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Learning Java, 4th Edition

Cover of Learning Java, 4th Edition by Daniel Leuck... Published by O'Reilly Media, Inc.
  1. Learning Java
  2. Preface
    1. Who Should Read This Book
    2. New Developments
      1. New in This Edition (Java 6 and 7)
    3. Using This Book
    4. Online Resources
    5. Conventions Used in This Book
    6. Using Code Examples
    7. Safari® Books Online
    8. How to Contact Us
    9. Acknowledgments
  3. 1. A Modern Language
    1. Enter Java
      1. Java’s Origins
      2. Growing Up
    2. A Virtual Machine
    3. Java Compared with Other Languages
    4. Safety of Design
      1. Simplify, Simplify, Simplify...
      2. Type Safety and Method Binding
      3. Incremental Development
      4. Dynamic Memory Management
      5. Error Handling
      6. Threads
      7. Scalability
    5. Safety of Implementation
      1. The Verifier
      2. Class Loaders
      3. Security Managers
    6. Application and User-Level Security
    7. A Java Road Map
      1. The Past: Java 1.0–Java 1.6
      2. The Present: Java 7
      3. The Future
      4. Availability
  4. 2. A First Application
    1. Java Tools and Environment
    2. Configuring Eclipse and Creating a Project
      1. Importing the Learning Java Examples
    3. HelloJava
      1. Classes
      2. The main() Method
      3. Classes and Objects
      4. Variables and Class Types
      5. HelloComponent
      6. Inheritance
      7. The JComponent Class
      8. Relationships and Finger Pointing
      9. Package and Imports
      10. The paintComponent() Method
    4. HelloJava2: The Sequel
      1. Instance Variables
      2. Constructors
      3. Events
      4. The repaint() Method
      5. Interfaces
    5. HelloJava3: The Button Strikes!
      1. Method Overloading
      2. Components
      3. Containers
      4. Layout
      5. Subclassing and Subtypes
      6. More Events and Interfaces
      7. Color Commentary
      8. Static Members
      9. Arrays
      10. Our Color Methods
    6. HelloJava4: Netscape’s Revenge
      1. Threads
      2. The Thread Class
      3. The Runnable Interface
      4. Starting the Thread
      5. Running Code in the Thread
      6. Exceptions
      7. Synchronization
  5. 3. Tools of the Trade
    1. JDK Environment
    2. The Java VM
    3. Running Java Applications
      1. System Properties
    4. The Classpath
      1. javap
    5. The Java Compiler
    6. JAR Files
      1. File Compression
      2. The jar Utility
      3. The pack200 Utility
    7. Policy Files
      1. The Default Security Manager
      2. The policytool Utility
      3. Using a Policy File with the Default Security Manager
  6. 4. The Java Language
    1. Text Encoding
    2. Comments
      1. Javadoc Comments
    3. Types
      1. Primitive Types
      2. Reference Types
      3. A Word About Strings
    4. Statements and Expressions
      1. Statements
      2. Expressions
    5. Exceptions
      1. Exceptions and Error Classes
      2. Exception Handling
      3. Bubbling Up
      4. Stack Traces
      5. Checked and Unchecked Exceptions
      6. Throwing Exceptions
      7. try Creep
      8. The finally Clause
      9. Try with Resources
      10. Performance Issues
    6. Assertions
      1. Enabling and Disabling Assertions
      2. Using Assertions
    7. Arrays
      1. Array Types
      2. Array Creation and Initialization
      3. Using Arrays
      4. Anonymous Arrays
      5. Multidimensional Arrays
      6. Inside Arrays
  7. 5. Objects in Java
    1. Classes
      1. Accessing Fields and Methods
      2. Static Members
    2. Methods
      1. Local Variables
      2. Shadowing
      3. Static Methods
      4. Initializing Local Variables
      5. Argument Passing and References
      6. Wrappers for Primitive Types
      7. Autoboxing and Unboxing of Primitives
      8. Variable-Length Argument Lists
      9. Method Overloading
    3. Object Creation
      1. Constructors
      2. Working with Overloaded Constructors
      3. Static and Nonstatic Initializer Blocks
    4. Object Destruction
      1. Garbage Collection
      2. Finalization
      3. Weak and Soft References
    5. Enumerations
      1. Enum Values
      2. Customizing Enumerations
  8. 6. Relationships Among Classes
    1. Subclassing and Inheritance
      1. Shadowed Variables
      2. Overriding Methods
      3. Special References: this and super
      4. Casting
      5. Using Superclass Constructors
      6. Full Disclosure: Constructors and Initialization
      7. Abstract Methods and Classes
    2. Interfaces
      1. Interfaces as Callbacks
      2. Interface Variables
      3. Subinterfaces
    3. Packages and Compilation Units
      1. Compilation Units
      2. Package Names
      3. Class Visibility
      4. Importing Classes
    4. Visibility of Variables and Methods
      1. Basic Access Modifiers
      2. Subclasses and Visibility
      3. Interfaces and Visibility
    5. Arrays and the Class Hierarchy
      1. ArrayStoreException
    6. Inner Classes
      1. Inner Classes as Adapters
      2. Inner Classes Within Methods
  9. 7. Working with Objects and Classes
    1. The Object Class
      1. Equality and Equivalence
      2. Hashcodes
      3. Cloning Objects
    2. The Class Class
    3. Reflection
      1. Modifiers and Security
      2. Accessing Fields
      3. Accessing Methods
      4. Accessing Constructors
      5. What About Arrays?
      6. Accessing Generic Type Information
      7. Accessing Annotation Data
      8. Dynamic Interface Adapters
      9. What Is Reflection Good For?
    4. Annotations
      1. Using Annotations
      2. Standard Annotations
      3. The apt Tool
  10. 8. Generics
    1. Containers: Building a Better Mousetrap
      1. Can Containers Be Fixed?
    2. Enter Generics
      1. Talking About Types
    3. “There Is No Spoon”
      1. Erasure
      2. Raw Types
    4. Parameterized Type Relationships
      1. Why Isn’t a List<Date> a List<Object>?
    5. Casts
    6. Writing Generic Classes
      1. The Type Variable
      2. Subclassing Generics
      3. Exceptions and Generics
      4. Parameter Type Limitations
    7. Bounds
      1. Erasure and Bounds (Working with Legacy Code)
    8. Wildcards
      1. A Supertype of All Instantiations
      2. Bounded Wildcards
      3. Thinking Outside the Container
      4. Lower Bounds
      5. Reading, Writing, and Arithmetic
      6. <?>, <Object>, and the Raw Type
      7. Wildcard Type Relationships
    9. Generic Methods
      1. Generic Methods Introduced
      2. Type Inference from Arguments
      3. Type Inference from Assignment Context
      4. Explicit Type Invocation
      5. Wildcard Capture
      6. Wildcard Types Versus Generic Methods
    10. Arrays of Parameterized Types
      1. Using Array Types
      2. What Good Are Arrays of Generic Types?
      3. Wildcards in Array Types
    11. Case Study: The Enum Class
    12. Case Study: The sort() Method
    13. Conclusion
  11. 9. Threads
    1. Introducing Threads
      1. The Thread Class and the Runnable Interface
      2. Controlling Threads
      3. Death of a Thread
    2. Threading an Applet
      1. Issues Lurking
    3. Synchronization
      1. Serializing Access to Methods
      2. Accessing class and instance Variables from Multiple Threads
      3. The wait() and notify() Methods
      4. Passing Messages
      5. ThreadLocal Objects
    4. Scheduling and Priority
      1. Thread State
      2. Time-Slicing
      3. Priorities
      4. Yielding
    5. Thread Groups
      1. Working with ThreadGroups
      2. Uncaught Exceptions
    6. Thread Performance
      1. The Cost of Synchronization
      2. Thread Resource Consumption
    7. Concurrency Utilities
      1. Executors
      2. Locks
      3. Synchronization Constructs
      4. Atomic Operations
    8. Conclusion
  12. 10. Working with Text
    1. Text-Related APIs
    2. Strings
      1. Constructing Strings
      2. Strings from Things
      3. Comparing Strings
      4. Searching
      5. Editing
      6. String Method Summary
      7. StringBuilder and StringBuffer
    3. Internationalization
      1. The java.util.Locale Class
      2. Resource Bundles
    4. Parsing and Formatting Text
      1. Parsing Primitive Numbers
      2. Tokenizing Text
    5. Printf-Style Formatting
      1. Formatter
      2. The Format String
      3. String Conversions
      4. Primitive and Numeric Conversions
      5. Flags
      6. Miscellaneous
    6. Formatting with the java.text Package
      1. MessageFormat
    7. Regular Expressions
      1. Regex Notation
      2. The java.util.regex API
  13. 11. Core Utilities
    1. Math Utilities
      1. The java.lang.Math Class
      2. Big/Precise Numbers
      3. Floating-Point Components
      4. Random Numbers
    2. Dates and Times
      1. Working with Calendars
      2. Time Zones
      3. Parsing and Formatting with DateFormat
      4. Printf-Style Date and Time Formatting
    3. Timers
    4. Collections
      1. The Collection Interface
      2. Iterator
      3. Collection Types
      4. The Map Interface
      5. Collection Implementations
      6. Hash Codes and Key Values
      7. Synchronized and Unsynchronized Collections
      8. Read-Only and Read-Mostly Collections
      9. WeakHashMap
      10. EnumSet and EnumMap
      11. Sorting Collections
      12. A Thrilling Example
    5. Properties
      1. Loading and Storing
      2. System Properties
    6. The Preferences API
      1. Preferences for Classes
      2. Preferences Storage
      3. Change Notification
    7. The Logging API
      1. Overview
      2. Logging Levels
      3. A Simple Example
      4. Logging Setup Properties
      5. The Logger
      6. Performance
    8. Observers and Observables
  14. 12. Input/Output Facilities
    1. Streams
      1. Basic I/O
      2. Character Streams
      3. Stream Wrappers
      4. Pipes
      5. Streams from Strings and Back
      6. Implementing a Filter Stream
    2. File I/O
      1. The java.io.File Class
      2. File Streams
      3. RandomAccessFile
      4. Resource Paths
    3. The NIO File API
      1. FileSystem and Path
      2. NIO File Operations
      3. Directory Operations
      4. Watching Paths
    4. Serialization
      1. Initialization with readObject()
      2. SerialVersionUID
    5. Data Compression
      1. Archives and Compressed Data
      2. Decompressing Data
      3. Zip Archive As a Filesystem
    6. The NIO Package
      1. Asynchronous I/O
      2. Performance
      3. Mapped and Locked Files
      4. Channels
      5. Buffers
      6. Character Encoders and Decoders
      7. FileChannel
      8. Scalable I/O with NIO
  15. 13. Network Programming
    1. Sockets
      1. Clients and Servers
      2. author="pat” timestamp="20120926T110720-0500” comment="one of those sections I hate to get rid of but is less relevant in terms of the example... should probably find a more modern example...”The DateAtHost Client
      3. The TinyHttpd Server
      4. Socket Options
      5. Proxies and Firewalls
    2. Datagram Sockets
      1. author="pat” timestamp="20120926T141346-0500” comment="I actually rewrote this as a standalone client but then decided to leave it as an applet”The HeartBeat Applet
      2. InetAddress
    3. Simple Serialized Object Protocols
      1. A Simple Object-Based Server
    4. Remote Method Invocation
      1. Real-World Usage
      2. Remote and Nonremote Objects
      3. An RMI Example
      4. RMI and CORBA
    5. Scalable I/O with NIO
      1. Selectable Channels
      2. Using Select
      3. LargerHttpd
      4. Nonblocking Client-Side Operations
  16. 14. Programming for the Web
    1. Uniform Resource Locators (URLs)
    2. The URL Class
      1. Stream Data
      2. Getting the Content as an Object
      3. Managing Connections
      4. Handlers in Practice
      5. Useful Handler Frameworks
    3. Talking to Web Applications
      1. Using the GET Method
      2. Using the POST Method
      3. The HttpURLConnection
      4. SSL and Secure Web Communications
      5. URLs, URNs, and URIs
    4. Web Services
      1. XML-RPC
      2. WSDL
      3. The Tools
      4. The Weather Service Client
  17. 15. Web Applications and Web Services
    1. Web Application Technologies
      1. Page-Oriented Versus “Single Page” Applications
      2. JSPs
      3. XML and XSL
      4. Web Application Frameworks
      5. Google Web Toolkit
      6. HTML5, AJAX, and More...
    2. Java Web Applications
      1. The Servlet Lifecycle
      2. Servlets
      3. The HelloClient Servlet
      4. The Servlet Response
      5. Servlet Parameters
      6. The ShowParameters Servlet
      7. User Session Management
      8. The ShowSession Servlet
      9. The ShoppingCart Servlet
      10. Cookies
      11. The ServletContext API
      12. Asynchronous Servlets
    3. WAR Files and Deployment
      1. Configuration with web.xml and Annotations
      2. URL Pattern Mappings
      3. Deploying HelloClient
      4. Error and Index Pages
      5. Security and Authentication
      6. Protecting Resources with Roles
      7. Secure Data Transport
      8. Authenticating Users
      9. Procedural Authorization
    4. Servlet Filters
      1. A Simple Filter
      2. A Test Servlet
      3. Declaring and Mapping Filters
      4. Filtering the Servlet Request
      5. Filtering the Servlet Response
    5. Building WAR Files with Ant
      1. A Development-Oriented Directory Layout
      2. Deploying and Redeploying WARs with Ant
    6. Implementing Web Services
      1. Defining the Service
      2. Our Echo Service
      3. Using the Service
      4. Data Types
    7. Conclusion
  18. 16. Swing
    1. Components
      1. Peers and Look-and-Feel
      2. The MVC Framework
      3. Painting
      4. Enabling and Disabling Components
      5. Focus, Please
      6. Other Component Methods
      7. Layout Managers
      8. Insets
      9. Z-Ordering (Stacking Components)
      10. The revalidate() and doLayout() Methods
      11. Managing Components
      12. Listening for Components
      13. Windows, Frames and Splash Screens
      14. Other Methods for Controlling Frames
      15. Content Panes
      16. Desktop Integration
    2. Events
      1. Event Receivers and Listener Interfaces
      2. Event Sources
      3. Event Delivery
      4. Event Types
      5. The java.awt.event.InputEvent Class
      6. Mouse and Key Modifiers on InputEvents
      7. Focus Events
    3. Event Summary
      1. Adapter Classes
      2. Dummy Adapters
    4. The AWT Robot!
    5. Multithreading in Swing
  19. 17. Using Swing Components
    1. Buttons and Labels
      1. HTML Text in Buttons and Labels
    2. Checkboxes and Radio Buttons
    3. Lists and Combo Boxes
    4. The Spinner
    5. Borders
    6. Menus
    7. Pop-Up Menus
      1. Component-Managed Pop Ups
    8. The JScrollPane Class
    9. The JSplitPane Class
    10. The JTabbedPane Class
    11. Scrollbars and Sliders
    12. Dialogs
      1. File Selection Dialog
      2. The Color Chooser
  20. 18. More Swing Components
    1. Text Components
      1. The TextEntryBox Application
      2. Formatted Text
      3. Filtering Input
      4. Validating Data
      5. Say the Magic Word
      6. Sharing a Data Model
      7. HTML and RTF for Free
      8. Managing Text Yourself
    2. Focus Navigation
      1. Trees
      2. Nodes and Models
      3. Save a Tree
      4. Tree Events
      5. A Complete Example
    3. Tables
      1. A First Stab: Freeloading
      2. Round Two: Creating a Table Model
      3. Round Three: A Simple Spreadsheet
      4. Sorting and Filtering
      5. Printing JTables
    4. Desktops
    5. Pluggable Look-and-Feel
    6. Creating Custom Components
      1. Generating Events
      2. A Dial Component
      3. Model and View Separation
  21. 19. Layout Managers
    1. FlowLayout
    2. GridLayout
    3. BorderLayout
    4. BoxLayout
    5. CardLayout
    6. GridBagLayout
      1. The GridBagConstraints Class
      2. Grid Coordinates
      3. The fill Constraint
      4. Spanning Rows and Columns
      5. Weighting
      6. Anchoring
      7. Padding and Insets
      8. Relative Positioning
      9. Composite Layouts
    7. Other Layout Managers
    8. Absolute Positioning
  22. 20. Drawing with the 2D API
    1. The Big Picture
    2. The Rendering Pipeline
    3. A Quick Tour of Java 2D
      1. Filling Shapes
      2. Drawing Shape Outlines
      3. Convenience Methods
      4. Drawing Text
      5. Drawing Images
      6. The Whole Iguana
    4. Filling Shapes
      1. Solid Colors
      2. Color Gradients
      3. Textures
      4. Desktop Colors
    5. Stroking Shape Outlines
    6. Using Fonts
      1. Font Metrics
    7. Displaying Images
      1. The Image Class
      2. Image Observers
      3. Scaling and Size
    8. Drawing Techniques
      1. Double Buffering
      2. Limiting Drawing with Clipping
      3. Offscreen Drawing
    9. Printing
  23. 21. Working with Images and Other Media
    1. Loading Images
      1. ImageObserver
      2. MediaTracker
      3. ImageIcon
      4. ImageIO
    2. Producing Image Data
      1. Drawing Animations
      2. BufferedImage Anatomy
      3. Color Models
      4. Creating an Image
      5. Updating a BufferedImage
    3. Filtering Image Data
      1. How ImageProcessor Works
      2. Converting an Image to a BufferedImage
      3. Using the RescaleOp Class
      4. Using the AffineTransformOp Class
    4. Saving Image Data
    5. Simple Audio
    6. Java Media Framework
  24. 22. JavaBeans
    1. What’s a Bean?
      1. What Constitutes a Bean?
    2. The NetBeans IDE
      1. Installing and Running NetBeans
    3. Properties and Customizers
    4. Event Hookups and Adapters
      1. Taming the Juggler
      2. Molecular Motion
    5. Binding Properties
      1. Constraining Properties
    6. Building Beans
      1. The Dial Bean
      2. Design Patterns for Properties
    7. Limitations of Visual Design
    8. Serialization Versus Code Generation
    9. Customizing with BeanInfo
      1. Getting Properties Information
    10. Handcoding with Beans
      1. Bean Instantiation and Type Management
      2. Working with Serialized Beans
      3. Runtime Event Hookups with Reflection
    11. BeanContext and BeanContextServices
    12. The Java Activation Framework
    13. Enterprise JavaBeans and POJO-Based Enterprise Frameworks
  25. 23. Applets
    1. The Politics of Browser-Based Applications
    2. Applet Support and the Java Plug-in
    3. The JApplet Class
      1. Applet Lifecycle
      2. The Applet Security Sandbox
      3. Getting Applet Resources
      4. The <applet> Tag
      5. Attributes
      6. Parameters
      7. ¿Habla Applet?
      8. The Complete <applet> Tag
      9. Loading Class Files
      10. Packages
      11. appletviewer
    4. Java Web Start
    5. Conclusion
  26. 24. XML
    1. The Butler Did It
    2. A Bit of Background
      1. Text Versus Binary
      2. A Universal Parser
      3. The State of XML
      4. The XML APIs
      5. XML and Web Browsers
    3. XML Basics
      1. Attributes
      2. XML Documents
      3. Encoding
      4. Namespaces
      5. Validation
      6. HTML to XHTML
    4. SAX
      1. The SAX API
      2. Building a Model Using SAX
      3. XMLEncoder/Decoder
    5. DOM
      1. The DOM API
      2. Test-Driving DOM
      3. Generating XML with DOM
      4. JDOM
    6. XPath
      1. Nodes
      2. Predicates
      3. Functions
      4. The XPath API
      5. XMLGrep
    7. XInclude
      1. Enabling XInclude
    8. Validating Documents
      1. Using Document Validation
      2. DTDs
      3. XML Schema
      4. The Validation API
    9. JAXB Code Binding and Generation
      1. Annotating Our Model
      2. Generating a Java Model from an XML Schema
      3. Generating an XML Schema from a Java Model
    10. Transforming Documents with XSL/XSLT
      1. XSL Basics
      2. Transforming the Zoo Inventory
      3. XSLTransform
      4. XSL in the Browser
    11. Web Services
    12. The End of the Book
  27. A. The Eclipse IDE
    1. The IDE Wars
    2. Getting Started with Eclipse
      1. Importing the Learning Java Examples
    3. Using Eclipse
      1. Getting at the Source
      2. The Lay of the Land
      3. Running the Examples
      4. Building the Ant-Based Examples
      5. Loner Examples
    4. Eclipse Features
      1. Coding Shortcuts
      2. Autocorrection
      3. Refactoring
      4. Diffing Files
      5. Organizing Imports
      6. Formatting Source Code
    5. Conclusion
  28. B. BeanShell: Java Scripting
    1. Running BeanShell
    2. Java Statements and Expressions
      1. Imports
    3. BeanShell Commands
    4. Scripted Methods and Objects
      1. Scripting Interfaces and Adapters
    5. Changing the Classpath
    6. Learning More . . .
  29. Glossary
  30. Index
  31. About the Authors
  32. Colophon
  33. Copyright
O'Reilly logo

Dates and Times

Working with dates and times without the proper tools can be a chore. Fortunately, Java has three classes that handle most of the work for you. The java.util.Date class encapsulates a point in time. The java.util.GregorianCalendar class, which extends the abstract java.util.Calendar, translates between a point in time and calendar fields like month, day, and year. Finally, the java.text.DateFormat class knows how to generate and parse string representations of dates and times in many languages.[32]

The separation of the Date and Calendar classes is analogous to having a class representing temperature and a class that translates that temperature to Celsius units. A Date represents an absolute point in time as defined by a number of milliseconds from the reference point: midnight, Jan 1, 1970, GMT. This is the same frame of reference used by the System.currentTimeMillis() call. A Calendar encapsulates a point in time and maps it to higher-level (and messier) notions like years, months, weeks, and days, and deals with discontinuities like leap years. Conceivably, we could define subclasses of Calendar other than the default GregorianCalendar, say JulianCalendar or LunarCalendar, that map time using other sociological or cultural conventions.[33]

Working with Calendars

The default GregorianCalendar constructor creates a calendar initialized to the current time, in the current time zone:

    GregorianCalendar now = new GregorianCalendar();

However, more generally we can just ask the Calendar class for an appropriate calendar instance without worrying about what type of calendar system the world is using this century:

    Calendar now = Calendar.getInstance();

In either case, all the real work is done through the main set() and get() methods of Calendar. These methods use static identifiers to refer to calendar fields and values. For example:

    Calendar birthday = Calendar.getInstance();
    birthday.set( Calendar.YEAR, 1972 );
    birthday.set( Calendar.MONTH, Calendar.MAY );
    birthday.set( Calendar.DATE, 20 );

Here, we set the year, month, and day values on the calendar, altering the internal Date of the Calendar object. Any remaining fields that we did not set are left as they were initialized (to the current date and time when it was constructed). In this case, we did not really specify a full date and time; we simply overrode individual fields in the calendar.

The Calendar class contains identifiers for all of the standard date and time fields, as well as values such as days of the week and months of the year. The following are the most common identifiers:

  • YEAR, MONTH

  • WEEK_OF_YEAR, WEEK_OF_MONTH

  • DATE, DAY_OF_YEAR, DAY_OF_MONTH, DAY_OF_WEEK

  • HOUR, HOUR_OF_DAY, AM_PM

  • MINUTE, SECOND, MILLISECOND

  • ZONE_OFFSET, DST_OFFSET

DATE and DAY_OF_MONTH are synonymous. HOUR is a 12-hour clock that can be combined with AM_PM. The values are just what you would expect, as shown in the following:

  • SUNDAY, MONDAY, TUESDAY...

  • JANUARY, FEBRUARY, MARCH...

  • AM, PM

In addition to the set() method for changing field values, the Calendar class has two additional methods for performing date math, add() and roll(). Using add(), you can move a calendar forward or backward in any unit of time easily, without having to calculate the other fields. For example, we can move our calendar forward four weeks:

    Calendar cal = Calendar.getInstance();
    System.out.println( cal.getTime() );
    // Thu Nov 04 16:39:06 CST 2004
     
    cal.add( Calendar.WEEK_OF_YEAR, 4 );
    System.out.println( cal.getTime() );
    // Thu Dec 02 16:39:06 CST 2004

The roll() method, by contrast, does not alter the other fields of the calendar, but arbitrarily adjusts individual fields. See the Spinner example in Chapter 17 for additional information about adding and subtracting time periods using the add() method.

Finally, you can always get the internal Date of the Calendar object or reinitialize the calendar to a specific Date using the getTime() and setTime() method:

    // Get the absolute time the Calendar references
    Date date = calendar.getTime();
     
    // Reinitialize this calendar to the current date and time
    Date now = new Date();
    calendar.setTime( now );

Time Zones

An instance of the TimeZone class represents a time zone and the knowledge of daylight savings time at that location. You can construct a time zone from a string specifier in a number of ways. The most general approach is to use an offset from GMT, but many human-readable formats are included. (For a list, use TimeZone.getAvailableIDs().)

    TimeZone.getTimeZone("US/Central");       // CST
    TimeZone.getTimeZone("GMT-06");           // CST
    TimeZone.getTimeZone("America/Chicago");  // CST

A Calendar inherits the default time zone from the platform on which it was created. You can set a different time zone with the setTimeZone() method:

    GregorianCalendar smokey = new
    GregorianCalendar();
    smokey.setTimeZone( TimeZone.getTimeZone("US/Mountain") );

It’s important to think about dates and time zones in the right way. Remember that a Date is an absolute point in time, while a Calendar translates that Date into localized fields that may depend on where you are. In a sense, it is meaningless to talk about the date “Nov 1, 2004,” without specifying a time zone because at any given moment on earth, “now” could be one of two different calendar days. Even specifying a date and time such as “Nov 1, 2004, 9:01 pm” is ambiguous, because that particular combination of calendar and time fields occurs at 24 separate times over the span of a day as the world turns (see Figure 11-1). Only a complete date, time, and time zone specifies an absolute point in time, such as “Nov 1, 2004, 9:01 pm EST.” So it’s important to remember that the Calendar class defaults all of these fields for you even if you haven’t set them.

Calendars translate an absolute point in time to a localized data and time

Figure 11-1. Calendars translate an absolute point in time to a localized data and time

The following example prints the day of the week for the same Date object in two different time zones:

    Date date = new Date(); // point in time
     
    TimeZone CST = TimeZone.getTimeZone( "America/Chicago" );
    Calendar usa = Calendar.getInstance( CST );
    usa.setTime( date );
    System.out.println( usa.get( Calendar.DAY_OF_WEEK ) );  // 1
     
    TimeZone GMT8 = TimeZone.getTimeZone( "GMT+08"); // Beijing
    Calendar china = Calendar.getInstance( GMT8 );
    china.setTime( date );
    System.out.println( china.get( Calendar.DAY_OF_WEEK ) ); // 2

In this example, we could also have simply changed the time zone on the calendar usa using the setTimeZone() method. Unlike the field set() methods, setting the time zone does not change the underlying Date value of the calendar, only the interpretation of the fields.

The meaning of the Date object and its relationship to Calendar become particularly important when dealing with APIs for things such as databases that construct dates from incomplete date and time fields. If, as is entirely possible, you end up sending your Date object from a client application in one part of the world to a server in another, you may be surprised that the calendar fields have changed. In these situations, it’s important to work with Calendars to translate the date fields and avoid the temptation to “fix” the problem by adding or subtracting real time from the date.

Locale

It should be clear now that Calendar is not just a fancy Date, but rather is something in between a time-keeping device and a time-formatting device. This point is brought home by the fact that the Calendar class is also locale-sensitive. In addition to the notion of a time zone, a Calendar has a Locale that governs conventions such as on which day the week begins and ends. You can specify an alternate locale with the setLocale() method. Most locale-specific details, however, are handled by the DateFormat class, which we’ll discuss next.

Parsing and Formatting with DateFormat

As its name suggests, the DateFormat class formats Date objects and not Calendars, so the first step in formatting dates and times from a Calendar is to get back to a Date with the getTime() method:

    Date birthDate = calendar.getTime();

To create string representations of dates and times, create a DateFormat object and apply its format() method to a Date object. Like the NumberFormat object we looked at in the previous chapter, DateFormat itself is abstract, but it has several static (“factory”) methods that return useful DateFormat subclass instances. To get a default DateFormat, simply call getInstance():

    DateFormat simple = DateFormat.getInstance();
    String now = simple.format( new Date() );         // 4/12/06 6:06 AM

You can generate a date string or a time string, or both, using the getDateInstance(), getTimeInstance(), and getDateTimeInstance() factory methods. The argument to these methods describes what level of detail you’d like to see. DateFormat defines four constants representing detail levels: they are SHORT, MEDIUM, LONG, and FULL. There is also a DEFAULT, which is the same as MEDIUM. The following code creates three DateFormat instances: one to format a date, one to format a time, and one to format a date and time together. getDateTimeInstance() requires two arguments: the first specifies how to format the date, the second how to format the time:

    // 12-Apr-06
    DateFormat df  = DateFormat.getDateInstance(DateFormat.DEFAULT);

    // 9:18:27 AM
    DateFormat tf  = DateFormat.getTimeInstance(DateFormat.DEFAULT);

    // Wednesday, April 12, 2006 9:18:27 o'clock AM EDT
    DateFormat dtf =
      DateFormat.getDateTimeInstance( DateFormat.FULL, DateFormat.FULL );

We’re showing only how to create the DateFormat objects here. In order to actually generate a String from a date, you’ll need to call the format() method of these objects, passing a Date as an argument.

Formatting dates and times for other countries is just as easy. Overloaded factory methods accept a Locale argument:

    // 12 avr. 06
    DateFormat df =
      DateFormat.getDateInstance( DateFormat.DEFAULT, Locale.FRANCE );

    // 9:27:49
    DateFormat tf =
      DateFormat.getTimeInstance( DateFormat.DEFAULT, Locale.GERMANY );

    // mercoledi 12 aprile 2006 9.27.49 GMT-04:00
    DateFormat dtf =
        DateFormat.getDateTimeInstance(
            DateFormat.FULL, DateFormat.FULL, Locale.ITALY );

To parse a string representing a date, we use the parse() method of the DateFormat class. The result is a Date object. The parsing algorithms are finicky, so it’s safest to parse dates and times that are in the same format produced by the DateFormat. The parse() method throws a ParseException if it doesn’t understand the string you give it. All of the following calls to parse() succeed except the last; we don’t supply a time zone, but the format for the time is LONG. Other exceptions are occasionally thrown from the parse() method. To cover all the bases, catch NullPointerExceptions and StringIndexOutOfBoundsExceptions also:

    try {
      Date d;
      DateFormat df;

      df = DateFormat.getDateTimeInstance(
               DateFormat.FULL, DateFormat.FULL);
      d = df.parse("Wednesday, April 12, 2006 2:22:22 o'clock PM EDT");

      df = DateFormat.getDateTimeInstance(
               DateFormat.MEDIUM, DateFormat.MEDIUM);
      d = df.parse("12-Apr-06 2:22:22 PM");

      df = DateFormat.getDateTimeInstance(
               DateFormat.LONG, DateFormat.LONG);
      d = df.parse("April 12, 2006 2:22:22 PM EDT");

      // throws a ParseException; detail level mismatch
      d = df.parse("12-Apr-06 2:22:22 PM");
    }
    catch (Exception e) { ... }

Printf-Style Date and Time Formatting

The printf-style formatting covered in Chapter 10 can render dates and times to strings in completely arbitrary ways, without having to resort to Calendar methods to get components.

All date and time format strings use the same conversion character, t or T, followed by a suffix character that identifies the actual format or date/time component to be generated. For example, the format string %tc turns a Date argument into the string equivalent of what you get with the standard Date toString() method:

    System.out.printf( "The date is %tc\n", new Date() );
    // The date is Thu Nov 04 22:32:00 CST 2004

As with other conversion characters, the only difference between t and T is that the latter forces all of the output to uppercase. All time and date formatting is locale-sensitive, including the names of days and months and the A.M./P.M. identifier. To format a Date for another language, simply pass the Locale as the first argument:

    System.out.printf( Locale.ITALIAN, "The date is %tc\n", new Date() );
    // The date is gio nov 04 22:32:00 CST 2004

There are two additional composite, date-only formats and three composite time-only formats, as shown in the following table. The format string description in the third column of Table 11-4 refers to date and time component formats discussed in Tables 11-5 and 11-6.

Table 11-4. Composite date and time formats

Format suffix

Example

Components

c

Thu Nov 04 22:32:00 CST 2004

%ta %tb %td %tT %tZ %tY

D

11/04/04

%tm/%td/%ty

F

2004-11-04

%tY-%tm-%td

r

10:32:00 PM

%tI:%tM:%tS %Tp

R

22:32

%tH:%tM

T

22:32:00

%tH:%tM:%tS

Table 11-5 lists formats for accessing date components.

Table 11-5. Date component formats

Format suffix

Examples

Description

a

Sun, Mon, Tue...

Abbreviated day of week

A

Sunday, Monday...

Full day of week

b

Jan, Feb, Mar, ...

Abbreviated month

B

January, February, ...

Full month

Y

1999, 2004

Four-digit year

C

2004 = 20

High two digits of year

y

1999 = 99

Low two digits of year

j

001 ... 366

Day of year

m

01 ... 13

Month of year

d

01 ... 31

Day of month

e

1 ... 31

Day of month, no leading zeros

Table 11-6 lists formats for accessing time components.

Table 11-6. Time component formats

Format suffix

Examples

Description

H

00 ... 23

24-hour clock

k

0 ... 23

24-hour clock, no leading zeros

I

01 ... 12

12-hour clock

l

1 ... 12

12-hour clock, no leading zeros

M

00 ... 59

Minute

S

00 ... 60[a]

Second

L

000 ... 999

Millisecond

p

am, pm

Morning or afternoon designator

Z

CST, EST

Time zone name

z

-0600

Time zone GMT offset

[a] The second value (60) is a convention used to support leap seconds.



[32] Prior to Java 1.1, the Date class handled some of the functions of a calendar as well. Most of these methods have now been deprecated. Today, the only purpose of the Date class is to represent a point in time.

[33] Java’s GregorianCalendar class is actually both a Julian and Gregorian calendar with a programmable cut-over date. For a wealth of information about time and world time-keeping conventions, see the U.S. Navy Directorate of Time.

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