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.
An API consists of the functions and variables
programmers use in their applications. The Java
API consists of all
protected methods of all
classes in the
An embedded Java application that runs in the context of an applet viewer, such as a Web browser.
HTML tag that specifies an applet run within a Web document.
Sun's application that implements the additional structure needed to run and display Java applets.
A Java program that runs standalone; i.e., it doesn't require an applet viewer.
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
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.
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
An electronic document used to verify 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.
An organization that is entrusted to issue certificates, taking whatever steps are necessary to verify the identity for which it is issuing the certificate.
A primitive Java data type; a variable of type
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.
class keyword is used to declare a class,
thereby defining a new object type. Its syntax is similar
struct keyword in C.
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.
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
The directory path specifying the location of compiled Java class files on the local system.
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.
The 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.
Any of the GUI primitives implemented in the
java.awt package as subclasses of
TextField (among many others) are components.
A component architecture is a methodology for building parts of an application. It is a way to build reusable objects that can be easily assembled to form applications.
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
A class that is called to parse a particular type of data and converts it to an appropriate object.
A packet of data sent to a receiving computer without warning, error checking, or other control information.
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
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.
The overall design of the mechanism for sending and receiving events. The Java event model changed between the 1.0 and 1.1 releases in order to allow more explicit control over how and when events are delivered from sources to listeners.
A signal that some unexpected condition has occurred in the
program. In Java, exceptions are objects that are subclasses of
themselves are subclasses of
in Java are "raised" with the
throw keyword and
received with the
catch keyword. See
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
and methods of the superclass (or, if the class being defined is in
the same package, it has access to all non-
variables and methods). If a class definition omits the
extends clause, its superclass is taken to be
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
applied to a class, it means that the class may never be
java.lang.System is an example of a
final class. When
applied to a variable, the variable is a constant; i.e., it can't be
finalize is not actually a Java keyword, but a
reserved method name. The
finalize() method 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
blocks provide exception handling and routine cleanup for code in
try block. The
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
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.
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").
A drawable surface represented by the
class. A graphics context contains contextual information about the
drawing area and provides methods for performing drawing operations in
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
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
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
An interface in the
java.awt.image package that
represents an image source (i.e., a source of pixel data).
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
abstract class, every method from each
specified interface must be implemented by the class or by one of its
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
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
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.
A class definition that is nested within another class. An inner class functions within the lexical scope of another class.
An object. When a class is instantiated to produce an object, we say the object is an instance of the class.
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
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.
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
static variable of a class. Copies of such
variables occur in every instance of the created class. See also class
A primitive Java data type that's a 32-bit two's-complement signed number (in all implementations).
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 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.
The process by which a Java Bean provides additional information about itself, supplementing information learned by reflection.
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.
Sun's Web browser-based tool written in Java for the development of Java applications.
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.
Individual Java Beans are Java classes that are built using certain design patterns and naming conventions.
A language, developed by Netscape, for creating dynamic Web pages. 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.
The standard Java API for talking to an SQL (structural query language) database.
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.
An object that controls the arrangement of components within the
display area of a container. The
contains a number of layout managers that provide different layout
A nickname for ISO8859-1.
A Java component that has no native peer in the AWT.
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).
A long number computed from 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. It is almost impossible to create two similar messages with the same digest.
The object-oriented programming term for a function or procedure.
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.
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
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.
This is a special value of the
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
native method should have a semicolon
instead of a body. A
native method cannot be
abstract, but all other method modifiers may be used
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.
A proposed HTML tag that may replace the widely used but nonstandard
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) in the package, and all
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.
HTML tag used within
</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
One of the Java data types:
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.
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
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.
Software that describes and enables the use of a new protocol. A
protocol handler consists of two classes: a
StreamHandler and a
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
or interface is visible everywhere. A non-
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
public modifiers are specified, a field is visible only within the package of which its
class is a part.
A cryptographic system that requires two keys, a public key and a private key. The private key can be used to decrypt messages encrypted with the corresponding public key, and vice versa. The public key can be made available to the public without compromising cryptographic security.
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. Reflection in Java allows a Java program to examine class files at run-time to find out about their methods and variables, and to invoke methods or modify variables dynamically.
RMI is a native Java distributed object system. With RMI you can pass references to objects on remote hosts and invoke methods in them just as if they were local objects.
The base of a hierarchy, such as a root class, whose descendants are
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.
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 so that only one may be executing at a given time.
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
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.
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 run-time privileges.
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.
static keyword is a modifier applied to method
and variable declarations within a class. A
variable is also known as a class variable as opposed to
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
static variables may be accessed by class
name or through an instance. Non-
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
A class that extends another. The subclass inherits the
protected methods and
variables of its superclass. See
super refers to the same value as
this: the instance of the class for which the
current method (these keywords are valid only within
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
protected methods and
variables are available to the subclass. See
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
synchronized class method, Java obtains a
lock on the class, to ensure that no other threads can modify the
class concurrently. Before running a
instance method, Java obtains a lock on the instance that invoked the
method, ensuring that no other threads can modify the object at the
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
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
that can handle the specified exception object. Note that
throw keyword must be followed by an exception
object, not an exception class.
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
RuntimeException must either be caught within the
method or declared in the method's
The property of the Java API that defines the look and feel of the user interface on a specific platform.
try keyword indicates a block of code to which
clauses apply. The
try statement itself performs no
special action. See the entries for
finally for more information on the
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
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.