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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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Types

The type system of a programming language describes how its data elements (variables and constants) are associated with storage in memory and how they are related to one another. In a statically typed language, such as C or C++, the type of a data element is a simple, unchanging attribute that often corresponds directly to some underlying hardware phenomenon, such as a register or a pointer value. In a more dynamic language such as Smalltalk or Lisp, variables can be assigned arbitrary elements and can effectively change their type throughout their lifetime. A considerable amount of overhead goes into validating what happens in these languages at runtime. Scripting languages such as Perl achieve ease of use by providing drastically simplified type systems in which only certain data elements can be stored in variables, and values are unified into a common representation, such as strings.

Java combines many of the best features of both statically and dynamically typed languages. As in a statically typed language, every variable and programming element in Java has a type that is known at compile time, so the runtime system doesn’t normally have to check the validity of assignments between types while the code is executing. Unlike traditional C or C++, Java also maintains runtime information about objects and uses this to allow truly dynamic behavior. Java code may load new types at runtime and use them in fully object-oriented ways, allowing casting and full polymorphism (extending of types). Java code may also “reflect” upon or examine its own types at runtime, allowing advanced kinds of application behavior such as interpreters that can interact with compiled programs dynamically.

Java data types fall into two categories. Primitive types represent simple values that have built-in functionality in the language; they are fixed elements, such as literal constants and numbers. Reference types (or class types) include objects and arrays; they are called reference types because they “refer to” a large data type that is passed “by reference,” as we’ll explain shortly. Generic types are really just a kind of composition (combination) of class types and are therefore reference types as well.

Primitive Types

Numbers, characters, and Boolean values are fundamental elements in Java. Unlike some other (perhaps more pure) object-oriented languages, they are not objects. For those situations where it’s desirable to treat a primitive value as an object, Java provides “wrapper” classes. The major advantage of treating primitive values as special is that the Java compiler and runtime can more readily optimize their implementation. Primitive values and computations can still be mapped down to hardware as they always have been in lower-level languages. Later we’ll see how Java can automatically convert between primitive values and their object wrappers as needed to partially mask the difference between the two. We’ll explain what that means in more detail in the next chapter when we discuss boxing and unboxing of primitive values.

An important portability feature of Java is that primitive types are precisely defined. For example, you never have to worry about the size of an int on a particular platform; it’s always a 32-bit, signed, two’s complement number. Table 4-2 summarizes Java’s primitive types.

Table 4-2. Java primitive data types

Type

Definition

boolean

true or false

char

16-bit, Unicode character

byte

8-bit, signed, two’s complement integer

short

16-bit, signed, two’s complement integer

int

32-bit, signed, two’s complement integer

long

64-bit, signed, two’s complement integer

float

32-bit, IEEE 754, floating-point value

double

64-bit, IEEE 754

Note

Those of you with a C background may notice that the primitive types look like an idealization of C scalar types on a 32-bit machine, and you’re absolutely right. That’s how they’re supposed to look. The 16-bit characters were forced by Unicode, and ad hoc pointers were deleted for other reasons. But overall, the syntax and semantics of Java primitive types derive from C.

Floating-point precision

Floating-point operations in Java follow the IEEE 754 international specification, which means that the result of floating-point calculations is normally the same on different Java platforms. However, Java allows for extended precision on platforms that support it. This can introduce extremely small-valued and arcane differences in the results of high-precision operations. Most applications would never notice this, but if you want to ensure that your application produces exactly the same results on different platforms, you can use the special keyword strictfp as a class modifier on the class containing the floating-point manipulation (we cover classes in the next chapter). The compiler then prohibits these platform-specific optimizations.

Variable declaration and initialization

Variables are declared inside of methods and classes with a type name followed by one or more comma-separated variable names. For example:

    int foo;
    double d1, d2;
    boolean isFun;

Variables can optionally be initialized with an expression of the appropriate type when they are declared:

    int foo = 42;
    double d1 = 3.14, d2 = 2 * 3.14;
    boolean isFun = true;

Variables that are declared as members of a class are set to default values if they aren’t initialized (see Chapter 5). In this case, numeric types default to the appropriate flavor of zero, characters are set to the null character (\0), and Boolean variables have the value false. Local variables, which are declared inside a method and live only for the duration of a method call, on the other hand, must be explicitly initialized before they can be used. As we’ll see, the compiler enforces this rule so there is no danger of forgetting.

Integer literals

Integer literals can be specified in octal (base 8), decimal (base 10), or hexadecimal (base 16). A decimal integer is specified by a sequence of digits beginning with one of the characters 1–9:

    int i = 1230;

Octal numbers are distinguished from decimal numbers by a leading zero:

    int i = 01230;             // i = 664 decimal

A hexadecimal number is denoted by the leading characters 0x or 0X (zero “x”), followed by a combination of digits and the characters a–f or A–F, which represent the decimal values 10–15:

    int i = 0xFFFF;            // i = 65535 decimal

Integer literals are of type int unless they are suffixed with an L, denoting that they are to be produced as a long value:

    long l = 13L;
    long l = 13;       // equivalent: 13 is converted from type int

(The lowercase letter l is also acceptable but should be avoided because it often looks like the number 1.)

When a numeric type is used in an assignment or an expression involving a “larger” type with a greater range, it can be promoted to the bigger type. In the second line of the previous example, the number 13 has the default type of int, but it’s promoted to type long for assignment to the long variable. Certain other numeric and comparison operations also cause this kind of arithmetic promotion, as do mathematical expressions involving more than one type. For example, when multiplying a byte value by an int value, the compiler promotes the byte to an int first:

    byte b = 42;
    int i = 43;
    int result = b * i;  // b is promoted to int before multiplication

A numeric value can never go the other way and be assigned to a type with a smaller range without an explicit cast, however:

    int i = 13;
    byte b = i;          // Compile-time error, explicit cast needed
    byte b = (byte) i;   // OK

Conversions from floating-point to integer types always require an explicit cast because of the potential loss of precision.

Finally, we should note that if you are using Java 7 or later, you can add a bit of formatting to your numeric literals by utilizing the “_” underscore character between digits. So if you have particularly large strings of digits, you can break them up as in the following examples:

    int RICHARD_NIXONS_SSN = 567_68_0515;
    int for_no_reason = 1___2___3;
    int JAVA_ID = 0xCAFE_BABE;

Underscores may only appear between digits, not at the beginning or end of a number or next to the “L” long integer signifier.

Floating-point literals

Floating-point values can be specified in decimal or scientific notation. Floating-point literals are of type double unless they are suffixed with an f or F denoting that they are to be produced as a float value. And just as with integer literals, in Java 7 you may use “_” underscore characters to format floating-point numbers—but only between digits, not at the beginning, end, or next to the decimal point or “F” signifier of the number.

    double d = 8.31;
    double e = 3.00e+8;
    float f = 8.31F;
    float g = 3.00e+8F;
    float pi = 3.14_159_265_358;

Binary literals

A new feature of Java 7 is the introduction of binary literal values. This allows you to write out binary values directly by prefixing the number with a “0b” or “0B” (zero B).

    byte one =     (byte)0b00000001;
    byte two =     (byte)0b00000010;
    byte four =    (byte)0b00000100;
    byte sixteen = (byte)0b00001000;
    int cafebabe = 0b11001010111111101011101010111110;
    long lots_o_ones = (long)0b11111111111111111111111111111111111111111111111L;

Character literals

A literal character value can be specified either as a single-quoted character or as an escaped ASCII or Unicode sequence:

    char a = 'a';
    char newline = '\n';
    char smiley = '\u263a';

Reference Types

In an object-oriented language like Java, you create new, complex data types from simple primitives by creating a class. Each class then serves as a new type in the language. For example, if we create a new class called Foo in Java, we are also implicitly creating a new type called Foo. The type of an item governs how it’s used and where it can be assigned. As with primitives, an item of type Foo can, in general, be assigned to a variable of type Foo or passed as an argument to a method that accepts a Foo value.

A type is not just a simple attribute. Classes can have relationships with other classes and so do the types that they represent. All classes in Java exist in a parent-child hierarchy, where a child class or subclass is a specialized kind of its parent class. The corresponding types have the same relationship, where the type of the child class is considered a subtype of the parent class. Because child classes inherit all of the functionality of their parent classes, an object of the child’s type is in some sense equivalent to or an extension of the parent type. An object of the child type can be used in place of an object of the parent’s type. For example, if you create a new class, Cat, that extends Animal, the new type, Cat, is considered a subtype of Animal. Objects of type Cat can then be used anywhere an object of type Animal can be used; an object of type Cat is said to be assignable to a variable of type Animal. This is called subtype polymorphism and is one of the primary features of an object-oriented language. We’ll look more closely at classes and objects in Chapter 5.

Primitive types in Java are used and passed “by value.” In other words, when a primitive value like an int is assigned to a variable or passed as an argument to a method, it’s simply copied. Reference types (class types), on the other hand, are always accessed “by reference.” A reference is simply a handle or a name for an object. What a variable of a reference type holds is a “pointer” to an object of its type (or of a subtype, as described earlier). When the reference is assigned to a variable or passed to a method, only the reference is copied, not the object to which it’s pointing. A reference is like a pointer in C or C++, except that its type is so strictly enforced. The reference value itself can’t be explicitly created or changed. A variable acquires a reference value only through assignment to an appropriate object.

Let’s run through an example. We declare a variable of type Foo, called myFoo, and assign it an appropriate object:[7]

    Foo myFoo = new Foo();
    Foo anotherFoo = myFoo;

myFoo is a reference-type variable that holds a reference to the newly constructed Foo object. (For now, don’t worry about the details of creating an object; we’ll cover that in Chapter 5.) We declare a second Foo type variable, anotherFoo, and assign it to the same object. There are now two identical references : myFoo and anotherFoo, but only one actual Foo object instance. If we change things in the state of the Foo object itself, we see the same effect by looking at it with either reference.

Object references are passed to methods in the same way. In this case, either myFoo or anotherFoo would serve as equivalent arguments:

    myMethod( myFoo );

An important, but sometimes confusing, distinction to make at this point is that the reference itself is a value and that value is copied when it is assigned to a variable or passed in a method call. Given our previous example, the argument passed to a method (a local variable from the method’s point of view) is actually a third reference to the Foo object, in addition to myFoo and anotherFoo. The method can alter the state of the Foo object through that reference (calling its methods or altering its variables), but it can’t change the caller’s notion of the reference to myFoo: that is, the method can’t change the caller’s myFoo to point to a different Foo object; it can change only its own reference. This will be more obvious when we talk about methods later. Java differs from C++ in this respect. If you need to change a caller’s reference to an object in Java, you need an additional level of indirection. The caller would have to wrap the reference in another object so that both could share the reference to it.

Reference types always point to objects, and objects are always defined by classes. However, two special kinds of reference types—arrays and interfaces—specify the type of object they point to in a slightly different way.

Arrays in Java have a special place in the type system. They are a special kind of object automatically created to hold a collection of some other type of object, known as the base type. Declaring an array type reference implicitly creates the new class type designed as a container for its base type, as you’ll see in the next chapter.

Interfaces are a bit sneakier. An interface defines a set of methods and gives it a corresponding type. An object that implements the methods of the interface can be referred to by that interface type, as well as its own type. Variables and method arguments can be declared to be of interface types, just like other class types, and any object that implements the interface can be assigned to them. This adds flexibility in the type system and allows Java to cross the lines of the class hierarchy and make objects that effectively have many types. We’ll cover interfaces in the next chapter as well.

Generic types or parameterized types, as we mentioned earlier, are an extension of the Java class syntax that allows for additional abstraction in the way classes work with other Java types. Generics allow for specialization of classes by the user without changing any of the original class’s code. We cover generics in detail in Chapter 8.

A Word About Strings

Strings in Java are objects; they are therefore a reference type. String objects do, however, have some special help from the Java compiler that makes them look more like primitive types. Literal string values in Java source code are turned into String objects by the compiler. They can be used directly, passed as arguments to methods, or assigned to String type variables:

    System.out.println( "Hello, World..." );
    String s = "I am the walrus...";
    String t = "John said: \"I am the walrus...\"";

The + symbol in Java is “overloaded” to perform string concatenation as well as regular numeric addition. Along with its sister +=, this is the only overloaded operator in Java:

    String quote = "Four score and " + "seven years ago,";
    String more = quote + " our" + " fathers" +  " brought...";

Java builds a single String object from the concatenated strings and provides it as the result of the expression. We discuss the String class and all things text-related in great detail in Chapter 10.



[7] The comparable code in C++ would be:

    Foo& myFoo = *(new Foo());
    Foo& anotherFoo = myFoo;

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