glossary

Question 1

abstract class

Answer 1

A class that defines acommonmessage protocol andcommonset of instance variables fori ts subclasses. In Javaanabstract classcannot be instantiated.

Question 2

access level

Answer 2

The scope foraccess that exists for a softwarecomponent such as amethod. There are four access levels in Java: public, private, protected and default. The default level occurs whennoaccess modifier is specified

Question 3

access modifier

Answer 3

Oneofthree Java keywords (public, private, protected)which specify thev isibility of variables andmethods.

Question 4

actual argument

Answer 4

An actual argument is a value used in a message that is copied to a formal argument for use inside the corresponding method. Actual arguments must be of a compatible type with their corresponding formal arguments. They are sent to a method or constructor e.g. add(no1,no2)

Question 5

annotation

Answer 5

a piece of code in a source file that indicates a programmer’s intention to the compiler. The only annotation we use in M250 is @Override.
All annotations begin with the character @ in Java.

Question 6

array

Answer 6

An indexable, fixed-size collection whose elements are all of the same type.

Question 7

array initialiser

Answer 7

An expression that can be used to initialise an array using assignment, a shortcut way of initialising array elements.

Question 8

assert

Answer 8

The keyword in a Java assertion statement used to indicate a condition that should be true in order for the code to be correct, particularly used in private methods.

Question 9

assertion

Answer 9

A statement in the Java language that enables you to test your assumptions about your program; a condition that a programmer believes should be true.
assign See assignment.

Question 10

assignment

Answer 10

the process that results in the variable on the left-hand side of the assignment operator receiving a copy of the value on the right-hand side of the assignment operator. The value may be a primitive value or a reference to an object. If the right-hand side of the assignment operator is an expression, it is evaluated first.

Question 11

assignment operator

Answer 11

An operator (=) used to assign a value to a variable in an assignment statement.

Question 12

attribute

Answer 12

Some property or characteristic of an object that can be accessed using a getter method. Attributes are generally implemented by instance variables. Examples of such attributes are position and colour for Frog objects, and balance and holder for Account objects.

Question 13

auto-boxing

Answer 13

This is the automatic process of wrapping primitive values when called for by the context of usage in a Java program.

Question 14

auto-unboxing

Answer 14

This is the automatic process of unwrapping primitive values when called for by the context of usage in a Java program.

Question 15

bucket

Answer 15

A data structure that can contain zero or more unsorted elements. A HashSet uses buckets to implement a set. The hash code of each incoming element is used to decide which bucket to store it in.

Question 16

buffer

Answer 16

Could refer to either an area used for temporary storage as data is transferred between a data source and a data sink, or a sequence of unfilled components that allows a StringBuilder object to grow.

Question 17

bug

Answer 17

The cause of a run-time error.

Question 18

call stack

Answer 18

A representation of the order of methods called at some point during the execution of a program, shown as a stack of method names, with the first called method on the bottom and the last called method on the top.

Question 19

catch

Answer 19

The process of catching an exception, and the keyword introducing the clause in a try-catch statement that handles an exception.

Question 20

chaining

Answer 20

Constructors are said to be xxxed, meaning that when an object is created, the constructors of all its superclasses are also called, either explicitly or implicitly.

Question 21

checked exception

Answer 21

An exception that the compiler requires the programmer to explicitly handle or declare may be thrown in a method header using a throws clause. These exceptions are ones that a programmer may reasonably be expected to catch or should be made aware of.checked at compile time ioexception

Question 22

header

Answer 22

The line in a class definition which gives its access modifier, name and, optionally, the name of a class it extends and the name(s) of any interface(s) it implements. Example usage: public class WeatherFrog extends Frog implements WeatherClient class method

Question 23

class variable

Answer 23

a variable declared with the keyword static. associated with a class rather than with any of its instances, although each instance of a class can access its own class’s class variables. A class only ever has one copy of each of its class variables. See qualified for details of how class variables are accessed.

Question 24

collection

Answer 24

consists of a number of, possibly zero, objects (strictly object references) or primitives. The objects or primitives within a collection are referred to as elements.

Question 25

Collection Class

Answer 25

A set of library classes used to create various kinds of collection such as tree structures and queues. When capitalised, this phrase usually refers to classes that are part of Java’s Collections Framework, which excludes array types. When used in lower case, the same phrase generally means any class that implements a collection (in the looser English language sense), which includes array types.

Question 26

Collections Framework

Answer 26

Framework includes a set of collection interfaces (Collection, Set, List, Map, and others), a set of collection classes (e.g. HashSet and HashMap) that implement them, and some supporting utility classes (e.g. Collections).

Question 27

common message protocol

Answer 27

A set of messages shared by a number of classes.

Question 28

Comparable

Answer 28

An interface that allows an element type to be sorted. It contains a single method: compareTo().

Question 29

compareTo()

Answer 29

The sole method of the Comparable interface. Allows classes of objects implementing this interface to be sorted (i.e. gives them a natural ordering).
comparison operators A set of operators used for comparing values of expressions, including ==, > and < .

Question 30

compilation error

Answer 30

An error detected by a compiler when code is compiled (which is known as compile-time). No bytecode is generated if there are

Question 31

compiler

Answer 31

Software which checks that text written in a high-level language is correctly formed and, as far as can be determined before compilation, that it is meaningful source code for the language.

Question 32

component type

Answer 32

The type of the components of an array object; this determines the types of the object or primitive value that can be stored in the array.

Question 33

concrete class

Answer 33

A class which is not abstract; a class for which instances can be created.

Question 34

constant instance variable

Answer 34

are usually declared as final static variables. However, sometimes it makes more sense to define a constant as a final instance variable.

Question 35

constructor

Answer 35

A programming construct, similar to a method, used to initialise a newly created object.

Question 36

constructor chaining

Answer 36

The process whereby constructors use super() to invoke constructors higher up their inheritance hierarchy.

Question 37

data hiding

Answer 37

The use of access modifiers to restrict access to class members, particularly instance variables, so that an object becomes a ‘black box

Question 38

declaration

Answer 38

A statement in which memory is reserved for a variable of some type that is formed by writing the type of the variable followed by some name (its identifier

Question 39

default constructor

Answer 39

A zero-argument constructor that simply invokes super(). This constructor is provided automatically by the compiler when the programmer has not specified any constructor in a class.

Question 40

defensive programming

Answer 40

A programming technique in which a method provides alternative paths to deal with expected and unexpected arguments. The method may return a Boolean to indicate success or failure, or (when necessary) use an exception to signal a failure.

Question 41

delimiter

Answer 41

A character used to mark where some part of a program starts or ends. For example, in Java, the character ; marks the end of a statement and the character { marks the beginning of a statement block.

Question 42

design by contract (DbC)

Answer 42

A programming technique in which a specification states a precondition for the correct use of a method, and a postcondition that the method will achieve in the event that its precondition is met. An exception is thrown by a method when its precondition is not met.

Question 43

destructive operation

Answer 43

An operation on a collection that changes the contents of the collection in some way is said to be destructive. You may need to examine the Java API to determine whether a method is destructive or non-destructive.

Question 44

dynamic binding

Answer 44

The postponing of what method to select for execution in response to a message until run-time. The method selected depends on the class of object that receives the corresponding message, rather than on the type of the variable that references it.

Question 45

effective length

Answer 45

The number of meaningful elements that a fixed-size collection actually holds.

Question 46

encapsulation

Answer 46

the parcelling up of information and behaviour into a reusable component. incorporating into a single entity (the object) both the data (instance variables) and the behaviour (methods) defined for that data.

Question 47

equals()

Answer 47

A method that determines whether two objects are equal or not. Since it is implemented by the class Object, all classes of object understand the message equals(), but the method is overridden to define versions of equality appropriate to particular classes. For many classes, such as String, the method equals() is defined effectively to mean ‘has the same state as’.

Question 48

exception

Answer 48

An object that is thrown by a method or the JVM as the result of a run-time error

Question 49

failing fast

Answer 49

The programming philosophy which recommends that errors be signalled as soon as possible so that their cause can be more easily traced, and so that errors are not allowed to be passed on to other parts of a software system, potentially causing problems later. In Java this is done by throwing an exception.

Question 50

final

Answer 50

prevents a variable from ever having its value reassigned once it has an initial value

Question 51

finally

Answer 51

introduces a block of code in a try-catch statement that will always be executed, regardless of whether an exception occurs or not.

Question 52

final

Answer 52

prevents a variable from ever having its value reassigned once it has an initial value.

Question 53

floating point

Answer 53

A type capable of representing a decimal number (to a restricted level of accuracy).

Question 54

flushing

Answer 54

The process of emptying a buffer so that no data remains in it, the data being transferred to a sink.

Question 55

formal argument

Answer 55

A typed identifier used in a method signature between parentheses to stand for a valuethat is passed into the method body by a message.

Question 56

hiding

Answer 56

The situation in which a superclass member is not directly accessible in a subclass due to the subclass defining a member with the same name (for a variable) or same signature (for a method).

Question 57

helper method

Answer 57

A method whose purpose is to do some internal work of an object and which therefore should have private access so that it cannot be used by objects of other classes.

Question 58

homogeneous collection

Answer 58

A collection in which all the elements are of the same type

Question 59

implements

Answer 59

A keyword in Java used in a class header to specify that the class implements a particular interface

Question 60

indirect subclass

Answer 60

if it inherits from that class via one or more intermediate classes.

Question 61

indirect superclass

Answer 61

if it is above that class in the class hierarchy but not directly above it.

Question 62

inheritance

Answer 62

A relationship between classes by which they are organised into a hierarchy. According to the Java Language Specification (JLS), classes lower in a hierarchy are said to inherit all the accessible members from classes higher in the hierarchy, other than those they override or hide. However, in general object-oriented terms, an object of class T is said to inherit all the members of the superclasses of T.

Question 63

instance method

Answer 63

Code that is executed as the result of a message being sent to an object.

Question 64

intersection

Answer 64

a mathematical operation on two or more sets that results in a set containing only the elements common to both sets. The term is used to refer both to the result and to the operation.

Question 65

lazy evaluation

Answer 65

A type of evaluation used in compound Boolean expressions involving the && and || operators whereby the second operand is evaluated only when it is necessary to do so in order to determine the truth value of the whole expression.

Question 66

map

Answer 66

A collection type that relates a set of keys to their corresponding values

Question 67

message

Answer 67

a request for an object to do something. The only way to make an object do something is to send it a message. For example, the position of a Frog object changes when it is sent left()

Question 68

overloading

Answer 68

overloaded when there are other methods, defined in the same class, or inherited (in the JLS sense) from some superclass, with the same name but a different method signature, i.e. different types and/or numbers of arguments.

Question 69

overriding

Answer 69

The process of redefining (replacing) a method that would have been inherited
(JLS definition) from a superclass so as to cause it to have different behaviour.

Question 70

polymorphic method

Answer 70

A method with the same signature as some other method, but which typically defines different behaviour. For example the method home() defines different behaviour for Frog, Toad and HoverFrog objects

Question 71

postcondition

Answer 71

A condition that is true after some code (particularly a method) is executed.

Question 72

precondition

Answer 72

A condition that must be true before some code is executed in order for that code to behave correctly. The user of a method typically checks its precondition before using it.

Question 73

private

Answer 73

An access modifier that restricts access to a class member to objects of the class to which it belongs.

Question 74

protected

Answer 74

Java access modifier used to restrict access of a member of a class X to subclasses of X and classes within the same package as class X.

Question 75

protocol

Answer 75

The set of messages an object understands.

Question 76

reference type variable

Answer 76

A variable declared to hold a reference to an object of the declared type (or a compatible type).

Question 77

run-time error

Answer 77

A programming error that becomes apparent only when the program is run

Question 78

Scanner

Answer 78

A class used to read string tokens from a source

Question 79

scope

Answer 79

describes the areas of program code from which the variable may be used.

Question 80

static

Answer 80

A Java keyword that defines a variable or method as belonging to a class rather than its instances.

Question 81

super()

Answer 81

Used within a constructor to invoke the constructor without any arguments in the direct superclass. May also be used with n arguments to invoke a superclass constructor with n formal arguments.

Question 82

this()

Answer 82

Used within a constructor to invoke the constructor without any arguments in the current class. May also be used with n arguments to invoke a constructor with n formal arguments in the current class.

Question 83

unchecked exception

Answer 83

An exception for which the compiler does not require any explicit exception handling code and that the compiler does not require the programmer to declare as thrown in a method header. These are errors that may not be easily recovered from, or should not be recovered from; instead, program logic or other issues may need to be resolved

Question 84

utility class

Answer 84

A utility class is one that provides methods (usually static) that perform common functions. The Math class is an example.

Question 85

Writer

Answer 85

which are used to write 16-bit character streams.

Question 86

Reader

Answer 86

are used to read 16-bit character streams