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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.

API (Application Programming Interface)

An API consists of the functions and variables programmers are use in their applications. The Java API consists of all public and protected methods of all public classes in the java.applet, java.awt, java.awt.image, java.awt.peer, java.io, java.lang, java.net, and java.util packages.


An embedded Java application that runs in the context of an applet viewer, such as a Web browser.

<applet> tag

HTML tag that specifies an applet run within a Web document.

applet viewer

An application that implements the additional structure needed to run and display Java applets. An applet viewer can be a Web browser like HotJava or Netscape Navigator, or a separate program like Sun's appletviewer.


A Java program that runs standalone; i.e., it doesn't require an applet viewer.

AWT (Abstract Windowing Toolkit)

Java's platform-independent windowing, graphics, and user-interface toolkit.


A primitive Java data type that contains a truth value. The two possible values of a boolean variable are true and false.


A primitive Java data type that's an 8-bit two's-complement signed number (in all implementations).


A behavior that is defined by one object and then later invoked by another object when a particular event occurs.


A technique that explicitly converts one data type to another.


The catch statement introduces an exception-handling block of code following a try statement. The catch keyword is followed by an exception type and argument name in parentheses, and a block of code within curly braces.


A primitive Java data type; a variable of type char holds a single 16-bit Unicode character.


a) An encapsulated collection of data and methods to operate on the data. A class may be instantiated to produce an object that's an instance of the class.

b) The class keyword is used to declare a class, thereby defining a new object type. Its syntax is similar to the struct keyword in C.

class loader

An object in the Java security model that is responsible for loading Java binary classes from the network into the local interpreter. A class loader keeps its classes in a separate namespace, so that loaded classes cannot interact with system classes and breach system security.

class method

A method declared static. Methods of this type are not passed implicit this references 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.

class path

The directory path specifying the location of compiled Java class files on the local system.

class 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.


The application that initiates a conversation as part of a networked client/server application. See server.

compilation unit

The source code for a Java class. A compilation unit normally contains a single class definition, and in most current development environments is just a file with a .java extension.


A program that translates source code into executable code.


Any of the GUI primitives implemented in the java.awt package as subclasses of Component. The classes Button, Choice, and TextField (among many others) are components.


Using objects as part of 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 inheritance.


A 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.


One of the java.awt classes that "contain" GUI components. Components in a container appear within the boundaries of the container. The classes Dialog, Frame, Panel, and Window are containers.

content handler

A class that recognizes the content type of particular data, parses it, and converts it to an appropriate object.


A packet of data sent to a receiving computer without warning, error checking, or other control information.

data hiding

See encapsulation.


A Java primitive data type;a double value is a 64-bit (double-precision) floating-point number.


An object-oriented programming technique that makes an object's data private or protected (i.e., hidden) and allows programmers to access and manipulate that data only through method calls. Done well, encapsulation reduces bugs and promotes reusability and modularity of classes. This technique is also known as data hiding.


A user's action, such as a mouse click or key press.


A signal that some unexpected condition has occurred in the program. In Java, exceptions are objects that are subclasses of Exception or Error (which themselves are subclasses of Throwable). Exceptions in Java are "raised" with the throw keyword and received with the catch keyword. See throw, throws, and catch.


The extends keyword is used in a class declaration to specify the superclass of the class being defined. The class being defined has access to all the public and protected variables 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 extends clause, its superclass is taken to be java.lang.Object.


The final keyword is a modifier that may be applied to classes, methods, and variables. It has a similar, but not identical meaning in each case. When final is applied to a class, it means that the class may never be subclassed. java.lang.System is an example of a final class. When final is applied to a variable, the variable is a constant; i.e., it can't be modified.


finalize is not actually a Java keyword, but a reserved method name. The finalizer is called 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 not handled by Java's garbage-collection system.


This keyword introduces the finally block of a try/catch/finally construct. catch and finally blocks provide exception handling and routine cleanup for code in a try block. The finally block is optional, and appears after the try block, and after zero or more catch blocks. The code in a finally block is executed once, regardless of how the code in the try block executes. In normal execution, control reaches the end of the try block and proceeds to the finally block, which generally performs any necessary cleanup.


A Java primitive data type; a float value 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.


An abbreviation for garbage collection or garbage collector (or occasionally "graphics context").

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 GUI is a user interface constructed from graphical push buttons, text fields, pull-down menus, dialog boxes, and other standard interface components. In Java, GUIs are implemented with the classes in the java.awt package.


An arbitrary-looking identifying number used as a kind of signature for an object. A hashcode stores an object in a hashtable. See hashtable.


An object that is like a dictionary or an associative array. A hashtable stores and retrieves elements using key values called hashcodes. See hashcode.


The name given to an individual computer attached to the Internet.


A WWW browser written in Java that is capable of downloading and running Java applets.


An interface for receiving image data from an image source. Image consumers are usually implemented by the awt.peer interface, so they are largely invisible to programmers.


An interface in the java.awt.image package that receives information about the status of an image being constructed by a particular ImageConsumer.


An interface in the java.awt.image package that represents an image source (i.e., a source of pixel data).


The implements keyword is 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-abstract class, every method from each specified interface must be implemented by the class or by one of its superclasses.


The import statement makes Java classes available to the current class under an abbreviated name. (Java classes are always available by their fully qualified name, assuming the appropriate class file can be found relative to the CLASSPATH environment 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 import statements may appear in a Java program. They must appear, however, after the optional package statement 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. That is, an object implicitly contains all the non-private variables of its superclass and can invoke all the non-private methods of its superclass. Java supports single inheritance of classes and multiple inheritance of interfaces.


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-static method of a class. Such a method is passed an implicit this reference to the object that invoked it. See also class method and static.


instanceof is a Java operator that returns true if the object on its left-hand side is an instance of the class (or implements the interface) specified on its right-hand side. instanceof returns false if the object is not an instance of the specified class or does not implement the specified interface. It also returns false if the specified object is null.

instance variable

A non-static variable of a class. Copies of such variables occur in every instance of the created class. See also class variable and static.


A primitive Java data type that's a 32-bit two's-complement signed number (in all implementations).


The interface keyword is used to declare an interface. More generally, an interface defines a list of methods that enables a class to implement the interface itself.


The module that decodes and executes Java bytecode.


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 4-byte character encoding that includes all of the world's national standard character encodings. Also known as UCS. The 2-byte Unicode character set maps to the range 0x00000000 to 0x0000FFFF of ISO 10646.

Java WorkShop

Sun's Web browser-based tool written in Java for the development of Java applications.

JDK (Java 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, appletviewer, stub file generator, and documentation generator.


A language for creating dynamic Web pages developed by Netscape. From a programmer's point of view, it's unrelated to Java, although some of its capabilities are similar. Internally, there may be a relationship, but even that is unclear.

layout manager

An object that controls the arrangement of components within the display area of a container. The java.awt package contains a number of layout managers that provide different layout styles.


A nickname for ISO8859-1.

local variable

A variable that is declared inside a single method. A local variable can be seen only by code within that method.


A primitive Java data type that's a 64-bit two's-complement signed number (in all implementations).


The object-oriented programming term for a function or procedure.

method overloading

Providing definitions of more than one method with the same name but with different argument lists or return values. When an overloaded method is called, the compiler determines which one is intended by examining the supplied argument types.

method overriding

Defining a method that exactly matches (i.e., same name, same argument types, and same return type) 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.


A keyword placed before a class, variable, or method that alters the item's accessibility, behavior, or semantics. See abstract, final, native, private, private protected, protected, public, static, and synchronized.

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.

NaN (not-a-number)

This is a special value of the double and float data types that represents an undefined result of a mathematical operation, such as zero divided by zero.


native is a modifier that may be applied to method declarations. It indicates that the method is implemented (elsewhere) in C, or in some other platform-dependent fashion. A native method should have a semicolon instead of a body. A native method cannot be abstract, but all other method modifiers may be used with native methods.

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).


null is a special value that indicates a variable doesn't refer to any object. The value null may be assigned to any class or interface variable. It cannot be cast to any integral type, and should not be considered equal to zero, as in C.


An instance of a class. A class models a group of things; an object models a particular member of that group.


The package statement specifies which package the code in the file is part of. Java code that is part of a particular package has access to all classes (public and non-public) in the package, and all non-private methods 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 CLASSPATH directory 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, or during development because it means the code can be interpreted from the current directory.

<param> tag

HTML tag used within <applet> ... </applet> to specify a named parameter and string value to an applet within a Web page.


The actual implementation of a GUI component on a specific platform. Peer components reside within a Toolkit object. See Toolkit.

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.


The private keyword is a visibility modifier that can be applied to method and field variables of classes. A private field is not visible outside its class definition.

private protected

When the private and protected visibility modifiers are both applied to a variable or method in a class, they indicate the field is visible only within the class itself and within subclasses of the class. Note that subclasses can access only private protected fields within themselves or within other objects that are subclasses; they cannot access those fields within instances of the superclass.


The protected keyword is a visibility modifier that can be applied to method and field variables of classes. A protected field is visible only within its class, within subclasses, or within the package of which its class is a part. Note that subclasses in different packages can access only protected fields within themselves or within other objects that are subclasses; they cannot access protected fields within instances of the superclass.

protocol handler

Software that describes and enables the use of a new protocol. A protocol handler consists of two classes: a StreamHandler and a URLConnection.


The public keyword is a visibility modifier that can be applied to classes and interfaces and to the method and field variables of classes and interfaces. A public class or interface is visible everywhere. A non-public class or interface is visible only within its package. A public method or variable is visible everywhere its class is visible. When none of the private, protected or public modifiers is specified, a field is visible only within the package of which its class is a part.

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 base of a hierarchy, such as a root class, whose descendants are subclasses. The java.lang.Object class serves as the root of the Java class hierarchy.


The Java class that defines the methods the system calls to check whether a certain operation is permitted in the current environment.


The application that accepts a request for a conversation as part of a networked client/server application. See client.


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 super keyword to refer to the shadowed variable, or refer to it by casting the object to the type of the superclass.


A primitive Java data type that's a 16-bit two's-complement signed number (in all implementations).


An interface that listens for connections from clients on a data port and connects the client data stream with the receiving application.


The static keyword is a modifier applied to method and variable declarations within a class. A static variable 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. static variables may be accessed by class name or through an instance. Non-static variables can be accessed only through an instance.


A flow of data, or a channel of communication. All fundamental I/O in Java is based on streams.


A class used to represent textual information. The String class includes many methods for operating on string objects. Java overloads the + operator for string concatenation.


A class that extends another. The subclass inherits the public and protected methods and variables of its superclass. See extends.


The keyword super refers to the same value as this: the instance of the class for which the current method (these keywords are valid only within non-static methods) was invoked. While the type of this is the type of the class in which the method appears, the type of super is the type of the superclass of the class in which the method appears. super is usually used to refer to superclass variables shadowed by variables in the current class. Using super in this way is equivalent to casting this to the type of the superclass.


A class extended by some other class. The superclass's public and protected methods and variables are available to the subclass. See extends.


The synchronized keyword is 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 synchronized class method, Java obtains a lock on the class, to ensure that no other threads can modify the class concurrently. Before running a synchronized instance 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 synchronized statement that serves to specify a "critical section" of code. The synchronized keyword 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.


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, this refers 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.


A single, 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 Thread object.


The throw statement signals that an exceptional condition has occurred by throwing a specified exception object. This statement stops program execution and resumes it at the nearest containing catch statement that can handle the specified exception object. Note that the throw keyword must be followed by an exception object, not an exception class.


The throws keyword 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 Error or RuntimeException must either be caught within the method or declared in the method's throws clause.


The property of the Java API that defines the look and feel of the user interface on a specific platform.


The try keyword indicates a block of code to which subsequent catch and finally clauses apply. The try statement itself performs no special action. See the entries for catch and finally for more information on the try/catch/finally construct.

UCS (universal character set)

A synonym for ISO10646.


User Datagram Protocol. A connectionless unreliable protocol. UDP describes a network data connection based on datagrams with little packet control.


A 16-bit character encoding that includes all of the world's commonly used alphabets and ideographic character sets in a "unified" form (i.e., a form from which duplications among national standards have been removed). ASCII and Latin-1 characters may be trivially mapped to Unicode characters. Java uses Unicode for its char and 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.


A dynamic array of elements.


A theorem prover that steps through the Java byte-code before it is run and makes sure that it is well-behaved. The byte-code verifier is the first line of defense in Java's security model.

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