The abstract keyword is used to declare abstract methods and classes. An abstract method has no implementation defined; it is declared with arguments and a return type as usual, but the body enclosed in curly braces is replaced with a semicolon. The implementation of an abstract method is provided by a subclass of the class in which it is defined. If an abstract method appears in a class, the class is also abstract.
Metadata added to Java source code using the @ tag syntax. Annotations can be used by the compiler or at runtime to augment classes, provide data or mappings, or flag additional services.
A popular, XML-based build tool for Java applications. Ant builds can compile, package, and deploy Java source code as well as generate documentation and perform other activities through pluggable "targets."
- API (Application Programmer Interface)
An API consists of the methods and variables programmers use to work with a component or tool in their applications. The Java language APIs consist of the classes and methods of the java.lang, java.util, java.io, java.text, and java.net packages and many others.
An embedded Java application that runs in the context of an applet viewer, such as a web browser.
- <applet> tag
An HTML tag that embeds an applet within a web document.
Sun's application that runs and displays Java applets outside of a web browser.
A Java program that runs standalone, as compared with an applet.
- apt (Annotation Processing Tool)
A frontend for the Java compiler that processes annotations via a pluggable factory architecture, allowing users to implement custom compile-time annotations.
A language feature used to test for conditions that should be guaranteed by program logic. If a condition checked by an assertion is found to be false, a fatal error is thrown. For added performance, assertions can be disabled when an application is deployed.
Discrete or transactional in the sense that an operation happens as a unit, in an all-or-nothing fashion. Certain operations in the Java virtual machine (VM) and provided by the Java concurrency API are atomic.
- AWT (Abstract Window Toolkit)
This is the Title of the Book, eMatter Edition Copyright © 2005 O'Reilly & Associates, Inc. All rights reserved.
- AWT (Abstract Window Toolkit)
Java's original platform-independent windowing, graphics, and user interface toolkit.
An open source, lightweight, Java-compatible scripting language that can be used for Java experimentation, teaching, application extension, configuration, and debugging.
The mystical, spectral, alter-ego of a Snark. From the Lewis Carroll poem "The Hunting of the Snark," 1876.
A primitive Java data type that contains a true or falsevalue.
In Java generics, a limitation on the type of a type parameter. An upper bound specifies that a type must extend (or is assignable to) a specific Java class. A lower bound is used to indicate that a type must be a supertype of (or is assignable from) the specified type.
Wrapping of primitive types in Java by their object wrapper types. See also unboxing.
A primitive Java data type that's an 8-bit two's-complement signed number.
A behavior that is defined by one object and then later invoked by another object when a particular event occurs. The Java event mechanism is a kind of callback.
The changing of the apparent type of a Java object from one type to another, specified type. Java casts are checked both statically by the Java compiler and at runtime.
The Java catchstatement introduces an exception-handling block of code following a trystatement. The catchkeyword is followed by an exception type and argument name in parentheses and a block of code within curly braces.
An electronic document using a digital signature to assert the identity of a person, group, or organization. Certificates attest to the identity of a person or group and contain that organization's public key. A certificate is signed by a certificate authority with its digital signature.
- certificate authority (CA)
An organization that is entrusted to issue certificates, taking whatever steps are necessary to verify the real-world identity for which it is issuing the certificate.
A primitive Java data type; a variable of type charholds a single 16-bit Unicode character.
1. The fundamental unit that defines an object in most object-oriented programming languages. A class is an encapsulated collection of variables and methods that may have privileged access to one another. Usually a class can be instantiated to produce an object that's an instance of the class, with its own unique set of data. 2. The classkeyword is used to declare a class, thereby defining a new object type.
An instance of the class java.lang.ClassLoader, which is responsible for loading Java binary classes into the Java VM. Classloaders help partition classes based on their source for both structural and security purposes and can also be chained in a parent-child hierarchy.
- class method
See static method.
The sequence of path locations specifying directories and archive files containing compiled Java class files and resources, which are searched in order to find components of a Java application.
- class variable
See static variable.
The consumer of a resource or the party that initiates a conversation in the case of a networked client/server application. See also server.
- Collections API
Classes in the core java.utilpackage for working with and sorting structured collections or maps of items. This API includes the Vectorand Hashtableclasses as well as newer items such as List, Map, and Queue.
- compilation unit
The unit of source code for a Java class. A compilation unit normally contains a single class definition and, in most current development environments, is simply a file with a .java extension.
A program that translates source code into executable code.
- component architecture
A methodology for building parts of an application. It is a way to build reusable objects that can be easily assembled to form applications.
Combining existing objects to create another, more complex object. When you compose a new object, you create complex behavior by delegating tasks to the internal objects. Composition is different from inheritance, which defines a new object by changing or refining the behavior of an old object. See also inheritance.
A special method that is invoked automatically when a new instance of a class is created. Constructors are used to initialize the variables of the newly created object. The constructor method has the same name as the class and no explicit return value.
- content handler
A class that is called to parse a particular type of data and that converts it to an appropriate object.
A packet of data normally sent using a connectionless protocol such as UDP, which provides no guarantees about delivery or error checking and provides no control information.
- data hiding
- deep copy
A duplicate of an object along with all of the objects that it references, transitively. A deep copy duplicates the entire "graph" of objects, instead of just duplicating references. See also shallow copy.
- DOM (Document Object Model)
An in-memory representation of a fully parsed XML document using objects with names like Element, Attribute, and Text. The Java XML DOM API binding is standardized by the World Wide Web Consortium (W3C).
A Java primitive data type; a doublevalue is a 64-bit (double-precision) floating-point number.
- DTD (Document Type Definition)
A document containing specialized language that expresses constraints on the structure of XML tags and tag attributes. DTDs are used to validate an XML document and can constrain the order and nesting of tags as well as the allowed values of attributes.
- EJB (Enterprise JavaBeans)
A server-side business component architecture named for, but not significantly related to, the JavaBeans component architecture. EJBs represent business services and database components and provide declarative security and transactions.
The object-oriented programming technique of limiting the exposure of variables and methods to simplify the API of a class or package. Using the private and protected keywords, a programmer can limit the exposure of internal ("black box") parts of a class. Encapsulation reduces bugs and promotes reusability and modularity of classes. This technique is also known as data hiding.
The Java keyword for declaring an enumerated type. An enum holds a list of constant object identifiers that can be used as a typesafe alternative to numeric constants that serve as identifiers or labels.
The implementation technique used by Java generics in which generic type information is removed (erased) and distilled to raw Java types at compilation. Erasure provides backward compatibility with nongeneric Java code, but introduces some difficulties in the language.
1. A user's action, such as a mouse-click or key press. 2. The Java object delivered to a registered event listener in response to a user action or other activity in the system.
A signal that some unexpected condition has occurred in the program. In Java, exceptions are objects that are subclasses of Exceptionor Error(which themselves are subclasses of Throwable). Exceptions in Java are "raised" with the tHRowkeyword and handled with the catchkeyword. See also catch, throw, and throws.
- exception chaining
The design pattern of catching an exception and throwing a new, higher level, or more appropriate exception that contains the underlying exception as its cause. The "cause" exception can be retrieved if necessary.
A keyword used in a classdeclaration to specify the superclass of the class being defined. The class being defined has access to all the publicand protectedvariables and methods of the superclass (or, if the class being defined is in the same package, it has access to all non-private variables and methods). If a class definition omits the extendsclause, its superclass is taken to be java.lang.Object.
A keyword modifier that may be applied to classes, methods, and variables. It has a similar, but not identical, meaning in each case. When finalis applied to a class, it means that the class may never be subclassed. java.lang.System is an example of a finalclass. When finalis applied to a variable, the variable is a constanti.e., it can't be modified.
A reserved method name. The finalize( ) method is called by the Java VM when an object is no longer being used (i.e., when there are no further references to it) but before the object's memory is actually reclaimed by the system. A finalizer should perform cleanup tasks and free system resources before the object is discarded by Java's garbage-collection system.
A keyword that introduces the finally block of a try/catch/finallyconstruct. catch and finallyblocks provide exception handling and routine cleanup for code in a tryblock. The finallyblock is optional and appears after the TRyblock, and after zero or more catchblocks. The code in a finallyblock is executed once, regardless of how the code in the try block executes. In normal execution, control reaches the end of the TRyblock and proceeds to the finallyblock, which generally performs any necessary cleanup.
A Java primitive data type; a floatvalue is a 32-bit (single-precision) floating-point number represented in IEEE 754 format.
- garbage collection
The process of reclaiming the memory of objects no longer in use. An object is no longer in use when there are no references to it from other objects in the system and no references in any local variables on the method call stack.
The syntax and implementation of parameterized types in the Java language, added in Java 5.0. Generic types are Java classes that are parameterized by the user on one or more additional Java types to specialize the behavior of the class. Generics are sometimes referred to as templates in other languages.
- generic class
A class that uses the Java generics syntax and is parameterized by one or more type variables, which represent class types to be substituted by the user of the class. Generic classes are particularly useful for container objects and collections that can be specialized to operate on a specific type of element.
- generic method
A method that uses the Java generics syntax and has one or more arguments or return types that refers to type variables representing the actual type of data element the method will use. The Java compiler can often infer the types of the type variables from the usage context of the method.
- graphics context
A drawable surface represented by the java.awt.Graphics class. A graphics context contains contextual information about the drawing area and provides methods for performing drawing operations in it.
- GUI (graphical user interface)
A traditional, visual user interface consisting of a window containing graphical items such as buttons, text fields, pull-down menus, dialog boxes, and other standard interface components.
A random-looking identifying number, based on the data content of an object, used as a kind of signature for the object. A hashcode is used to store an object in a hashtable (or hash map). See also hash table.
- hash table
An object that is like a dictionary or an associative array. A hash table stores and retrieves elements using key values called hashcodes. See also hashcode.
The human-readable name given to an individual computer attached to the Internet.
An early web browser written in Java, capable of downloading and running Java applets.
- HTTP (Hypertext Transfer Protocol)
The protocol used by web browsers or other clients to talk to web servers. The simplest form of the protocol uses the commands GETto request a file and POST to send data.
- IDE (Integrated Development Environment)
A GUI tool such as NetBeans or Eclipse that provides source editing, compiling, running, debugging, and deployment functionality for developing Java applications.
A keyword used in class declarations to indicate that the class implements the named interface or interfaces. The implements clause is optional in class declarations; if it appears, it must follow the extends clause (if any). If an implements clause appears in the declaration of a non-abstractclass, every method from each specified interface must be implemented by the class or by one of its superclasses.
The importstatement makes Java classes available to the current class under an abbreviated name or disambiguates classes imported in bulk by other import statements. (Java classes are always available by their fully qualified name, assuming the appropriate class file can be found relative to the CLASSPATHenvironment variable and that the class file is readable. import doesn't make the class available; it just saves typing and makes your code more legible.) Any number of importstatements may appear in a Java program. They must appear, however, after the optional packagestatement at the top of the file, and before the first class or interface definition in the file.
An important feature of object-oriented programming that involves defining a new object by changing or refining the behavior of an existing object. Through inheritance an object implicitly contains all of the non-privatevariables and methods of its superclass. Java supports single inheritance of classes and multiple inheritance of interfaces.
- inner class
A class definition that is nested within another class or a method. An inner class functions within the lexical scope of another class.
An occurrence of something, usually an object. When a class is instantiated to produce an object, we say the object is an instance of the class.
- instance method
A non-staticmethod of a class. Such a method is passed an implicit thisreference to the object that invoked it. See also static; static method.
A Java operator that returns TRueif the object on its left side is an instance of the class (or implements the interface) specified on its right side. instanceofreturns false if the object isn't an instance of the specified class or doesn't implement the specified interface. It also returns falseif the specified object is null.
- instance variable
A non-staticvariable of a class. Each instance of a class has an independent copy of all of the instance variables of the class. See also class variable; static.
A primitive Java data type that's a 32-bit two's-complement signed number.
1. A keyword used to declare an interface. 2. A collection of abstract methods that collectively define a type in the Java language. Classes implementing the methods may declare that they implement the interface type and instances of them may be treated as that type.
The process of making an application accessible to people who speak a variety of languages. Sometimes abbreviated I18N.
The module that decodes and executes Java bytecode. Most Java bytecode is not, strictly speaking, interpreted any longer but compiled to native code dynamically by the Java VM.
The process by which a JavaBean provides additional information about itself, supplementing information learned by reflection.
- ISO 8859-1
An 8-bit character encoding standardized by the ISO. This encoding is also known as Latin-1 and contains characters from the Latin alphabet suitable for English and most languages of western Europe.
A component architecture for Java. It is a way to build interoperable Java objects that can be manipulated easily in a visual application builder environment.
- Java beans
Java classes that are built following the JavaBeans design patterns and conventions.
A language developed early in the history of the Web by Netscape for creating dynamic web pages. From a programmer's point of view, it's unrelated to Java, although some of its syntax is similar.
- JAXB (Java API for XML Binding)
A Java API that allows for generation of Java classes from XML DTD or Schema descriptions and the generation of XML from Java classes.
- JAXP (Java API for XML Parsers)
The Java API that allows for pluggable implementations of XML and XSL engines. This API provides an implementation- neutral way to construct parsers and transforms.
The Java API for XML Remote Procedure Calls, used by web services.
- JDBC (Java Database Connectivity)
The standard Java API for talking to an SQL (structural query language) database.
A native Java XML DOM created by Jason Hunter and Brett McLaughlin. JDOM is easier to use than the standard DOM API for Java. It uses the Java collections API and standard Java conventions. Available at http://www.jdom.org/.
- JWSDP (Java Web Services Developer Pack)
A bundle of standard extension APIs packaged as a group with an installer from Sun. The JWSDP includes JAXB, JAX-RPC, and other XML and web services -related packages.
A nickname for ISO 8859-1.
- layout manager
An object that controls the arrangement of components within the display area of a Swing or AWT container.
- lightweight component
A pure Java GUI component that has no native peer in the AWT.
- local variable
A variable that is declared inside a method. A local variable can be seen only by code within that method.
- Logging API
The Java API for structured logging and reporting of messages from within application components. The Logging API supports logging levels indicating the importance of messages, as well as filtering and output capabilities.
A primitive Java data type that's a 64-bit two's-complement signed number.
- message digest
A cryptographically computed number based on the content of a message, used to determine whether the message's contents have been changed in any way. A change to a message's contents will change its message digest. When implemented properly, it is almost impossible to create two similar messages with the same digest.
The object-oriented programming term for a function or procedure.
- method overloading
Provides definitions of more than one method with the same name but with different argument lists. When an overloaded method is called, the compiler determines which one is intended by examining the supplied argument types.
- method overriding
Defines a method that matches the name and argument types of a method defined in a superclass. When an overridden method is invoked, the interpreter uses "dynamic method lookup" to determine which method definition is applicable to the current object. In Java 5.0, overridden methods can have different return types, with restrictions.
- Model-View-Controller (MVC) framework
A user-interface design that originated in Smalltalk. In MVC, the data for a display item is called the "model." A "view" displays a particular representation of the model, and a "controller" provides user interaction with both. Java incorporates many MVC concepts.
A keyword placed before a class, variable, or method that alters the item's accessibility, behavior, or semantics. See also abstract; final; native; private; protected; public; static; synchronized.
- NaN (not-a-number)
This is a special value of the doubleand float data types that represents an undefined result of a mathematical operation, such as zero divided by zero.
- native method
A method that is implemented in a native language on a host platform, rather than being implemented in Java. Native methods provide access to such resources as the network, the windowing system, and the host filesystem.
new is a unary operator that creates a new object or array (or raises an OutOfMemoryException if there is not enough memory available).
The Java "new" I/O package. A core package introduced in Java 1.4 to support asynchronous, interruptible, and scalable I/O operations. The NIO API supports non-threadbound "select" style I/O handling.
null is a special value that indicates a reference- type variable doesn't refer to any object instance. Static and instance variables of classes default to the value nullif not otherwise assigned.
The fundamental structural unit of an object-oriented programming language, encapsulating a set of data and behavior that operates on that data.
An instance of a class, having the structure of the class but its own copy of data elements. See also instance.
- <object> tag
An HTML tag used to embed media objects and applications into web browsers like the <applet>tag.
The packagestatement specifies the Java package for a Java class. Java code that is part of a particular package has access to all classes (publicand non-public) in the package, and all non-privatemethods and fields in all those classes. When Java code is part of a named package, the compiled class file must be placed at the appropriate position in the CLASSPATHdirectory hierarchy before it can be accessed by the Java interpreter or other utilities. If the package statement is omitted from a file, the code in that file is part of an unnamed default package. This is convenient for small test programs run from the command line, or during development because it means the code can be interpreted from the current directory.
- <param> tag
An HTML tag used within <applet> ... </applet> to specify a named parameter and string value to an applet within a web page.
- parameterized type
A class, using Java generics syntax, that is dependent on one or more types to be specified by the user. The user-supplied parameter types fill in type values in the class and adapt it for use with the specified types.
A modular application component for a web browser designed to extend the browser's capabilities to handle a specific type of data (MIME type). The Java Plug-in supports Java applets in browsers that do not have up-to-date Java runtime support.
One of the fundamental principles of an object-oriented language. Polymorphism states that a type that extends another type is a "kind of" the parent type and can be used interchangeably with the original type, but augmenting or refining its capabilities.
- Preferences API
The Java API for storing small amounts of information on a per-user or systemwide basis across executions of the Java VM. The Preferences API is analogous to a small database or the Windows registry.
- primitive type
One of the Java data types: boolean, char, byte, short, int, long, float, double. Primitive types are manipulated, assigned, and passed to methods "by value" (i.e., the actual bytes of the data are copied). See also reference type.
A style of text formatting originating in the C language, relying on an embedded identifier syntax and variable-length argument lists to supply parameters.
The privatekeyword is a visibility modifier that can be applied to method and field variables of classes. A private method or field is not visible outside its class definition and cannot be accessed by subclasses.
A keyword that is a visibility modifier; it can be applied to method and field variables of classes. A protectedfield is visible only within its class, within subclasses, and within the package of which its class is a part. Note that subclasses in different packages can access only protectedfields within themselves or within other objects that are subclasses; they cannot access protected fields within instances of the superclass.
- protocol handler
A URL component that implements the network connection required to access a resource for a type of URL scheme (such as HTTP or FTP). A Java protocol handler consists of two classes: a StreamHandler and a URLConnection.
A keyword that is a visibility modifier; it can be applied to classes and interfaces and to the method and field variables of classes and interfaces. A publicclass or interface is visible everywhere. A non-publicclass or interface is visible only within its package. A publicmethod or variable is visible everywhere its class is visible. When none of the private, protected, or publicmodifiers are specified, a field is visible only within the package of which its class is a part.
- public-key cryptography
A cryptographic system that requires public and private keys. The private key can decrypt messages encrypted with the corresponding public key, and vice versa. The public key can be made available to the public without compromising security and used to verify that messages sent by the holder of the private key must be genuine.
A list-like data structure normally used in a first-in-first-out fashion to buffer work items.
- raw type
In Java generics, the plain Java type of a class without any generic type parameter information. This is the true type of all Java classes after they are compiled. See also erasure.
- reference type
Any object or array. Reference types are manipulated, assigned, and passed to methods "by reference." In other words, the underlying value is not copied; only a reference to it is. See also primitive type.
The ability of a programming language to interact with structures of the language itself at runtime. Reflection in Java allows a Java program to examine class files at runtime to find out about their methods and variables, and to invoke methods or modify variables dynamically.
- regular expression
A compact yet powerful syntax for describing a pattern in text. Regular expressions can be used to recognize and parse most kinds of textual constructs allowing for wide variation in their form.
- Regular Expression API
The core java.util.regexpackage for using regular expressions. The regex package can be used to search and replace text based on sophisticated patterns.
- Remote Method Invocation (RMI)
RMI is a native Java distributed object system. With RMI, you can pass references to objects on remote hosts and invoke methods in them as if they were local objects.
- SAX (Simple API for XML)
SAX is an event-driven API for parsing XML documents in which the client receives events in response to activities such as the opening of tags, character data, and the closing of tags.
XML Schemas are a replacement for DTDs. Introduced by the W3C, XML Schema is an XML-based language for expressing constraints on the structure of XML tags and tag attributes, as well as the structure and type of the data content. Other types of XML schema languages have different syntaxes.
- SDK (Software Development Kit)
A package of software distributed by Sun Microsystems for Java developers. It includes the Java interpreter, Java classes, and Java development tools: compiler, debugger, disassembler, applet viewer, stub file generator, and documentation generator. Also called the JDK.
The Java class that defines the methods the system calls to check whether a certain operation is permitted in the current environment.
To serialize means to put in order or make sequential. A serialized object is an object that has been packaged so that it can be stored or transmitted over the network. Serialized methods are methods that have been synchronized with respect to threads so that only one may be executing at a given time.
The party that provides a resource or accepts a request for a conversation in the case of a networked client/server application. See also client.
A Java application component that implements the javax.servlet.ServletAPI allowing it to run inside a servlet container or web server. Servlets are widely used in web applications to process user data and generate HTML or other forms of output.
- servlet context
In the Servlet API, it is the web application environment of a servlet that provides server and application resources. The base URL path of the web application is also often referred to as the servlet context.
To declare a variable with the same name as a variable defined in a superclass. We say the variable "shadows" the superclass's variable. Use the superkeyword to refer to the shadowed variable or refer to it by casting the object to the type of the superclass.
- shallow copy
A copy of an object that duplicates only values contained in the object itself. References to other objects are repeated as references and are not duplicated themselves. See also deep copy.
A primitive Java data type that's a 16-bit two's-complement signed number.
Referring to a digital signature. A combination of a message's message digest, encrypted with the signer's private key, and the signer's certificate, attesting to the signer's identity. Someone receiving a signed message can get the signer's public key from the certificate, decrypt the encrypted message digest, and compare that result with the message digest computed from the signed message. If the two message digests agree, the recipient knows that the message has not been modified and that the signer is who he or she claims to be.
Referring to a Java method. The method name and argument types and possibly return type, collectively uniquely identifying the method in some context.
- signed applet
An applet packaged in a JAR file signed with a digital signature, allowing for authentication of its origin and validation of the integrity of its contents.
- signed class
A Java class (or Java archive) that has a signature attached. The signature allows the recipient to verify the class's origin and that it is unmodified. The recipient can therefore grant the class greater runtime privileges.
A networking API originating in BSD Unix. A pair of sockets provide the endpoints for communication between two parties on the network. A server socket listens for connections from clients and creates individual server-side sockets for each conversation.
A GUI component that displays a value and a pair of small up and down buttons that increment or decrement the value. The Swing JSpinnercan work with number ranges and dates as well as arbitrary enumerations.
A keyword that is a modifier applied to method and variable declarations within a class. A staticvariable is also known as a class variable as opposed to non-static instance variables. While each instance of a class has a full set of its own instance variables, there is only one copy of each static class variable, regardless of the number of instances of the class (perhaps zero) that are created. staticvariables may be accessed by class name or through an instance. Non-staticvariables can be accessed only through an instance.
- static import
A statement, similar to the class and package import, that imports the names of static methods and variables of a class into a class scope. The static import is a convenience that provides the effect of global methods and constants.
- static method
A method declared static. Methods of this type are not passed implicit thisreferences and may refer only to class variables and invoke other class methods of the current class. A class method may be invoked through the class name, rather than through an instance of the class.
- static variable
A variable declared static. Variables of this type are associated with the class, rather than with a particular instance of the class. There is only one copy of a static variable, regardless of the number of instances of the class that are created.
A flow of data, or a channel of communication. All fundamental I/O in Java is based on streams. The NIO package uses channels, which are packet oriented.
A sequence of character data and the Java class used to represent this kind of character data. The Stringclass includes many methods for operating on string objects.
A class that extends another. The subclass inherits the publicand protected methods and variables of its superclass. See also extends.
A keyword used by a class to refer to variables and methods of its parent class. The special reference superis used in the same way as the special reference thisis used to qualify references to the current object context.
A parent class, extended by some other class. The superclass's publicand protected methods and variables are available to the subclass. See also extends.
A keyword used in two related ways in Java: as a modifier and as a statement. First, it is a modifier applied to class or instance methods. It indicates that the method modifies the internal state of the class or the internal state of an instance of the class in a way that is not thread-safe. Before running a synchronizedclass method, Java obtains a lock on the class, to ensure that no other threads can modify the class concurrently. Before running a synchronizedinstance method, Java obtains a lock on the instance that invoked the method, ensuring that no other threads can modify the object at the same time.
Java also supports a synchronizedstatement that serves to specify a "critical section" of code. The synchronizedkeyword is followed by an expression in parentheses and a statement or block of statements. The expression must evaluate to an object or array. Java obtains a lock on the specified object or array before executing the statements.
- TCP (Transmission Control Protocol)
A connection-oriented, reliable protocol. One of the protocols on which the Internet is based.
Within an instance method or constructor of a class, thisrefers to "this object" the instance currently being operated on. It is useful to refer to an instance variable of the class that has been shadowed by a local variable or method argument. It is also useful to pass the current object as an argument to static methods or methods of other classes. There is one additional use of this: when it appears as the first statement in a constructor method, it refers to one of the other constructors of the class.
An independent stream of execution within a program. Since Java is a multithreaded programming language, more than one thread may be running within the Java interpreter at a time. Threads in Java are represented and controlled through the Threadobject.
- thread pool
A group of "recyclable" threads used to service work requests. A thread is allocated to handle one item and then returned to the pool.
The throwstatement signals that an exceptional condition has occurred by throwing a specified Throwable(exception) object. This statement stops program execution and passes it to the nearest containing catch statement that can handle the specified exception object.
The throwskeyword is used in a method declaration to list the exceptions the method can throw. Any exceptions a method can raise that are not subclasses of Erroror RuntimeExceptionmust either be caught within the method or declared in the method's throwsclause.
The trykeyword indicates a guarded block of code to which subsequent catch and finallyclauses apply. The trystatement itself performs no special action. See also catch and finally for more information on the try/catch/finallyconstruct.
- type instantiation
In Java generics, the point at which a generic type is applied by supplying actual or wildcard types as its type parameters. A generic type is instantiated by the user of the type, effectively creating a new type in the Java language specialized for the parameter types.
- type invocation
See type instantiation. The term "type invocation" is sometimes used by analogy with the syntax of method invocation.
- UDP (User Datagram Protocol)
A connectionless unreliable protocol. UDP describes a network data connection based on datagrams with little packet control.
Unwrapping a primitive value that is held in its object wrapper type and retrieving the value as a primitive.
A universal standard for text character encoding, accommodating the written forms of almost all languages. Unicode is standardized by the Unicode Consortium. Java uses Unicode for its charand String types.
- UTF-8 (UCS transformation format 8-bit form)
An encoding for Unicode characters (and more generally, UCS characters) commonly used for transmission and storage. It is a multibyte format in which different characters require different numbers of bytes to be represented.
- variable-length argument list
A method in Java may indicate that it can accept any number of a specified type of argument after its initial fixed list of arguments. The arguments are handled by packaging them as an array.
See variable-length argument list.
A dynamic array of elements.
A kind of theorem prover that steps through the Java bytecode before it is run and makes sure that it is well-behaved and does not violate the Java security model. The bytecode verifier is the first line of defense in Java's security model.
- WAR file (Web Applications Resources file)
A JAR file with additional structure to hold classes and resources for web applications. A WAR file includes a WEB-INF directory for classes, libraries, and the web.xml deployment file.
- web application
An application that runs on a web server or application server, normally using a web browser as a client.
- web service
An application-level service that runs on a server and is accessed in a standard way using XML for data marshaling and HTTP as its network transport.
- wildcard type
In Java generics, a "*" syntax used in lieu of an actual parameter type for type instantiation to indicate that the generic type represents a set or supertype of many concrete type instantiations.
An XML standard and Java API for inclusion of XML documents.
- XML (Extensible Markup Language)
A universal markup language for text and data, using nested tags to add structure and meta-information to the content.
An XML standard and Java API for matching elements and attributes in XML using a hierarchical, regex-like expression language.
- XSL/XSLT (Extensible Stylesheet Language/XSLTransformations)
An XML-based language for describing styling and transformation of XML documents. Styling involves simple addition of markup, usually for presentation. XSLT allows complete restructuring of documents, in addition to styling.