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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
      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


In this section, we’ll take a look at the Java Reflection API, supported by the classes in the java.lang.reflect package. As its name suggests, reflection is the ability for a class or object to examine itself. Reflection lets Java code look at an object (more precisely, the class of the object) and determine its structure. Within the limits imposed by the security manager, you can find out what constructors, methods, and fields a class has, as well as their attributes. You can even change the value of fields, dynamically invoke methods, and construct new objects, much as if Java had primitive pointers to variables and methods. And you can do all this on objects that your code has never even seen before. The Annotations API also has the ability to preserve metadata about source code in the compiled classes and we can retrieve this information with the Reflection API.

We don’t have room here to cover the Reflection API fully. As you might expect, the reflect package is complex and rich in details. But reflection has been designed so that you can do a lot with relatively little effort; 20% of the effort gives you 80% of the fun.

The Reflection API can be used to determine the capabilities of objects at runtime. It’s used by object serialization to tear apart and build objects for transport over streams or into persistent storage. Obviously, the power to pick apart objects and see their internals must be zealously guarded by the security manager. The general rule is that your code is not allowed to do anything with the Reflection API that it couldn’t do with static (ordinary, compiled) Java code. In short, reflection is a powerful tool, but it isn’t an automatic loophole. By default, an object can’t use it to work with fields or methods that it wouldn’t normally be able to access (for example, another object’s private fields), although those privileges can be granted, as we’ll discuss later.

The three primary features of a class are its fields (variables), methods, and constructors. For purposes of describing and accessing an object, these three features are represented by separate classes in the Reflection API: java.lang.reflect.Field, java.lang.reflect.Method , and java.lang.reflect.Constructor . We can look up these members of a class through the Class object.

The Class class provides two pairs of methods for getting at each type of feature. One pair allows access to a class’s public features (including those inherited from its superclasses) while the other pair allows access to any public or nonpublic item declared directly within the class (but not features that are inherited), subject to security considerations. Some examples:

  • getFields() returns an array of Field objects representing all a class’s public variables, including those it inherits.

  • getDeclaredFields() returns an array representing all the variables declared in the class, regardless of their access modifiers, but not including inherited variables.

  • For constructors, the distinction between “all constructors” and “declared constructors” is not meaningful (classes do not inherit constructors), so getConstructors() and getDeclaredConstructors() differ only in that the former returns public constructors while the latter returns all the class’s constructors.

Each set of methods includes the methods for listing all the items at once (for example, getFields()) and an additional method for looking up a particular item by name and—for methods and constructors—by signature (for example, getField(), which takes the field name as an argument).

The following listing shows the methods in the Class class:

Field [] getFields();

Get all public variables, including inherited ones.

Field getField(String name);

Get the specified public variable, which may be inherited.

Field [] getDeclaredFields();

Get all public and nonpublic variables declared in this class (not including those inherited from superclasses).

Field getDeclaredField(String name);

Get the specified variable, public or nonpublic, declared in this class (inherited variables not considered).

Method [] getMethods();

Get all public methods, including inherited ones.

Method getMethod(String name, Class ... argumentTypes);

Get the specified public method that has arguments that match the types listed in argumentTypes. The method may be inherited.

Method [] getDeclaredMethods();

Get all public and nonpublic methods declared in this class (not including those inherited from superclasses).

Method getDeclaredMethod(String name, Class ... argumentTypes);

Get the specified method, public or nonpublic, that has arguments that match the types listed in argumentTypes, and which is declared in this class (inherited methods not considered).

Constructor [] getConstructors();

Get all public constructors of this class.

Constructor getConstructor(Class ... argumentTypes);

Get the specified public constructor of this class that has arguments that match the types listed in argumentTypes.

Constructor [] getDeclaredConstructors();

Get all public and nonpublic constructors of this class.

Constructor getDeclaredConstructor(Class ... argumentTypes);

Get the specified constructor, public or nonpublic, that has arguments that match the types listed in argumentTypes.

Class [] getDeclaredClasses();

Get all public and nonpublic inner classes declared within this class.

Constructor [] getInterfaces();

Get all interfaces implemented by this class, in the order in which they are declared.

As you can see, the four getMethod() and getConstructor() methods take advantage of the Java variable-length argument lists to allow you to pass in the argument types. In older versions of Java, you have to pass an array of Class types in their place. We’ll show an example later.

As a quick example, we’ll show how easy it is to list all the public methods of the java.util.Calendar class:

    for ( Method method : Calendar.class.getMethods() )
        System.out.println( method );

Here, we’ve used the .class notation to get a reference to the Class of Calendar. Remember the discussion of the Class class; the reflection methods don’t belong to a particular instance of Calendar itself; they belong to the java.lang.Class object that describes the Calendar class. If we wanted to start from an instance of Calendar (or, say, an unknown object), we could have used the getClass() method of the object instead:

    Method [] methods = myUnknownObject.getClass().getMethods();

Modifiers and Security

All of the types of members of a Java class—fields, methods, constructors, and inner classes—have a method getModifiers() that returns a set of flags indicating whether the member is private, protected, default level, or publicly accessible. You can test for these with the java.lang.reflect.Modifier class, like so:

    Method method = Object.class.getDeclaredMethod( "clone" ); // no arguments
    int perms = method.getModifiers();
    System.out.println( Modifier.isPublic( perms ) ); // false
    System.out.println( Modifier.isProtected( perms ) ); // true
    System.out.println( Modifier.isPrivate( perms ) ); // false

In this example, the clone() method in Object is protected.

Access to the Reflection API is governed by a security manager. A fully trusted application has access to all the previously discussed functionality; it can gain access to members of classes at the level of restriction normally granted code within its scope. It is, however, possible to grant special access to code so that it can use the Reflection API to gain access to private and protected members of other classes in a way that the Java language ordinarily disallows.

The Field, Method, and Constructor classes all extend from a base class called AccessibleObject. The AccessibleObject class has one important method called setAccessible(), which allows you to deactivate normal security when accessing that particular class member. That may sound too easy. It is indeed simple, but whether that method allows you to disable security or not is a function of the Java security manager and security policy. You can do this in a normal Java application running without any security policy, but not, for example, in an applet or other secure environment. For example, to be able to use the protected clone() method of the Object class, all we have to do (given no contravening security manager) is:

    Method method = Object.class.getDeclaredMethod( "clone" );
    method.setAccessible( true );

Accessing Fields

The class java.lang.reflect.Field represents static variables and instance variables. Field has a full set of overloaded accessor methods for all the base types (for example, getInt() and setInt(), getBoolean() and setBoolean()), and get() and set() methods for accessing fields that are reference types. Let’s consider this class:

    class BankAccount {
        public int balance;

With the Reflection API, we can read and modify the value of the public integer field balance:

    BankAccount myBankAccount = ...;
    try {
        Field balanceField = BankAccount.class.getField("balance");
        // read it
        int mybalance = balanceField.getInt( myBankAccount );
        // change it
        balanceField.setInt( myBankAccount, 42 );
    } catch ( NoSuchFieldException e ) {
        ... // there is no "balance" field in this class
    } catch ( IllegalAccessException e2) {
        ... // we don't have permission to access the field

In this example, we are assuming that we already know the structure of a BankAccount object. In general, we could gather that information from the object itself.

All the data access methods of Field take a reference to the particular object instance that we want to access. In this example, the getField() method returns a Field object that represents the balance of the BankAccount class; this object doesn’t refer to any specific BankAccount. Therefore, to read or modify any specific BankAccount, we call getInt() and setInt() with a reference to myBankAccount, which is the particular object instance that contains the field with which we want to work. For a static field, we’d use the value null here. An exception occurs if we try to access a field that doesn’t exist, or if we don’t have the proper permission to read or write to the field. If we make balance a private field, we can still look up the Field object that describes it, but we won’t be able to read or write its value.

Therefore, we aren’t doing anything that we couldn’t have done with static code at compile time; as long as balance is a public member of a class that we can access, we can write code to read and modify its value. What’s important is that we’re accessing balance at runtime, and we could just as easily use this technique to examine the balance field in a class that was dynamically loaded or that we just discovered by iterating through the class’s fields with the getDeclaredFields() method.

Accessing Methods

The class java.lang.reflect.Method represents a static or instance method. Subject to the normal security rules, a Method object’s invoke() method can be used to call the underlying object’s method with specified arguments. Yes, Java does have something like a method pointer!

As an example, we’ll write a Java application called Invoke that takes as command-line arguments the name of a Java class and the name of a method to invoke. For simplicity, we’ll assume that the method is static and takes no arguments (quite a limitation):

    import java.lang.reflect.*;

    class Invoke {
      public static void main( String [] args ) {
        try {
          Class clas = Class.forName( args[0] );
          Method method = clas.getMethod( args[1] ); // Named method, 
                                                     // no arguments
          Object ret =  method.invoke( null );  // Invoke a static method

              "Invoked static method: " + args[1]
              + " of class: " + args[0]
              + " with no args\nResults: " + ret );
        } catch ( ClassNotFoundException e ) {
          // Class.forName() can't find the class
        } catch ( NoSuchMethodException e2 ) {
          // that method doesn't exist
        } catch ( IllegalAccessException e3 ) {
          // we don't have permission to invoke that method
        } catch ( InvocationTargetException e4 ) {
          // an exception occurred while invoking that method
              "Method threw an: " + e4.getTargetException() );

We can run invoke to fetch the value of the system clock:

    % java Invoke java.lang.System currentTimeMillis
    Invoked static method: currentTimeMillis of class:
    java.lang.System with no args
    Results: 861129235818

Our first task is to look up the specified Class by name. To do so, we call the forName() method with the name of the desired class (the first command-line argument). We then ask for the specified method by its name. getMethod() has two arguments: the first is the method name (the second command-line argument), and the second is an array of Class objects that specifies the method’s signature. (Remember that any method may be overloaded; you must specify the signature to make it clear which version you want.) Because our simple program calls only methods with no arguments, we create an anonymous empty array of Class objects. Had we wanted to invoke a method that takes arguments, we would have passed an array of the classes of their respective types, in the proper order. For primitive types, we would have used the standard wrappers (Integer, Float, Boolean, etc.) to hold the values. The classes of primitive types in Java are represented by special static TYPE fields of their respective wrappers; for example, use Integer.TYPE for the Class of an int. As shown in comments in the code, starting in Java 5.0, the getMethod() and invoke() methods accept variable-length argument lists, which means that we can omit the arguments entirely and Java will make the empty array for us.

Once we have the Method object, we call its invoke() method. This calls our target method and returns the result as an Object. To do anything nontrivial with this object, you must cast it to something more specific. Presumably, because we’re calling the method, we know what kind of object to expect. But if we didn’t, we could use the Method getReturnType() method to get the Class of the return type at runtime. If the returned value is a primitive type such as int or boolean, it will be wrapped in the standard wrapper class for its type. If the method returns void, invoke() returns a java.lang.Void object. This is the “wrapper” class that represents void return values.

The first argument to invoke() is the object on which we would like to invoke the method. If the method is static, there is no object, so we set the first argument to null. That’s the case in our example. The second argument is an array of objects to be passed as arguments to the method. The types of these should match the types specified in the call to getMethod(). Because we’re calling a method with no arguments, we can pass null for the second argument to invoke(). As with the return value, you must use wrapper classes for primitive argument types.

The exceptions shown in the previous code occur if we can’t find or don’t have permission to access the method. Additionally, an InvocationTargetException occurs if the method being invoked throws some kind of exception itself. You can find what it threw by calling the getTargetException() method of InvocationTargetException.

Accessing Constructors

The java.lang.reflect.Constructor class represents an object constructor in the same way that the Method class represents a method. You can use it, subject to the security manager, of course, to create a new instance of an object, even with constructors that require arguments. Recall that you can create instances of a class with Class.newInstance(), but you cannot specify arguments with that method. This is the solution to that problem, if you really need to do it.

Here, we’ll create an instance of java.util.Date,[20] passing a string argument to the constructor:

try {
        Constructor<Date> cons =
            Date.class.getConstructor( String.class );
        Date date = cons.newInstance( "Jan 1, 2006" );
        System.out.println( date );
    } catch ( NoSuchMethodException e ) {
        // getConstructor() couldn't find the constructor we described
    } catch ( InstantiationException e2 ) {
        // the class is abstract
    } catch ( IllegalAccessException e3 ) {
        // we don't have permission to create an instance
    } catch ( InvocationTargetException e4 ) {
        // the construct threw an exception

The story is much the same as with a method invocation; after all, a constructor is really no more than a method with some strange properties. We look up the appropriate constructor for our Date class—the one that takes a single String as its argument—by passing getConstructor() the String.class type. Here, we are using the Java 5.0 variable argument syntax. If the constructor required more arguments, we would pass additional Classes representing the class of each argument. We can then invoke newInstance(), passing it a corresponding argument object. Again, to pass primitive types, we would wrap them in their wrapper types first. Finally, we print the resulting object to a Date. Note that we’ve slipped in another strange construct using generics here. The Constructor<Date> type here simply allows us to specialize the Constructor for the Date type, alleviating the need to cast the result of the newInstance() method, as before.

The exceptions from the previous example apply here, too, along with IllegalArgumentException and InstantiationException. The latter is thrown if the class is abstract and, therefore, can’t be instantiated.

What About Arrays?

The Reflection API allows you to create and inspect arrays of base types using the java.lang.reflect.Array class. The process is very much the same as with the other classes, so we won’t cover it in detail. The primary feature is a static method of Array called newInstance(), which creates an array that allows you to specify a base type and length. You can also use it to construct multidimensional array instances by specifying an array of lengths (one for each dimension). For more information, look in your favorite Java language reference.

Accessing Generic Type Information

In Chapter 8, we’ll discuss generics, which first appeared in Java 5.0. Generics is a major addition that adds new dimensions (literally) to the concept of types in the Java language. With the addition of generics, types are no longer simply one-to-one with Java classes and interfaces but can be parameterized on one or more types to create a new, generic type. To make matters more complicated, these new types do not actually generate new classes, but instead are artifacts of the compiler. To keep the generic information, Java adds information to the compiled class files.

The Reflection API can accommodate all of this, mainly through the addition of the new java.lang.reflect.Type class, which is capable of representing generic types. Covering this in detail is a bit outside the scope of this book and because we won’t even get to generics until Chapter 8, we won’t devote much more space to this topic here. However, the following code snippets may guide you later if you return to the topic of accessing generic type information reflectively:

    // public interface List<E> extends Collection<E> { ... }

    TypeVariable [] tv = List.class.getTypeParameters();
    System.out.println( tv[0].getName() ); // "E"

This snippet gets the type parameter of the java.util.List class and prints its name:

    class StringList extends ArrayList<String> { }

    Type type = StringList.class.getGenericSuperclass();

    System.out.println( type );  //
    // "java.util.ArrayList<java.lang.String>"

    ParameterizedType pt = (ParameterizedType)type;
    System.out.println( pt.getActualTypeArguments()[0] ); //
    // "class java.lang.String"

This second snippet gets the Type for a class that extends a generic type and then prints the actual type on which it was parameterized.

Accessing Annotation Data

Later in this chapter, we discuss annotations, a feature that allows metadata to be added to Java classes, methods, and fields. Annotations can optionally be retained in the compiled Java classes and accessed through the Reflection API. This is one of several intended uses for annotations, allowing code at runtime to see the metadata and provide special services for the annotated code. For example, a property (field or setter method) on a Java object might be annotated to indicate that it is expecting a container application to set its value or export it in some way.

Covering this in detail is outside the scope of this book; however, getting annotation data through the Reflection API is easy. Java classes, as well as Method and Field objects, have the following pairs of methods (and some other related ones):

    public <A extends Annotation> A getAnnotation(Class<A> annotationClass)
    public Annotation[] getDeclaredAnnotations()

These methods (the first is a generic method, as covered in Chapter 8) return java.lang.annotation.Annotation type objects that represent the metadata.

Dynamic Interface Adapters

Ideally, Java reflection would allow us to do everything at runtime that we can do at compile time (without forcing us to generate and compile source into bytecode). But that is not entirely the case. Although we can dynamically load and create instances of objects at runtime using the Class.forName() method, there is no general way to create new types of objects—for which no class files preexist—on the fly.

The java.lang.reflect.Proxy class, however, takes a step toward solving this problem by allowing the creation of adapter objects that implement arbitrary Java interfaces at runtime. The Proxy class is a factory that can generate an adapter class, implementing any interface (or interfaces) you want. When methods are invoked on the adapter class, they are delegated to a single method in a designated InvocationHandler object. You can use this to create dynamic implementations of any kind of interface at runtime and handle the method calls anywhere you want. For example, using a Proxy, you could log all of the method calls on an interface, delegating them to a “real” implementation afterward. This kind of dynamic behavior is important for tools that work with Java beans, which must register event listeners. (We’ll mention this again in Chapter 22.) It’s also useful for a wide range of problems.

In the following snippet, we take an interface name and construct a proxy implementing the interface. It outputs a message whenever any of the interface’s methods are invoked:

    import java.lang.reflect.*;

    InvocationHandler handler =
        new InvocationHandler() {
            Object invoke( Object proxy, Method method, Object[] args ) {
                    "Method: {[QUOTE-REPLACEMENT]}+ method.getName() +"()"
                    +" of interface: "+ interfaceName
                    + " invoked on proxy!"
                return null;

    Class clas = Class.forName( MyInterface );

    MyInterface interfaceProxy =
            clas.getClassLoader(), new Class[] { class }, handler );

    // use MyInterface
    myInterface.anyMethod(); // Method: anyMethod() ... invoked on proxy!

The resulting object, interfaceProxy, is cast to the type of the interface we want. It will call our handler whenever any of its methods are invoked.

First, we make an implementation of InvocationHandler. This is an object with an invoke() method that takes as its argument the Method being called and an array of objects representing the arguments to the method call. We then fetch the class of the interface that we’re going to implement using Class.forName(). Finally, we ask the proxy to create an adapter for us, specifying the types of interfaces (you can specify more than one) that we want implemented and the handler to use. invoke() is expected to return an object of the correct type for the method call. If it returns the wrong type, a special runtime exception is thrown. Any primitive types in the arguments or in the return value should be wrapped in the appropriate wrapper class. (The runtime system unwraps the return value, if necessary.)

What Is Reflection Good For?

Reflection, although in some sense a “back door” feature of the Java language, is finding more and more important uses. In this chapter, we mentioned that reflection is used to access runtime annotations. In Chapter 22, we’ll see how reflection is used to dynamically discover capabilities and features of JavaBean objects. Those are pretty specialized applications—what can reflection do for us in everyday situations?

We could use reflection to go about acting as if Java had dynamic method invocation and other useful capabilities; in Chapter 22, we’ll see a dynamic adapter class that uses reflection to make calls for us. As a general coding practice however, dynamic method invocation is a bad idea. One of the primary features of Java (and what distinguishes it from some similar languages) is its strong type safety. You abandon much of that when you take a dip in the reflecting pool. And although the performance of the Reflection API is very good, it is not precisely as fast as compiled method invocations in general.

More appropriately, you can use reflection in situations where you need to work with objects that you can’t know about in advance. Reflection puts Java on a higher plane of programming languages, opening up possibilities for new kinds of applications. As we’ve mentioned, it makes possible one use of Java annotations at runtime, allowing us to inspect classes, methods, and fields for metadata. Another important and growing use for reflection is integrating Java with scripting languages. With reflection, you can write language interpreters in Java that can access the full Java APIs, create objects, invoke methods, modify variables, and do all the other things a Java program can do at compile time, while the program is running. In fact, you could go so far as to reimplement the Java language in Java, allowing completely dynamic programs that can do all sorts of things. Sound crazy? Well, someone has already done this—one of the authors of this book! We’ll explain next.

The BeanShell Java scripting language

I (Pat) can’t resist inserting a plug here for BeanShell—my free, open source, lightweight Java scripting language. BeanShell is just what we alluded to in the previous section—a Java application that uses the Reflection API to execute Java statements and expressions dynamically. You can use BeanShell interactively to quickly try out some of the examples in this book or take it further to start using Java syntax as a scripting language in your applications. BeanShell exercises the Java Reflection API to its fullest and serves as a demonstration of how dynamic the Java runtime environment really is.

You can find a copy of BeanShell at its own website. See Appendix B for more information on getting started. We hope you find it both interesting and useful!

[20] This Date constructor is deprecated but will serve us for this example.

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