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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
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Data Compression

The java.util.zip package contains classes you can use for data compression in streams or files. The classes in the java.util.zip package support two widespread compression formats: GZIP and ZIP. In this section, we’ll talk about how to use these classes. We’ll also present two useful example programs that build on what you have learned in this chapter. After that, we’ll talk about a higher-level way to work with ZIP archives—as filesystems—introduced with Java 7.

Archives and Compressed Data

The java.util.zip package provides two filter streams for writing compressed data. The GZIPOutputStream is for writing data in GZIP compressed format. The ZIPOutputStream is for writing compressed ZIP archives, which can contain one or many files. To write compressed data in the GZIP format, simply wrap a GZIPOutputStream around an underlying stream and write to it. The following is a complete example that shows how to compress a file using the GZIP format, but the stream could just as well be sent over a network connection or to any other type of stream destination. Our GZip example is a command line utility that compresses a file.

    import java.io.*;
    import java.util.zip.*;

    public class GZip {
      public static int sChunk = 8192;

      public static void main(String[] args) {
        if (args.length != 1) {
          System.out.println("Usage: GZip source");
          return;
        }
        // create output stream
        String zipname = args[0] + ".gz";
        GZIPOutputStream zipout;
        try {
          FileOutputStream out = new FileOutputStream(zipname);
          zipout = new GZIPOutputStream(out);
        }
        catch (IOException e) {
          System.out.println("Couldn't create " + zipname + ".");
          return;
        }
        byte[] buffer = new byte[sChunk];
        // compress the file
        try {
          FileInputStream in = new FileInputStream(args[0]);
          int length;
          while ((length = in.read(buffer, 0, sChunk)) != -1)
            zipout.write(buffer, 0, length);
          in.close();
        }
        catch (IOException e) {
          System.out.println("Couldn't compress " + args[0] + ".");
        }
        try { zipout.close(); }
        catch (IOException e) {}
      }
    }

First, we check to make sure we have a command-line argument representing a filename. We then construct a GZIPOutputStream wrapped around a FileOutputStream representing the given filename, with the .gz suffix appended. With this in place, we open the source file. We read chunks of data and write them into the GZIPOutputStream. Finally, we clean up by closing our open streams.

Zip archives

While GZIP is simple compression format for a stream or file, a ZIP archive is a file that is actually a collection of files, some (or all) of which may be compressed. Writing data to a ZIP archive file is a little more involved than simply wrapping a stream, but not difficult. Each item in the ZIP file is represented by a ZipEntry object. When writing to a ZipOutputStream, you’ll need to call putNextEntry() before writing the data for each item. The following example shows how to create a ZipOutputStream. You’ll notice that it starts out with a stream wrapper just like it did when creating a GZIPOutputStream:

    ZipOutputStream zipout;
    try {
      FileOutputStream out = new FileOutputStream("archive.zip");
      zipout = new ZipOutputStream(out);
    }
    catch (IOException e) {}

Let’s say we have two files we want to write into this archive. Before we begin writing, we need to call putNextEntry() to set the name of the file within the archive and initialize the stream to the correct position for it. Here we create a simple ZipEntry with just a file name. You can set other ZIP format specific fields in ZipEntry, but most of the time, you won’t need to bother with them.

    try {
      ZipEntry entry = new ZipEntry("first.dat");
      zipout.putNextEntry(entry);
      zipout.write( ... ) // Write data for first file

      ZipEntry entry = new ZipEntry("second.dat");
      zipout.putNextEntry(entry);
      zipout.write( ... ) // Write data for second file
      . . .
      zipout.close();
    }
    catch (IOException e) {}

Decompressing Data

To decompress data in the GZIP format, simply wrap a GZIPInputStream around an underlying FileInputStream and read from it. The following example complements our earlier GZip example and shows how to decompress a GZIP file:

    import java.io.*;
    import java.util.zip.*;

    public class GUnzip {
      public static int sChunk = 8192;
      public static void main(String[] args) {
        if (args.length != 1) {
          System.out.println("Usage: GUnzip source");
          return;
        }
        // create input stream
        String zipname, source;
        if (args[0].endsWith(".gz")) {
          zipname = args[0];
          source = args[0].substring(0, args[0].length() - 3);
        }
        else {
          zipname = args[0] + ".gz";
          source = args[0];
        }
        GZIPInputStream zipin;
        try {
          FileInputStream in = new FileInputStream(zipname);
          zipin = new GZIPInputStream(in);
        }
        catch (IOException e) {
          System.out.println("Couldn't open " + zipname + ".");
          return;
        }
        byte[] buffer = new byte[sChunk];
        // decompress the file
        try {
          FileOutputStream out = new FileOutputStream(source);
          int length;
          while ((length = zipin.read(buffer, 0, sChunk)) != -1)
            out.write(buffer, 0, length);
          out.close();
        }
        catch (IOException e) {
          System.out.println("Couldn't decompress " + args[0] + ".");
        }
        try { zipin.close(); }
        catch (IOException e) {}
      }
    }

First, we check to make sure we have a command-line argument representing a filename. If the argument ends with .gz, we figure out what the filename for the uncompressed file should be. Otherwise, we use the given argument and assume the compressed file has the .gz suffix. Then we construct a GZIPInputStream wrapped around a FileInputStream that represents the compressed file. With this in place, we open the target file. We read chunks of data from the GZIPInputStream and write them into the target file. Finally, we clean up by closing our open streams.

Reading a ZIP archive is also the mirror of writing. When reading from a ZipInputStream, you should call getNextEntry() before reading each item. When getNextEntry() returns null, there are no more items to read. The following example shows how to create a ZipInputStream:

    ZipInputStream zipin;
    try {
      FileInputStream in = new FileInputStream("archive.zip");
      zipin = new ZipInputStream(in);
    }
    catch (IOException e) {}

Suppose we want to read two files from this archive. Before we begin reading, we need to call getNextEntry(). At the very least, the entry gives us a name of the item we are reading from the archive:

    try {
      ZipEntry first = zipin.getNextEntry();
      zipin.read( ... ) // Read the file data
    } catch (IOException e) {}

Now, you can read the contents of the first item in the archive. When you come to the end of the item, the read() method returns -1. At this point, you can call getNextEntry() again to read the second item from the archive. If you call getNextEntry() and it returns null, there are no more items and you have reached the end of the archive.

Zip Archive As a Filesystem

One of the benefits of the new java.nio.file package introduce with Java 7 is the ability to implement custom filesystems in Java. (We talked about the File API for the NIO file package earlier in this chapter and we’ll return to the more general NIO facilities in the next section.) Java 7 ships with one such custom filesystem implementation bundled within it: the Zip Filesystem Provider.[35] Using the Zip Filesystem Provider, we can open a ZIP archive and treat it like a filesystem: reading, writing, copying, and renaming files using all of the standard java.nio.file APIs, except that all of these operations happen inside the ZIP archive file instead of on the host computer filesystem (as you might otherwise expect).

The key to making this possible is that the NIO File API starts with a FileSystem abstraction that serves as a factory for Path objects. In our previous discussion of the NIO File API we always simply asked for the default filesystem using Filesystems.getDefault(). This time, we are going to target a particular custom filesystem type and destination by constructing a special URI for our ZIP archive. (As we’ll discuss in the networking chapters, a URI is kind of like a URL except that it can be more abstract).

        // Construct the URI pointing to the ZIP archive
        URI zipURI = URI.create("jar:file:/Users/pat/tmp/MyArchive.zip");

        // Open or create it and write a file
        Map<String, String> env = new HashMap<>();
        env.put("create", "true");
        try ( FileSystem zipfs = FileSystems.newFileSystem( zipURI, env ) )
        {
            Path path = zipfs.getPath("/README.txt");
            OutputStream out = Files.newOutputStream( path );
            try ( PrintWriter pw = new PrintWriter( 
                new OutputStreamWriter( out ) ) ) {

                pw.println("Hello World!");
            }
        }

In this snippet, we constructed a URI for our ZIP archive using the URIcreate() method and the special jar:file: prefix. (The Java JAR format is really just the ZIP format with some additional conventions.) We then used that URI with the Filesystems newFileSystem() method to create the right kind of filesystem reference for us. The FileSystem it returns will perform all of its operations on entries within the ZIP, but otherwise will behave just like we’ve seen previously. The other argument to the newFileSystem() method is a Map containing string properties that are specific to the provider. In this case, we pass in the value “create” as “true,” indicating that we want the ZIP filesystem provider to create the archive if it does not already exist. In order to know what properties can be passed, you’ll have to consult the documentation for the particular filesystem provider.

In our preceding snippet, we then create a Path for a file /README.txt at the root folder of the filesystem and write a string to it. Because we are using try-with-resources clauses to encapsulate opening the filesystem and writing to the file, the resources will be automatically closed for us when the operation is complete.

Other operations proceed just as with “normal” files. For example, we can move a file by creating a path for the existing file and a path for the new location and then using the standard Files move() method.

        // Move the file
        try ( FileSystem zipfs = FileSystems.newFileSystem( fsURI, env ) )
        {
            Path path = zipfs.getPath("/README.txt");
            Path toPath = zipfs.getPath("/README2.txt");
            Files.move( path, toPath );
        }


[35] The Zip Filesystem Provider is also supplied as an example along with sample source code even though it’s unclear if Oracle intends it to be a standard. But at the time of this writing, it is bundled with the JDK and JRE of Java 7 on all platforms.

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