You are previewing Learning Java, 4th Edition.

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
      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 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

Printf-Style Formatting

A standard feature that Java adopted from the C language is printf-style string formatting. printf-style formatting utilizes special format strings embedded into text to tell the formatting engine where to place arguments and give detailed specification about conversions, layout, and alignment. The printf formatting methods also make use of variable-length argument lists, which makes working with them much easier. Here is a quick example of printf-formatted output:

    System.out.printf( "My name is %s and I am %d years old\n", name, age );

The printf formatting draws its name from the C language printf() function, so if you’ve done any C programming, this will look familiar. Java has extended the concept, adding some additional type safety and convenience features. Although Java has had some text formatting capabilities in the past (we’ll discuss the java.text package and MessageFormat later), printf formatting was not really feasible until variable-length argument lists and autoboxing of primitive types were added in Java 5.0. (We mention this to explain why these similar APIs both exist in Java.)


The primary new tool in our text formatting arsenal is the java.util.Formatter class and its format() method. Several convenience methods can hide the Formatter object from you and you may not need to create a Formatter directly. First, the static String.format() method can be used to format a String with arguments (like the C language sprintf() method):

    String message =
        String.format("My name is %s and I am %d years old.", name, age );

Next, the and classes, which are used for writing text to streams, have their own format() method. We discuss streams in Chapter 12, but this simply means that you can use this same printf-style formatting for writing strings to any kind of stream, whether it be to System.out standard console output, to a file, or to a network connection.

In addition to the format() method, PrintStream and PrintWriter also have a version of the format method that is actually called printf(). The printf() method is identical to and, in fact, simply delegates to the format() method. It’s there solely as a shout-out to the C programmers and ex-C programmers in the audience.

The Format String

The syntax of the format string is compact and a bit cryptic at first, but not bad once you get used to it. The simplest format string is just a percent sign (%) followed by a conversion character. For example, the following text has two embedded format strings:

    "My name is %s and I am %d years old."

The first conversion character is s, the most general format, which represents a string value; and the second is d, which represents an integer value. There are about a dozen basic conversion characters corresponding to different types and primitives and there are a couple of dozen more that are specifically used for formatting dates and times. We cover the basics here and return to date and time formatting in Chapter 11.

At first glance, some of the conversion characters may not seem to do much. For example, the %s general string conversion in our previous example would actually have handled the job of displaying the numeric age argument just as well as %d. However, these specialized conversion characters accomplish three things. First, they add a level of type safety. By specifying %d, we ensure that only an integer type is formatted at that location. If we make a mistake in the arguments, we get a runtime IllegalFormatConversionException instead of garbage in our string (and your IDE may flag it as well). Second, the format method is Locale-sensitive and capable of displaying numbers, percentages, dates, and times in many different languages just by specifying a Locale as an argument. By telling the Formatter the type of argument with type-specific conversion characters, printf can take into account language-specific localizations. Third, additional flags and fields can be used to govern layout with different meanings for different types of arguments. For example, with floating-point numbers, you can specify a precision in the format string.

The general layout of the embedded format string is as follows:


Following the literal % are a number of optional items before the conversion type character. We’ll discuss these as they come up, but here’s the rundown. The argument index can be used to reorder or reuse individual arguments in the variable-length argument list by referring to them by number. The flags field holds one or more special flag characters governing the format. The width and precision fields control the size of the output for text and the number of digits displayed for floating-point numbers.

String Conversions

The conversion characters s represents the general string conversion type. Ultimately, all of the conversion types produce a String. What we mean is that the general string conversion takes the easy route to turning its argument into a string. Normally, this simply means calling toString() on the object. Since all of the arguments in the variable argument list are autoboxed, they are all Objects. Any primitives are represented by the results of calling toString() on their wrapper classes, which generally return the value as you’d expect. If the argument is null, the result is the String “null.”

More interesting are objects that implement the java.util.Formattable interface. For these, the argument’s formatTo() method is invoked, passing it the flags, width, and precision information and allowing it to return the string to be used. In this way, objects can control their own printf string representation, just as an object can do so using toString().

Width, precision, and justification

For simple text arguments, you can think of the width and precision as a minimum and maximum number of characters to be output. As we’ll see later, for floating-point numeric types, the precision changes meaning slightly and controls the number of digits displayed after the decimal point. We can see the effect on a simple string here:

    System.out.printf("String is '%5s'\n", "A");
    // String is '    A'
    System.out.printf("String is '%.5s'\n", "Happy Birthday!");
    // String is 'Happy'

In the first case, we specified a width of five characters, resulting in spaces being added to pad our argument. In the second example, we used the literal . followed by the precision value of 5 characters to limit the length of the string displayed, so our “Happy Birthday” string is truncated after the first five characters.

When our string was padded, it was right-justified (leading spaces added). You can control this with the flag character literal minus (-). Reversing our example:

    System.out.printf("String is '%-5s'\n", "A");
    // String is 'A    '

And, of course, we can combine all three, specifying a justification flag and a minimum and maximum width. Here is an example that prints words of varying lengths in two columns:

    String [] words =
       new String [] { "abalone", "ape", "antidisestablishmentarianism" };
    System.out.printf( "%-10s %s\n", "Word", "Length" );
    for ( String word : words )
       System.out.printf( "%-10.10s %s\n", word, word.length() );

    // output
    Word       Length
    abalone    7
    ape        3
    antidisest 28


The s conversion’s big brother S indicates that the output of the conversion should be forced to uppercase. Several other primitive and numeric conversion characters follow this pattern, as we’ll see later. For example:

    String word = "abalone";
    System.out.println(" The lucky word is: %S\n", word );
    // The lucky word is: ABALONE

Numbered arguments

You can refer to an arbitrary argument by number from a format string using the %n$ notation. For example, the following code snippet uses the single argument three times:

    System.out.println( "A %1$s is a %1$s is a %1$S...", "rose" );
     // A rose is a rose is a ROSE...

Numbered arguments are useful for two reasons. The first, shown here, is simply for reusing the same argument in different places and with different conversions. The usefulness of this becomes more apparent when we look at Date and Time formatting in Chapter 11, where we may refer to the same item half a dozen times to get individual fields. The second advantage is that numbered arguments give the message the flexibility to reorder the arguments. This is important when you’re using formatting strings to lay out a message for internationalization or customization purposes where convention may dictate a different ordering.

    log.format("Error %d : %s\n", errNo, errMsg );
    // Error 42 : Low Power
    log.format("%2$s (Error %1$d)\n", errNo, errMsg );
    // Low Power (Error 42)

Primitive and Numeric Conversions

Table 10-3 shows character and Boolean conversion characters.

Table 10-3. Character and Boolean conversion characters




Example output



Formats the result as a Unicode character


b, B


Formats result as Boolean

true, FALSE

The c conversion character produces a Unicode character:

    System.out.printf("The first letter is: %c\n", 'a' );

The b and B conversion characters output the Boolean value of their arguments. If the argument is null, the output is false. Strangely, if the argument is of a type other than Boolean, the output is true. B is identical to b except that it forces the output to uppercase.

    System.out.printf( "The door is open: %b\n", ( door.status() == OPEN ) );

As for String types, a width value can be specified on c and b conversions to pad the result to a minimum length. Table 10-4 summarizes integer type conversion characters.

Table 10-4. Integer type conversion characters




Example output



Formats the result as an integer.


x, X


Formats result as hexadecimal.




Formats result as octal integer.

10, 010

h, H

Integer or object

Formats object as hexadecimal number. If object is not an integer, format its hashCode() value or “null” for null value.


The d, x, and o conversion characters handle the integer type values byte, short, int, and long. (The d apparently stands for decimal, which makes little sense in this context.) The h conversion is an oddity probably intended for debugging. Several important flags give additional control over the formatting of these numeric types. See the section Flags for details.

A width value can be specified on these conversions to pad the result. Precision values are not allowed on integer conversions.

Table 10-5 lists floating-point type conversion characters.

Table 10-5. Floating-point type conversion characters




Example output


Floating point

Formats result as decimal number.


e, E

Floating point

Formats result in scientific notation.


g, G

Floating point

Formats result in either decimal or scientific notation depending on value and precision.

3.14, 10.0e-15

a, A

Floating point

Formats result as hexadecimal floating-point number with significand and exponent.


The f conversion character is the primary floating-point conversion character. e and g conversions allow for values to be formatted in scientific notation. a complements the ability in Java to assign floating-point values using hexadecimal significand and exponent notation, allowing bit-for-bit floating-point values to be displayed without ambiguity.

As always, a width value may be used to pad results to a minimum length. The precision value of the conversion, as its name suggests, controls the number of digits displayed after the decimal point for floating-point values. The value is rounded as necessary. If no precision value is specified, it defaults to six digits:

    printf("float is %f\n",   1.23456789); // float is 1.234568
    printf("float is %.3f\n", 1.23456789); // float is 1.235
    printf("float is %.1f\n", 1.23456789); // float is 1.2
    printf("float is %.0f\n", 1.23456789); // float is 1

The g conversion character determines whether to use decimal or scientific notation. First, the value is rounded to the specified precision. If the result is less than 10−4 (less than .0001) or if the result is greater than 10precision (10 to the power of the precision value), it is displayed in scientific notation. Otherwise, decimal notation is displayed.


Table 10-6 summarizes supported flags to use in format strings.

Table 10-6. Flags for format strings


Arg types


Example output



Left-justifies result (pad space on the right)

'foo '



Prefixes a + sign on positive results


' '


Prefixes a space on positive results (aligning them with negative values)

' 1'



Pads number with leading zeros to accommodate width requirement




Formats numbers with commas or other Locale-specific grouping characters




Encloses negative numbers in parentheses (a convention used to show credits)




Uses an alternate form for octal and hexadecimal output

0xCAFE, 010

As mentioned earlier, the - flag can be used to left-justify formatted output. The remaining flags affect the display of numeric types as described.

The # alternate form flag can be used to print octal and hexadecimal values with their standard prefixes—0x for hexadecimal or 0 for octal:

    System.out.printf("%1$X, %1$#X", 0xCAFE, 0xCAFE ); // CAFE, 0xCAFE
    System.out.printf("%1$o, %1$#o", 8, 8 ); // 10, 010


Table 10-7 lists the remaining formatting items.

Table 10-7. Miscellaneous formatting items




Produces a literal % character (Unicode \u0025)


Produces the platform-specific line separator (e.g., newline or carriage-return, newline)

The best content for your career. Discover unlimited learning on demand for around $1/day.