### Introduction to Programming Using Java, Version 9, Swing Edition

## Glossary

abstraction. Abstraction refers, in general, to the idea of providing a simplified or higher level interface to a complex system. It is closely related to the idea of a "black box." Abstraction makes it possible to understand or use a system, while ignoring some of the details of what actually goes on in the system. For example, control abstractions such as if statements and while loops are actually implemented in machine language by jump and conditional jump instructions, but it is possible—and easier—to use if statements and while loops without knowing anything about their machine language implementation. Control abstractions make it possible to implement algorithms on a higher level than machine language. Abstraction is a fundamental concept in computer science.

abstract class. A class that cannot be used to create objects, and that exists only for the purpose of creating subclasses. Abstract classes in Java are defined using the modifier abstract.

abstract data type (ADT). A data type for which the possible values of the type and the permissible operations on those values are specified, without specifying how the values and operations are to be implemented.

access specifier. A modifier used on a method definition or variable specification that determines what classes can use that method or variable. The access specifiers in Java are public, protected, and private. A method or variable that has no access specifier is said to have "package" visibility.

activation record. A data structure that contains all the information necessary to implement a subroutine call, including the values of parameters and local variables of the subroutine and the return address to which the computer will return when the subroutine ends. Activation records are stored on a stack, which makes it possible for several subroutine calls to be active at the same time. This is particularly important for recursion, where several calls to the same subroutine can be active at the same time.

actual parameter. A parameter in a subroutine call statement, whose value will be passed to the subroutine when the call statement is executed. Actual parameters are also called "arguments".

address. Each location in the computer's memory has an address, which is a number that identifies that location. Locations in memory are numbered sequentially. In modern computers, each byte of memory has its own address. Addresses are used when information is being stored into or retrieved from memory.

algorithm. An unambiguous, step-by-step procedure for performing some task, which is guaranteed to terminate after a finite number of steps.

alpha color component. A component of a color that says how transparent or opaque that color is. The higher the alpha component, the more opaque the color.

ALU. Arithmetic Logic Unit. The ALU is the part of the CPU that performs arithmetic operations such as addition and subtraction and logical operations such as AND and OR.

API. Application Programming Interface. A specification of the interface to a software package or "toolbox." The API says what classes or subroutines are provided in the toolbox and how to use them.

applet. A type of Java program that is meant to run on a Web page in a Web browser, as opposed to a stand-alone application.

animation. An apparently moving picture created by rapidly showing a sequence of still images, called frames, one after the other. In Java, animations are often driven by Timer objects; a new frame of the animation is shown each time the timer fires.

antialiasing. Adjusting the color of pixels to reduce the "jagged" effect that can occur when shapes and text are represented by pixels. For antialiased drawing, when the shape covers only part of a pixel, the color of the shape is blended with the previous color of the pixel. The degree of blending depends on how much of the pixel is covered.

array. A list of items, sequentially numbered. Each item in the list can be identified by its index, that is, its sequence number. In Java, all the items in array must have the same type, called the base type of the array. An array is a random access data structure; that is, you can get directly at any item in the array at any time.

array type. A data type whose possible values are arrays. If Type is the name of a type, then Type[] is the array type for arrays that have base type Type.

assignment statement. A statement in a computer program that retrieves or computes a value and stores that value in a variable. An assignment statement in Java has the form: variable-name = expression;

asynchronous event. An event that can occur at an unpredictable time, outside the control of a computer program. User input events, such as pressing a button on the mouse, are asynchronous.

ASCII. American Standard Code for Information Interchange. A way of encoding characters using 7 bits for characters. ASCII code only supports 128 characters, with no accented letters, non-English alphabets, special symbols, or ideograms for non-alphabetic languages such as Chinese. Java uses the much larger and more complete Unicode code for characters.

base case. In a recursive algorithm, a simple case that is handled directly rather than by applying the algorithm recursively.

binary number. A number encoded as a sequence of zeros and ones. A binary number is represented in the "base 2" in the same way that ordinary numbers are represented in the "base 10."

binary tree. A linked data structure that is either empty or consists of a root node that contains pointers to two smaller (possibly empty) binary trees. The two smaller binary trees are called the left subtree and the right subtree.

bit. A single-digit binary number, which can be either 0 or 1.

black box. A system or component of a system that can be used without understanding what goes on inside the box. A black box has an interface and an implementation. A black box that is meant to be used as a component in a system is called a module.

block. In Java programming, a sequence of statements enclosed between a pair of braces, { and }. Blocks are used to group several statements into a single statement. A block can also be empty, meaning that it contains no statements at all and consists of just an empty pair of braces.

blocking operation. An operation, such as reading data from a network connection, is said to "block" if it has to wait for some event to occur. A thread that performs a blocking operation can be "blocked" until the required event occurs. A thread cannot execute any instructions while it is blocked. Other threads in the same program, however, can continue to run.

blocking queue. A queue in which the dequeue operation will block if the queue is empty, until an item is added to the queue. If the blocking queue has a limited capacity, the enqueue operation can also block, if the queue is full.

bottom-up design. An approach to software design in which you start by designing basic components of the system, then combine them into more complex components, and so on.

BufferedImage. A class representing "off-screen canvases," that is, images that are stored in the computer's memory and that can be used for drawing images off-screen.

branch. A control structure that allows the computer to choose among two or more different courses of action. Java has two branch statements: if statements and switch statements.

byte. A unit of memory that consists of eight bits. One byte of memory can hold an eight-bit binary number.

bytecode. "Java bytecode" is the usual name for the machine language of the Java Virtual Machine. Java programs are compiled into Java bytecode, which can then be executed by the JVM.

charset. A particular encoding of character data into binary form. Examples include UTF-8 and ISO-8859-1.

checked exception. An exception in Java that must be handled, either by a try..catch statement or by a throws clause on the method that can throw he exception. Failure to handle a checked exception in one way or the other is a syntax error.

class. The basic unit of programming in Java. A class is a collection of static and non-static methods and variables. Static members of a class are part of the class itself; non-static, or "instance," members constitute a blueprint for creating objects, which are then said to "belong" to the class.

class invariant. A statement about the state of a class, or of an object created from that class, that is always true. Any method in a class, to be correct, must preserve the truth of all class invariants.

class variables and class methods. Alternative terms for "static variables" and "static methods", which are part of the class itself rather than of objects.

client/server. A model of network communication in which a "server" waits at a known address on the network for connection requests that are sent to the server by "clients." This is the basic model for communication using the TCP/IP protocol.

command-line interface. A way of interacting with the computer in which the user types in commands to the computer and the computer responds to each command.

comment. In a computer program, text that is ignored by the computer. Comments are for human readers, to help them understand the program.

compiler. A computer program that translates programs written in some computer language (generally a high-level language) into programs written in machine language.

component. General term for a visual element of a GUI, such as a window, button, or menu.

constructor. A special kind of subroutine in a class whose purpose is to construct objects belonging to that class. A constructor is called using the new operator, and is not considered to be a "method."

container. A component, such as a BorderPane, that can contain other GUI components.

contract of a method. The semantic component of the method's interface. The contract specifies the responsibilities of the method and of the caller of the method. It says how to use the method correctly and specifies the task that the method will perform when it is used correctly. The contract of a method should be fully specified by its Javadoc comment.

control structure. A program structure such as an if statement or a while loop that affects the flow of control in a program (that is, the order in which the instructions in the program are executed).

CPU. Central Processing Unit. The CPU is the part of the computer that actually performs calculations and carries out programs.

CSS. Cascading Style Sheets, a language that can be used to control the visual appearance of components in JavaFX or of elements on a web page.

data structure. An organized collection of data, that can be treated as a unit in a program.

deadlock. A situation in which several threads hang indefinitely, for example because each of them is waiting for some resource that is locked by one of the other threads.

default method. A method in a Java interface that has an implementation. The default implementation is used in any class that implements the interface but does not override the method. Default methods are marked with the reserved word default. Not supported in Java 7 and earlier.

default package. The unnamed package. A class that does not declare itself to be in a named package is considered to be in the default package.

definite assignment. Occurs at a particular point in a program if it is definitely true that a given variable must have been assigned a value before that point in the program. It is only legal to use the value of a local variable if that variable has "definitely" been assigned a value before it is used. For this to be true, the compiler must be able to verify that every path through the program from the declaration of the variable to its use must pass through a statement that assigns a value to that variable.

deprecated. Considered to be obsolete, but still available for backwards compatibility. A deprecated Java class or method is still part of the Java language, but it is not advisable to use it in new code. Deprecated items might be removed in future versions of Java.

dialog box. A window that is dependent on another window, called its parent owner. Dialog boxes are usually popped up to get information from the user or to display a message to the user.

distributed computing. A kind of parallel processing in which several computers, connected by a network, work together to solve a problem.

dummy parameter. Identifier that is used in a subroutine definition to stand for the value of an actual parameter that will be passed to the subroutine when the subroutine is called. Dummy parameters are also called "formal parameters" (or sometimes just "parameters," when the term "argument" is used instead of actual parameter).

enum. Enumerated type. A type that is defined by listing every possible value of that type. An enum type in Java is a class, and the possible values of the type are objects.

event. In GUI programming, something that happens outside the control of the program, such as a mouse click, and that the program must respond to when it occurs.

exception. An error or exceptional condition that is outside the normal flow of control of a program. In Java, an exception can be represented by an object of type Throwable that can be caught and handled in a try..catch statement.

factory method. A method, usually a static function, that returns an object. Factory methods are an alternative to constructors.

fetch-and-execute cycle. The process by which the CPU executes machine language programs. It fetches (that is, reads) an instruction from memory and carries out (that is, executes) the instruction, and it repeats this over and over in a continuous cycle.

fill. A drawing operation that applies a color (or other type of fill) to each of the pixels inside a shape.

flag. A boolean value that is set to true to indicate that some condition or event is true. A single bit in a binary number can also be used as a flag.

formal parameter. Another term for "dummy parameter."

frame. One of the images that make up an animation. Also used as another name for activation record.

function. A subroutine that returns a value.

functional interface. A Java interface that defines only a single subroutine (where the term "interface" here means an interface that defines a Java type.)

garbage collection. The automatic process of reclaiming memory that is occupied by objects that can no longer be accessed.

generic programming. Writing code that will work with various types of data, rather than with just a single type of data. The Java Collection Framework, and classes that use similar techniques, are examples of generic programming in Java.

getter. An instance method in a class that is used to read the value of some property of that class. Usually the property is just the value of some instance variable. By convention, a getter is named getXyz() where xyz is the name of the property.

global variable. Another name for member variable, emphasizing the fact that a member variable in a class exists outside the methods of that class.

graphics context. The data and methods necessary for drawing to some particular destination. A graphics context in Swing is an object belonging to the Graphics class.

GUI. Graphical User Interface. The modern way of interacting with a computer, in which the computer displays interface components such as buttons and menus on a screen and the user interacts with them—for example by clicking on them with a mouse.

hash table. A data structure optimized for efficient search, insertion, and deletion of objects. A hash table consists of an array of locations, and the location in which an object is stored is determined by that object's "hash code," an integer that can be efficiently computed from the contents of the object.

heap. The section of the computer's memory in which objects are stored.

high level language. A programming language, such as Java, that is convenient for human programmers but that has to be translated into machine language before it can be executed.

HSB. A color system in which colors are specified by three numbers (in Java, real numbers in the range 0.0 to 1.0) giving the hue, saturation, and brightness.

IDE. Integrated Development Environment. A programming environment with a graphical user interface that integrates tools for creating, compiling, and executing programs.

identifier. A sequence of characters that can be used as a name in a program. Identifiers are used as names of variables, methods, and classes.

index. The position number of one item in an array.

implementation. The inside of a black box, such as the code that defines a subroutine.

immutable object. An immutable object cannot be modified after it is constructed, because all of its instance variables are final. (In my use of the term, an immutable object can contain pointers to other objects that are not immutable.)

infinite loop. A loop that never ends, because its continuation condition always evaluates to true.

inheritence. The fact that one class can extend another. It then inherits the data and behavior of the class that it extends.

instance of a class. An object that belongs to that class (or a subclass of that class). An object belongs to a class in this sense when the class is used as a template for the object when the object is created by a constructor defined in that class.

instance method. A non-static method in a class and hence a method in any object that is an instance of that class.

instance variable. A non-static variable in a class and hence a variable in any object that is an instance of that class.

interface. As a general term, how to use a black box such as a subroutine. Knowing the interface tells you nothing about what goes on inside the box. "Interface" is also a reserved word in Java; in this sense, an interface is a type that specifies one or more abstract methods. An object that implements the interface must provide definitions for those methods.

interpreter. A computer program that executes program written in some computer language by reading instructions from the program, one-by-one, and carrying each one out (by translating it into equivalent machine language instructions).

I/O. Input/Output, the way a computer program communicates with the rest of the world, such as by displaying data to the user, getting information from the user, reading and writing files, and sending and receiving data over a network.

I/O stream. An abstraction representing a source of input data or a destination for output data. Java has four basic IO stream classes representing input and output of character and binary data. These classes form the foundation for Java's input/output API.

iterator. An object associated with a collection, such a list or a set, that can be used to traverse that collection. The iterator will visit each member of the collection in turn.

Java Collection Framework (JCF). A set of standard classes that implement generic data structures, including ArrayList and TreeSet, for example.

JavaFX. A toolkit for GUI applications, which was introduced as a more modern alternative to the Swing GUI toolkit. JavaFX is not a standard part of Java and is not covered in this textbook.

JDK. Java Development Kit. Basic software that supports both compiling and running Java programs. A JDK includes a command-line programming environment as well as a JRE. You need a JDK if you want to compile Java source code, as well as executing pre-compiled programs.

JRE. Java Runtime Environment. Basic software that supports running standard Java programs that have already been compiled. A JRE includes a Java Virtual Machine and all the standard Java classes.

just-in-time compiler. A kind of combination interpreter/compiler that compiles parts of a program as it interprets them. This allows subsequent executions of the same parts of the program to be executed more quickly than they were the first time. This can result is greatly increased speed of execution. Modern JVMs use a just-in-time compiler.

JVM. Java Virtual Machine. The imaginary computer whose machine language is Java bytecode. Also used to refer to computer programs that act as interpreters for programs written in bytecode; to run Java programs on your computer, you need a JVM.

lambda expression. A notation that defines an anonymous method. More precisely, a lambda expression is a kind of literal that represents a value whose type is given by a functional interface.

linked data structure. A collection of data consisting of a number of objects that are linked together by pointers which are stored in instance variables of the objects. Examples include linked lists and binary trees.

linked list. A linked data structure in which nodes are linked together by pointers into a linear chain.

listener. In GUI programming, an object that can be registered to be notified when events of some given type occur. The object is said to "listen" for the events.

literal. A sequence of characters that is typed in a program to represent a constant value. For example, 'A' is a literal that represents the constant char value, A, when it appears in a Java program.

location (in memory). The computer's memory is made up of a sequence of locations. These locations are sequentially numbered, and the number that identifies a particular location is called the address of that location.

local class. A class that is defined inside a method definition. Local classes are often anonymous, but that is not required.

local variable. A variable declared within a method, for use only inside that method. A variable declared inside a block is valid from the point where it is declared until the end of block in which the declaration occurs.

loop. A control structure that allows a sequence of instructions to be executed repeatedly. Java has three kinds of loops: for loops, while loops, and do loops

loop control variable. A variable in a for loop whose value is modified as the loop is executed and is checked to determine whether or not to end the loop.

loop invariant. A statement such that, if the statement is true before a loop executes, then it will remain true after each execution of the loop, and therefore will still be true after the loop ends. Loop invariants can be a tool for proving correctness of loops.

machine language. A programming language consisting of instructions that can be executed directed by a computer. Instructions in machine language are encoded as binary numbers. Each type of computer has its own machine language. Programs written in other languages must be translated into a computer's machine language before they can be executed by that computer.

main memory. Programs and data can be stored in a computer's main memory, where they are available to the CPU. Other forms of memory, such as a disk drive, also store information, but only main memory is directly accessible to the CPU. Programs and data from a disk drive have to be copied into main memory before they can be used by the CPU.

map (data structure). An associative array; a data structure that associates an object from some collection to each object in some set. In Java, maps are represented by the generic interface Map<T,S>

map (stream operator). One of the fundamental operations on streams, defined as part of Java's stream API. A map operator applies a function to each element of a stream, producing a new stream consisting of the values output by the function.

member variable. A variable defined in a class but not inside a method, as opposed to a local variable, which is defined inside some method.

memory. Memory in a computer is used to hold programs and data.

method. Another term for subroutine, used in the context of object-oriented programming. A method is a subroutine that is contained in a class or in an object.

method reference. A notation for a lambda expression that represents a method that already exists in some class or object. A method reference uses the :: operator, such as Math::sqrt.

module. In general, a component of a larger system that interacts with the rest of the system in a simple, well-defined, straightforward manner. In Java 9 and later, a module is a collection of Java packages, allowing explicit control of dependencies between different modules, and the standard Java packages have been divided into q set of modules.

multitasking. Performing multiple tasks at once, either by switching rapidly back and forth from one task to another or by literally working on multiple tasks at the same time.

multiprocessing. Multitasking in which more than one processor is used, so that multiple tasks can literally be worked on at the same time.

mutual exclusion. Prevents two threads from accessing the same resource at the same time. In Java, this only applies to threads that access the resource in synchronized methods or synchronized statements. Mutual exclusion can prevent race conditions but introduces the possibility of deadlock.

MVC pattern. The Model/View/Controller pattern, a strategy for dividing responsibility in a GUI component. The model is the data for the component. The view is the visual presentation of the component on the screen. The controller is responsible for reacting to events by changing the model. According to the MVC pattern, these responsibilities should be handled by different objects.

NaN. Not a Number. Double.NaN is a special value of type double that represents an undefined or illegal value.

node. Common term for one of the objects in a linked data structure.

null. A special pointer value that means "not pointing to anything."

numerical analysis. The field that studies algorithms that use approximations, such as real numbers, and the errors that can result from such approximation.

off-by-one error. A common type of error in which one too few or one too many items are processed, often because counting is not being handled correctly or because the processing stops too soon or continues too long for some other reason.

object. An entity in a computer program that can have data (variables) and behaviors (methods). An object in Java must be created using some class as a template. The class of an object determines what variables and methods it contains.

object type. A type whose values are objects, as opposed to primitive types. Classes and interfaces are object types.

observable value. A value that generates an event when it is modified, so that observers of the value can be notified of the change and can react to it.

OOP. Object-Oriented Programming. An approach to the design and implementation of computer programs in which classes and objects are created to represent concepts and entities and their interactions.

operating system. The basic software that is always running on a computer, without which it would not be able to function. Examples include Linux, MacOS, and Windows Vista.

operator. A symbol such as "+", "<=", or "++" that represents an operation that can be applied to one or more values in an expression.

overloading (of operators). The fact that the same operator can be used with different types of data. For example, the "+" operator can be applied to both numbers and strings.

overloading (of method names). The fact that several methods that are defined in the same class can have the same name, as long as they have different signatures.

overriding. Redefining in a subclass. When a subclass provides a new definition of a method that is inherited from a superclass, the new definition is said to override the original definition.

package. In Java, a named collection of related classes and subpackages, such as java.io and javax.swing.

parallel processing. When several tasks are being performed simultaneously, either by multiple processors or by one processor that switches back and forth among the tasks.

parameter. Used to provide information to a subroutine when that subroutine is called. Values of "actual parameters" in the subroutine call statement are assigned to the "dummy parameters" in the subroutine definition before the code in the subroutine is executed.

parameterized type. A type such as ArrayList<String> that includes one or more type parameters (String in the example).

parsing. Determining the syntactical structure of a string in some language. To parse a string is to determine whether the string is legal according to the grammar of the language, and if so, how it can be created using the rules of the grammar.

partially full array. An array that is used to store varying numbers of items. A partially full array can be represented as a normal array plus a counter to keep track of how many items are actually stored.

pixel. A "picture element" on the screen or in an image. A picture consists of rows and columns of pixels. The color of each pixel can be individually set.

polymorphism. The fact that the meaning of a call to an instance method can depend on the actual type of the object that is used to make the call at run time. That is, if var is a variable of object type, then the method that is called by a statement such as var.action() depends on the type of the object to which var refers when the statement is executed at run time, not on the type of variable var.

pointer. A value that represents an address in the computer's memory, and hence can be thought of as "pointing" to the location that has that address. A variable in Java can never hold an object; it can only hold a pointer to the location where the object is stored. A pointer is also called a "reference."

pragmatics. Rules of thumb that describe what it means to write a good program. For example, style rules and guidelines about how to structure a program are part of the pragmatics of a programming language.

precedence. The precedence of operators determines the order in which they are applied, when several operators occur in an expression, in the absence of parentheses.

precondition. A condition that must be true at some point in the execution of a program, in order for the program to proceed correctly from that point. A precondition of a subroutine is something that must be true when the subroutine is called, in order for the subroutine to function properly. Subroutine preconditions are often restrictions on the values of the actual parameters that can be passed into the subroutine.

predicate. A function that outputs a boolean value. Predicates in Java can be represented by the parameterized functional interface Predicate<T>.

priority queue. A data structure representing a collection of items where each item has a "priority." A priority queue has operations add and remove. Items can be added in any order, but the remove operation always removes an item of minimal priority. (Some version of priority queue use maximum instead of minimum priority.)

postcondition. A condition that is known to be true at some point in the execution of a program, as a result of the computation that has come before that point. A postcondition of a subroutine is something that must be true after the subroutine finishes its execution. A postcondition of a function often describe the return value of the function.

primitive type. One of the eight basic built-in data types in Java, double, float, long, int, short, byte, boolean, and char. A variable of primitive type holds an actual value, as opposed to a pointer to that value.

priority of a thread. An integer associated with a thread that can affect the order in which threads are executed. A thread with greater priority is executed in preference to a thread with lower priority.

producer/consumer. A classic pattern in parallel programming in which one or more producers produce items that are consumed by one or more consumers, and the producers and consumers are meant to run in parallel. The problem is to get items safely and efficiently from the producers to the consumers. In Java, the producer/consumer pattern is implemented by blocking queues.

program. A set of instructions to be carried out by a computer, written in an appropriate programming language. Used as a verb, it means to create such a set of instructions.

programming language. A language that can be used to write programs for a computer. Programming languages range in complexity from machine language to high-level languages such as Java.

protocol. A specification of what constitutes legal communication in a give context. A protocol specifies the format of legal messages, when they can be sent, what kind of reply is expected, and so on.

pseudocode. Informal specification of algorithms, expressed in language that is closer to English than an actual programming language, and usually without filling in every detail of the procedure.

queue. A data structure consisting of a list of items, where items can only be added at one end and removed at the opposite end of the list.

race condition. A source of possible errors in parallel programming, where one thread can cause an error in another thread by changing some aspect of the state of the program that the second thread is depending on (such as the value of variable).

RAM. Random Access Memory. This term is often used as a synonym for the main memory of a computer. Technically, however, it means memory in which all locations are equally accessible at any given time. The term also implies that data can be written to the memory as well as read from it.

record. A simple data structure containing several data items, or fields, that are identified by name. The fields can be of different types. Java has record classes to represent (immutable) records.

recursion. Defining something in terms of itself. In particular, a recursive subroutine is one that calls itself, either directly, or indirectly through a chain of other subroutines. Recursive algorithms work by reducing a complex problem into smaller problems which can be solved either directly or by applying the same algorithm "recursively."

reduce (stream operator). One of the fundamental operations on a stream. A reduce operation combines all the elements from a stream in some way, such as by summing them or finding their maximum, producing a final result.

RGB. A color system in which colors are specified by three numbers (in Java, integers in the range 0 to 255) giving the red, green, and blue components of the color.

reference. Another term for "pointer."

return type of a function. The type of value that is returned by that function.

reserved word. A sequence of characters that looks like an identifier but can't be used as an identifier because it has a special meaning in the language. For example, class, public, and if are reserved words in Java.

resource. An image, sound, text, or other data file that is part of a program. Resource files for Java programs are stored on the same class path where the compiled class files for the program are stored.

robust program. A program is robust if it is not only correct, but also is capable of handling errors such as a non-existent file or a failed network connection in a reasonable way.

scene graph. In JavaFX, the hierarchical data structure that contains all the GUI components that are shown in a window.

set. A collection of objects which contains no duplicates. In Java, sets are represented by the generic interface Set<T>

scope. The region in a program where the declaration of an identifier is valid.

semantics. Meaning. The semantics rules of a language determine the meaning of strings of symbols (such as sentences or statements) in that language.

sentinel value. A special value that marks the end of a sequence of data values, to indicate the end of the data.

setter. An instance method in a class that is used to set the value of some property of that class. Usually the property is just the value of some instance variable. By convention, a setter is named setXyz() where xyz is the name of the property.

signature of a method. The name of the method, the number of formal parameters in its definition, and the type of each formal parameter. Method signatures are the information needed by a compiler to tell which method is being called by a given subroutine call statement.

socket. An abstraction representing one end of a connection between two computers on a network. A socket represents a logical connection between computer programs, not a physical connection between computers.

stack. A data structure consisting of a list of items where items can only be added and removed at one end of the list, which is known as the "top" of the stack. Adding an item to a stack is called "pushing," and removing an item is called "popping." The term stack also refers to the stack of activation records that is used to implement subroutine calls.

standard input. The standard source from which a program reads input data. It is represented by the object System.in. Usually, standard input comes from text typed by the user, but standard input can be "redirected" to read from another source, such as a file, instead.

standard output. The standard destination to which a program writes output text. It is represented by the object System.out. Usually, standard output is displayed to the user, but standard output can be "redirected" to write to another destination, such as a file, instead. There is also an object System.err that is meant for writing error messages.

state machine. A model of computation where an abstract "machine" can be in any of some finite set of different states. The behavior of the machine depends on its state, and the state can change in response to inputs or events. The basic logical structure of a GUI program can often be represented as a state machine.

step-wise refinement. A technique for developing an algorithm by starting with a general outline of the procedure, often expressed in pseudocode, and then gradually filling in the details.

stream. In Java 8, an abstraction representing a stream of values that can be processed. A stream can be created from a Collection, an array, or some other data source. Java's stream API includes many predefined operations that can be applied to streams. The term "stream" also refers to I/O streams, which are used for input and output.

stroke. A drawing operation that applies a color (or other type of paint) to pixels along the boundary of a shape.

source code. Text written in a high-level programming language, which must be translated into a machine language such as Java bytecode before it can be executed by a computer.

subclass. A class that extends another class, directly or indirectly, and therefore inherits its data and behaviors. The first class is said to be a subclass of the second.

subroutine. A sequence of program instructions that have been grouped together and given a name. The name can then be used to "call" the subroutine. Subroutines are also called methods in the context of object-oriented programming.

subroutine call statement. A statement in a program that calls a subroutine. When a subroutine call statement is executed, the computer executes the code that is inside the subroutine.

super. A special variable, automatically defined in any instance method, that refers to the object that contains the method, but considered as belonging to the superclass of the class in which the method definition occurs. super gives access to members of the superclass that are hidden by members of the same name in the subclass.

syntax. Grammar. The syntax rules of a language determine what strings of symbols are legal—that is, grammatical—in that language.

TCP/IP. Protocols that are used for network communication on the Internet.

this. A special variable, automatically defined in any instance method, that refers to the object that contains the method.

thread. An abstraction representing a sequence of instructions to be executed one after the other. It is possible for a computer to execute several threads in parallel.

thread pool. A collection of "worker threads" that are available to perform tasks. As tasks become available, they are assigned to threads in the pool. A thread pool is often used with a blocking queue that holds the tasks.

top-down design. An approach to software design in which you start with the problems, as a whole, subdivide it into smaller problems, divide those into even smaller problems, and so on, until you get to problems that can be solved directly.

type. Specifies some specific kind of data values. For example, the type int specifies integer data values that can be represented as 32-bit binary numbers. In Java, a type can be a primitive type, a class names, or an interface name. Type names are used to specify the types of variables, of dummy parameters in subroutines, and of return values of subroutines.

type cast. Forces the conversion of a value of one type into another type. For example, in (int)(6*Math.random()), the (int) is a type-cast operation that converts the double value (6*Math.random()) into an integer by discarding the fractional part of the real number.

Unicode. A way of encoding characters as binary numbers. The Unicode character set includes characters used in many languages, not just English. Unicode is the character set that is used internally by Java.

URL. Universal Resource Locator; an address for a resource on the Internet, such as a web page.

variable. A named memory location (or sequence of locations) that can be used to store data. A variable is created in a program, and a name is assigned to the variable, in a variable declaration statement. The name can then be used in that program to refer to the memory location, or to the data stored in that memory location, depending on context. In Java, a variable has a type, which specifies what kind of data it can hold.

wrapper class. A class such as Double or Integer that makes it possible to "wrap" a primitive type value in an object belonging to the wrapper class. This allows primitive type values to be used in contexts were objects are required, such as with the Java Collection Framework.

XML. eXtensible Markup Language. A very common and well-supported standard syntax for creating text-based data-representation languages.

David Eck