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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

File I/O

In this chapter, we’re going to talk about the Java file I/O API. To be more precise, we are going to talk about two file APIs: first, there is the core java.io File I/O facility that has been part of Java since the beginning. Then there is the “new” java.nio.file API introduced in Java 7. In general the NIO packages, which we’ll cover in detail later and which touch upon not only files but all types of network and channel I/O, were introduced to add advanced features that make Java more scaleable and higher performance. However, in the case of file NIO, the new package is also just somewhat of a “do-over” on the original API. In movie terms, you can think of the two APIs as the “classic” and the “reboot” of the series. The new API completely duplicates the functionality of the original, but because the core API is so fundamental (and in some cases simpler), it is likely that many people will prefer to keep using it. We’ll start with the classic API centering on java.io.File and later we’ll cover the new API, which centers on the analogous java.nio.Path.

Working with files in Java is easy, but poses some conceptual problems. Real-world filesystems can vary widely in architecture and implementation: think of the differences between Mac, PC, and Unix systems when it comes to filenames. Java tries to mask some of these differences and provide information to help an application tailor itself to the local environment, but it leaves a lot of the details of file access implementation dependent. We’ll talk about techniques for dealing with this as we go.

Before we leave File I/O we’ll also show you some tools for the special case of application “resource” files packaged with your app and loaded via the Java classpath.

The java.io.File Class

The java.io.File class encapsulates access to information about a file or directory. It can be used to get attribute information about a file, list the entries in a directory, and perform basic filesystem operations, such as removing a file or making a directory. While the File object handles these “meta” operations, it doesn’t provide the API for reading and writing file data; there are file streams for that purpose.

File constructors

You can create an instance of File from a String pathname:

    File fooFile = new File( "/tmp/foo.txt" );
    File barDir = new File( "/tmp/bar" );

You can also create a file with a relative path:

    File f = new File( "foo" );

In this case, Java works relative to the “current working directory” of the Java interpreter. You can determine the current working directory by reading the user.dir property in the System Properties list:

    System.getProperty("user.dir"); // e.g.,"/Users/pat"

An overloaded version of the File constructor lets you specify the directory path and filename as separate String objects:

    File fooFile = new File( "/tmp", "foo.txt" );

With yet another variation, you can specify the directory with a File object and the filename with a String:

    File tmpDir = new File( "/tmp" ); // File for directory /tmp
    File fooFile = new File ( tmpDir, "foo.txt" );

None of these File constructors actually creates a file or directory, and it is not an error to create a File object for a nonexistent file. The File object is just a handle for a file or directory whose properties you may wish to read, write, or test. For example, you can use the exists() instance method to learn whether the file or directory exists.

Path localization

One issue with working with files in Java is that pathnames are expected to follow the conventions of the local filesystem. Two differences are that the Windows filesystem uses “roots” or drive letters (for example, C:) and a backslash (\) instead of the forward slash (/) path separator that is used in other systems.

Java tries to compensate for the differences. For example, on Windows platforms, Java accepts paths with either forward slashes or backslashes. (On others, however, it only accepts forward slashes.)

Your best bet is to make sure you follow the filename conventions of the host filesystem. If your application has a GUI that is opening and saving files at the user’s request, you should be able to handle that functionality with the Swing JFileChooser class. This class encapsulates a graphical file-selection dialog box. The methods of the JFileChooser take care of system-dependent filename features for you.

If your application needs to deal with files on its own behalf, however, things get a little more complicated. The File class contains a few static variables to make this task possible. File.separator defines a String that specifies the file separator on the local host (e.g., / on Unix and Macintosh systems and \ on Windows systems); File.separatorChar provides the same information as a char.

You can use this system-dependent information in several ways. Probably the simplest way to localize pathnames is to pick a convention that you use internally, such as the forward slash (/), and do a String replace to substitute for the localized separator character:

    // we'll use forward slash as our standard
    String path = "mail/2004/june/merle";
    path = path.replace('/', File.separatorChar);
    File mailbox = new File( path );

Alternatively, you could work with the components of a pathname and build the local pathname when you need it:

    String [] path = { "mail", "2004", "june", "merle" };

       StringBuffer sb = new StringBuffer(path[0]);
    for (int i=1; i< path.length; i++)
        sb.append( File.separator + path[i] );
    File mailbox = new File( sb.toString() );

Note

One thing to remember is that Java interprets a literal backslash character (\) in source code as an escape character when used in a String. To get a backslash in a String, you have to use \\.

To grapple with the issue of filesystems with multiple “roots” (for example, C:\ on Windows), the File class provides the static method listRoots(), which returns an array of File objects corresponding to the filesystem root directories. Again, in a GUI application, a graphical file chooser dialog shields you from this problem entirely.

File operations

Once we have a File object, we can use it to ask for information about and perform standard operations on the file or directory it represents. A number of methods let us ask questions about the File. For example, isFile() returns true if the File represents a regular file, while isDirectory() returns true if it’s a directory. isAbsolute() indicates whether the File encapsulates an absolute path or relative path specification. An absolute path is a system-dependent notion that means that the path doesn’t depend on the application’s working directory or any concept of a working root or drive (e.g., in Windows, it is a full path including the drive letter: c:\\Users\pat\foo.txt).

Components of the File pathname are available through the following methods: getName(), getPath(), getAbsolutePath(), and getParent(). getName() returns a String for the filename without any directory information. If the File has an absolute path specification, getAbsolutePath() returns that path. Otherwise, it returns the relative path appended to the current working directory (attempting to make it an absolute path). getParent() returns the parent directory of the file or directory.

The string returned by getPath() or getAbsolutePath() may not follow the same case conventions as the underlying filesystem. You can retrieve the filesystem’s own or “canonical” version of the file’s path by using the method getCanonicalPath(). In Windows, for example, you can create a File object whose getAbsolutePath() is C:\Autoexec.bat but whose getCanonicalPath() is C:\AUTOEXEC.BAT; both actually point to the same file. This is useful for comparing filenames that may have been supplied with different case conventions or for showing them to the user.

You can get or set the modification time of a file or directory with lastModified() and setLastModified() methods. The value is a long that is the number of milliseconds since the epoch (Jan 1, 1970, 00:00:00 GMT). We can also get the size of the file in bytes with length().

Here’s a fragment of code that prints some information about a file:

    File fooFile = new File( "/tmp/boofa" );

    String type = fooFile.isFile() ? "File " : "Directory ";
    String name = fooFile.getName();
    long len = fooFile.length();
    System.out.println( type + name + ", " + len + " bytes " );

If the File object corresponds to a directory, we can list the files in the directory with the list() method or the listFiles() method:

    File tmpDir = new File("/tmp" );
    String [] fileNames = tmpDir.list();
    File [] files = tmpDir.listFiles();

list() returns an array of String objects that contains filenames. listFiles() returns an array of File objects. Note that in neither case are the files guaranteed to be in any kind of order (alphabetical, for example). You can use the Collections API to sort strings alphabetically like so:

    List list = Arrays.asList( sa );
    Collections.sort(list);

If the File refers to a nonexistent directory, we can create the directory with mkdir() or mkdirs(). The mkdir() method creates at most a single directory level, so any intervening directories in the path must already exist. mkdirs() creates all directory levels necessary to create the full path of the File specification. In either case, if the directory cannot be created, the method returns false. Use renameTo() to rename a file or directory and delete() to delete a file or directory.

Although we can create a directory using the File object, this isn’t the most common way to create a file; that’s normally done implicitly when we intend to write data to it with a FileOutputStream or FileWriter, as we’ll discuss in a moment. The exception is the createNewFile() method, which can be used to attempt to create a new zero-length file at the location pointed to by the File object. The useful thing about this method is that the operation is guaranteed to be “atomic” with respect to all other file creation in the filesystem. createNewFile() returns a Boolean value that tells you whether the file was created or not. This is sometimes used as a primitive locking feature—whoever creates the file first “wins.” (The NIO package supports true file locks, as we’ll see later.) This is useful in combination deleteOnExit(), which flags the file to be automatically removed when the Java VM exits. This combination allows you to guard resources or make an application that can only be run in a single instance at a time. Another file creation method that is related to the File class itself is the static method createTempFile(), which creates a file in a specified location using an automatically generated unique name. This, too, is useful in combination with deleteOnExit().

The toURL() method converts a file path to a file: URL object. URLs are an abstraction that allows you to point to any kind of object anywhere on the Net. Converting a File reference to a URL may be useful for consistency with more general utilities that deal with URLs. See Chapter 14 for details. File URLs also come into greater use with the NIO File API where they can be used to reference new types of filesystems that are implemented directly in Java code.

Table 12-1 summarizes the methods provided by the File class.

Table 12-1. File methods

Method

Return type

Description

canExecute()

Boolean

Is the file executable?

canRead()

Boolean

Is the file (or directory) readable?

canWrite()

Boolean

Is the file (or directory) writable?

createNewFile()

Boolean

Creates a new file.

createTempFile (String pfx, Stringsfx)

File

Static method to create a new file, with the specified prefix and suffix, in the default temp file directory.

delete()

Boolean

Deletes the file (or directory).

deleteOnExit()

Void

When it exits, Java runtime system deletes the file.

exists()

Boolean

Does the file (or directory) exist?

getAbsolutePath()

String

Returns the absolute path of the file (or directory).

getCanonicalPath()

String

Returns the absolute, case-correct path of the file (or directory).

getFreeSpace()

long

Get the number of bytes of unallocated space on the partition holding this path or 0 if the path is invalid.

getName()

String

Returns the name of the file (or directory).

getParent()

String

Returns the name of the parent directory of the file (or directory).

getPath()

String

Returns the path of the file (or directory). (Not to be confused with toPath()).

getTotalSpace()

long

Get the size of the partition that contains the file path in bytes or 0 if the path is invalid.

getUseableSpace()

long

Get the number of bytes of user-accessible unallocated space on the partition holding this path or 0 if the path is invalid. This method attempts to take into account user write permissions.

isAbsolute()

boolean

Is the filename (or directory name) absolute?

isDirectory()

boolean

Is the item a directory?

isFile()

boolean

Is the item a file?

isHidden()

boolean

Is the item hidden? (System-dependent.)

lastModified()

long

Returns the last modification time of the file (or directory).

length()

long

Returns the length of the file.

list()

String []

Returns a list of files in the directory.

listFiles()

File[]

Returns the contents of the directory as an array of File objects.

listRoots()

File[]

Returns array of root filesystems if any (e.g., C:/, D:/).

mkdir()

boolean

Creates the directory.

mkdirs()

boolean

Creates all directories in the path.

renameTo(File dest )

boolean

Renames the file (or directory).

setExecutable()

boolean

Sets execute permissions for the file.

setLastModified()

boolean

Sets the last-modified time of the file (or directory).

setReadable()

boolean

Sets read permissions for the file.

setReadOnly()

boolean

Sets the file to read-only status.

setWriteable()

boolean

Sets the write permissions for the file.

toPath()

java.nio.file.Path

Convert the File to an NIO File Path (see the NIO File API). (Not to be confused with getPath().)

toURL()

java.net.URL

Generates a URL object for the file (or directory).

File Streams

OK, you’re probably sick of hearing about files already and we haven’t even written a byte yet! Well, now the fun begins. Java provides two fundamental streams for reading from and writing to files: FileInputStream and FileOutputStream. These streams provide the basic byte-oriented InputStream and OutputStream functionality that is applied to reading and writing files. They can be combined with the filter streams described earlier to work with files in the same way as other stream communications.

You can create a FileInputStream from a String pathname or a File object:

    FileInputStream in = new FileInputStream( "/etc/passwd" );

When you create a FileInputStream, the Java runtime system attempts to open the specified file. Thus, the FileInputStream constructors can throw a FileNotFoundException if the specified file doesn’t exist or an IOException if some other I/O error occurs. You must catch these exceptions in your code. Wherever possible, it’s a good idea to get in the habit of using the new Java 7 try-with-resources construct to automatically close files for you when you are finished with them:

try ( FileInputStream fin = new FileInputStream( "/etc/passwd" ) ) {
    ....
    // Fin will be closed automatically if needed upon exiting the try clause.
}

When the stream is first created, its available() method and the File object’s length() method should return the same value.

To read characters from a file as a Reader, you can wrap an InputStreamReader around a FileInputStream. If you want to use the default character-encoding scheme for the platform, you can use the FileReader class instead, which is provided as a convenience. FileReader is just a FileInputStream wrapped in an InputStreamReader with some defaults. For some crazy reason, you can’t specify a character encoding for the FileReader to use, so it’s probably best to ignore it and use InputStreamReader with FileInputStream.

The following class, ListIt , is a small utility that sends the contents of a file or directory to standard output:

    //file: ListIt.java
    import java.io.*;

    class ListIt {
        public static void main ( String args[] ) throws Exception {
            File file =  new File( args[0] );

            if ( !file.exists() || !file.canRead() ) {
                System.out.println( "Can't read " + file );
                return;
            }

            if ( file.isDirectory() ) {
                String [] files = file.list();
                for ( String file : files )
                    System.out.println( file );
            } else
                try {
                    Reader ir = new InputStreamReader( 
                        new FileInputStream( file ) );

                    BufferedReader in = new BufferedReader( ir );
                    String line;
                    while ((line = in.readLine()) != null)
                        System.out.println(line);
                }
                catch ( FileNotFoundException e ) {
                    System.out.println( "File Disappeared" );
                }
        }
    }

ListIt constructs a File object from its first command-line argument and tests the File to see whether it exists and is readable. If the File is a directory, ListIt outputs the names of the files in the directory. Otherwise, ListIt reads and outputs the file, line by line.

For writing files, you can create a FileOutputStream from a String pathname or a File object. Unlike FileInputStream, however, the FileOutputStream constructors don’t throw a FileNotFoundException. If the specified file doesn’t exist, the FileOutputStream creates the file. The FileOutputStream constructors can throw an IOException if some other I/O error occurs, so you still need to handle this exception.

If the specified file does exist, the FileOutputStream opens it for writing. When you subsequently call the write() method, the new data overwrites the current contents of the file. If you need to append data to an existing file, you can use a form of the constructor that accepts a Boolean append flag:

    FileInputStream fooOut =
        new FileOutputStream( fooFile ); // overwrite fooFile
    FileInputStream pwdOut =
        new FileOutputStream( "/etc/passwd", true ); // append

Another way to append data to files is with RandomAccessFile, which we’ll discuss shortly.

Just as with reading, to write characters (instead of bytes) to a file, you can wrap an OutputStreamWriter around a FileOutputStream. If you want to use the default character-encoding scheme, you can use the FileWriter class instead, which is provided as a convenience.

The following example reads a line of data from standard input and writes it to the file /tmp/foo.txt:

    String s = new BufferedReader(
        new InputStreamReader( System.in ) ).readLine();
    File out = new File( "/tmp/foo.txt" );
    FileWriter fw = new FileWriter ( out );
    PrintWriter pw = new PrintWriter( fw )
    pw.println( s );pw.close();

Notice how we wrapped the FileWriter in a PrintWriter to facilitate writing the data. Also, to be a good filesystem citizen, we called the close() method when we’re done with the FileWriter. Here, closing the PrintWriter closes the underlying Writer for us. We also could have used try-with-resources here.

RandomAccessFile

The java.io.RandomAccessFile class provides the ability to read and write data at a specified location in a file. RandomAccessFile implements both the DataInput and DataOutput interfaces, so you can use it to read and write strings and primitive types at locations in the file just as if it were a DataInputStream and DataOutputStream. However, because the class provides random, rather than sequential, access to file data, it’s not a subclass of either InputStream or OutputStream.

You can create a RandomAccessFile from a String pathname or a File object. The constructor also takes a second String argument that specifies the mode of the file. Use the string r for a read-only file or rw for a read/write file.

    try {
        RandomAccessFile users = new RandomAccessFile( "Users", "rw" )
     } catch (IOException e) { ... }

When you create a RandomAccessFile in read-only mode, Java tries to open the specified file. If the file doesn’t exist, RandomAccessFile throws an IOException. If, however, you’re creating a RandomAccessFile in read/write mode, the object creates the file if it doesn’t exist. The constructor can still throw an IOException if another I/O error occurs, so you still need to handle this exception.

After you have created a RandomAccessFile, call any of the normal reading and writing methods, just as you would with a DataInputStream or DataOutputStream. If you try to write to a read-only file, the write method throws an IOException.

What makes a RandomAccessFile special is the seek() method. This method takes a long value and uses it to set the byte offset location for reading and writing in the file. You can use the getFilePointer() method to get the current location. If you need to append data to the end of the file, use length() to determine that location, then seek() to it. You can write or seek beyond the end of a file, but you can’t read beyond the end of a file. The read() method throws an EOFException if you try to do this.

Here’s an example of writing data for a simplistic database:

    users.seek( userNum * RECORDSIZE );
    users.writeUTF( userName );
    users.writeInt( userID );
    ...

In this naive example, we assume that the String length for userName, along with any data that comes after it, fits within the specified record size.

Resource Paths

A big part of packaging and deploying an application is dealing with all of the resource files that must go with it, such as configuration files, graphics, and application data. Java provides several ways to access these resources. One way is to simply open files and read the bytes. Another is to construct a URL pointing to a well-known location in the filesystem or over the network. (We’ll discuss working with URLs in detail in Chapter 14.) The problem with these methods is that they generally rely on knowledge of the application’s location and packaging, which could change or break if it is moved. What is really needed is a universal way to access resources associated with our application, regardless of how it’s installed. The Class class’s getResource() method and the Java classpath provides just this. For example:

    URL resource = MyApplication.class.getResource("/config/config.xml");

Instead of constructing a File reference to an absolute file path, or relying on composing information about an install directory, the getResource() method provides a standard way to get resources relative to the classpath of the application. A resource can be located either relative to a given class file or to the overall system classpath. getResource() uses the classloader that loads the application’s class files to load the data. This means that no matter where the application classes reside—a web server, the local filesystem, or even inside a JAR file or other archive—we can load resources packaged with those classes consistently.

Although we haven’t discussed URLs yet, we can tell you that many APIs for loading data (for example, images) accept a URL directly. If you’re reading the data yourself, you can ask the URL for an InputStream with the URL openStream() method and treat it like any other stream. A convenience method called getResourceAsStream() skips this step for you and returns an InputStream directly.

getResource() takes as an argument a slash-separated resource path for the resource and returns a URL. There are two kinds of resource paths: absolute and relative. An absolute path begins with a slash (for example, /config/config.xml). In this case, the search for the object begins at the “top” of the classpath. By the “top” of the classpath, we mean that Java looks within each element of the classpath (directory or JAR file) for the specified file. Given /config/config.xml, it would check each directory or JAR file in the path for the file config/config.xml. In this case, the class on which getResource() is called doesn’t matter as long as it’s from a class loader that has the resource file in its classpath. For example:

    URL data = AnyClass.getResource("/config/config.xml");

On the other hand, a relative URL does not begin with a slash (for example, mydata.txt). In this case, the search begins at the location of the class file on which getResource() is called. In other words, the path is relative to the package of the target class file. For example, if the class file foo.bar.MyClass is located at the path foo/bar/MyClass.class in some directory or JAR of the classpath and the file mydata.txt is in the same directory (foo/bar/mydata.txt), we can request the file via MyClass with:

    URL data = MyClass.getResource("mydata.txt");

In this case, the class and file come from the same logical directory. We say logical because the search is not limited to the classpath element from which the class was loaded. Instead, the same relative path is searched in each element of the classpath—just as with an absolute path—until it is found. Although we’d expect the file mydata.txt to be packaged physically with MyClass.class, it might be found in another JAR file or directory at the same relative and corresponding location.

For example, here’s an application that looks up some resources:

    package mypackage;
    import java.net.URL;
    import java.io.IOException;
      
    public class FindResources {
      public static void main( String [] args ) throws IOException {
        // absolute from the classpath
        URL url = FindResources.class.getResource("/mypackage/foo.txt");
        // relative to the class location
        url = FindResources.class.getResource("foo.txt");
        // another relative document
        url = FindResources.class.getResource("docs/bar.txt");
      }
    }

The FindResources class belongs to the mypackage package, so its class file will live in a mypackage directory somewhere on the classpath. FindResources locates the document foo.txt using an absolute and then a relative URL. At the end, FindResources uses a relative path to reach a document in the mypackage/docs directory. In each case, we refer to the FindResources’s Class object using the static .class notation. Alternatively, if we had an instance of the object, we could use its getClass() method to reach the Class object.

Again, getResource() returns a URL for whatever type of object you reference. This could be a text file or properties file that you want to read as a stream, or it might be an image or sound file or some other object. You can open a stream to the URL to parse the data yourself or hand the URL over to an API that deals with URLs. We discuss URLs in depth in Chapter 14. We should also emphasize that loading resources in this way completely shields your application from the details of how it is packaged or deployed. You may start with your application in loose files and then package it into a JAR file and the resources will still be loaded. Java applets (discussed in a later chapter) may even load files in this way over the network because the applet class loader treats the server as part of its classpath.

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