample Program

 "keytool" Commands and "keystore" Files

 KeyStore and Certificate Classes

 Secret Key Generation and Management

 Cipher - Encryption and Decryption

 The SSL (Secure Socket Layer) Protocol

 SSL Socket Communication Testing Programs

 SSL Client Authentication

 HTTPS (Hypertext Transfer Protocol Secure)

 Outdated Tutorials

 References

 Full Version in PDF/EPUB