Glossary

Question 1

wrapping

Answer 1

The process of converting a primitive value to an object equivalent, using a wrapper class.

Question 2

state

Answer 2

The values of the attributes of an object constitute its state. The state of an object can vary over time as the values of its attributes change.

A behaviour of an object in response to a message may be dependent on its state.
Example: sending an HoverFrog instance the message upBy()

Question 3

message

Answer 3

A message is 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 the message left() or right(); to obtain information on the value of a Frog object’s colour attribute, you send it the message getColour().

A message must be in the protocol of the receiver.

May return a message answer and/or change the state of the receiver

Question 4

protocol

Answer 4

The set of messages an object understands.

Question 5

public protocol

Answer 5

The part of the protocol of a class or object that is accessible from outside of the class itself.

Question 6

private protocol

Answer 6

The part of the protocol of a class or object that is inaccessible from a context outside of the class itself.

Question 7

message answer

Answer 7

When a message is sent to an object, then, depending on what the message is, a message answer may be returned. A message answer is a value or an object; it is not a message. Sometimes a message answer is used, sometimes it is ignored. A message answer may be used subsequently as the receiver or argument of another message. Getter messages return the value of an attribute, as with the message getColour(), which returns a value such as OUColour.GREEN.

Question 8

message expression

Answer 8

A message-send that evaluates to some value, i.e. the message returns a message answer. We also say that it returns a value.

Question 9

message name

Answer 9

The name of a message, including the following parentheses, but excluding any arguments. For example, the name of the message left() is left(), and the name of the message upBy(6) is upBy().

Question 10

message-send

Answer 10

The code that sends a message to an object – for example, frog1.right(), which consists of the receiver followed by a full stop and then the message.

Question 11

collaboration

Answer 11

The achievement of a software solution using two or more communicating objects.

Question 12

receiver

Answer 12

The object to which a message is sent.

Question 13

class

Answer 13

A class is like a blueprint for the creation of objects and ensures that all its instances have the same instance variables and behave in response to messages in an identical manner.

Question 14

class header

Answer 14

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

Question 15

class method

Answer 15

A class method is a method declared with the keyword static and is associated with a class rather than with any of its instances. Classes are not objects so code in a class method cannot use the expression this or the keyword super. Class methods are invoked directly on the name of the class, using static binding, and so do not exhibit polymorphism.

Question 16

class variable

Answer 16

A class variable is a variable declared with the keyword static. A class variable is 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.

Question 17

qualified

Answer 17

The qualified name of a member is its simple name prefixed by the name of its enclosing class, as in OUDialog.request. The parts of such names are separated by dots. This means that different classes can have members with the same names – the qualified names allow them to be distinguished from one another.

Question 18

superclass

Answer 18

If B is a subclass of A, then A is the superclass of B. In the Java programming language a subclass has only one direct superclass.

Question 19

subclass

Answer 19

A subclass is any class which extends (inherits from) another class. In Java all classes except Object are subclasses of some other class.

Question 20

subclassing

Answer 20

Defining a class as a subclass of an existing class.

Question 21

instance

Answer 21

An object that belongs to a given class is described as an instance of that class.

Question 22

instance method

Answer 22

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

Question 23

instance variable

Answer 23

A variable whose identifier and type is common to all the instances of a class, but whose value is specific to each instance. Each instance variable either contains a reference to an object or contains a value of some primitive type. For example, Frog objects have the instance variables colour and position. The values of the instance variables of a particular object represent the state of that object.

Question 24

invocation

Answer 24

Executing code in a method. Class (static) methods are invoked directly on a class, whereas instance methods are invoked by sending messages to objects. Also called method invocation.

Question 25

getter message

Answer 25

A message that returns as its message answer the value of one of the receiver’s attributes. See also setter message.

Question 26

getter method

Answer 26

A method whose purpose is to return the value of one of an object’s attributes as its message answer. For example, the method getPosition() is a getter method that returns the value of the instance variable position held by instances of the Frog class.

Question 27

behaviour

Answer 27

A term used to describe the way in which an object behaves in response to the messages in its protocol.

Question 28

primitive data type

Answer 28

A set of values together with operations that can be performed on them. The primitive data types in Java provide a set of basic building blocks from which all of the more complex types of data can be built. Java’s primitive data types include int, char and boolean.

Question 29

primitive type variable

Answer 29

A variable declared to hold a value of the declared or compatible primitive data type. Less formally we refer to a primitive variable.

int counter; // declaration
counter = 0; // initialisation

Question 30

primitive value

Answer 30

A value of some primitive type; for example, true, -1.3, 42, or ‘x’ are all primitive values of different primitive types.

Question 31

Integer types

Answer 31

byte - 8 bits
short - 16 bits
int - 32 bits
long - 64 bits

Question 32

declaration statement

Answer 32

Reserves memory for a variable of some type.

int myNumber;
String myString;

Question 33

reference type variable

Answer 33

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

Frog kermit; // declaration
kermit = new Frog(); // initialisation

Question 34

null

Answer 34

built-in literal

Can be assigned to reference variables

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

declaration

Answer 37

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 38

garbage collection

Answer 38

The process of destroying objects that have become unreachable because they are no longer referenced by variables, in order to reclaim their space in memory. In certain programming languages, including Java, this process is automatic.

Question 39

keyword

Answer 39

A Java keyword is a word reserved for use in the language. Keywords cannot be used as variable identifiers.

Question 40

evaluate

Answer 40

To find the value of something. We say that expressions have a value or evaluate to some value.

Question 41

expression

Answer 41

Code that evaluates to a single value. Expressions can be formed from literals, variables, operators and messages.

Question 42

literal

Answer 42

A textual representation of a primitive value or object. For example, ‘X’ is a char literal, 4.237 is a double literal and “hello there!” is a String literal.

Question 43

operator

Answer 43

A symbol used as a function, that is, it has one or more values it operates on and returns a value.

Unary operator:
only one operand i.e. -(-7)

Question 44

precedence rules

Answer 44

Rules for determining the order in which various operators are evaluated in a complex expression.

Question 45

compound expression

Answer 45

An expression built up using other sub-expressions, for example, the following is a compound expression: (3 + 2) * (6 - 3).

Question 46

string

Answer 46

A sequence of characters (sequence = in a particular order )enclosed in quotation marks; an object of type String.

Question 47

string interning

Answer 47

A process used by Java to create a pool of strings that can be reused when possible, which may result in the sharing of string literals.

Question 48

concatenation

Answer 48

The joining of two strings. In Java the string concatenation operator is + (the plus sign). For example, “Milton “ + “Keynes” evaluates to “Milton Keynes”.

“Hallo” + 4 evaluates to “Hallo4”

Question 49

Char

Answer 49

primitive type
can be represented by an unsigned integer
you can add 1 to get the next character in a character set

Question 50

toString()

Answer 50

returns textual representation of an object
defaults to class and memoryadress

Question 51

type

Answer 51

A type is a set of values and set of operations that are permitted on those values. Java has tow different kind of types - primitive data types and reference types.

Question 52

variable

Answer 52

• Named memory location

* Must follow the rules for Java identifiers

Question 53

method body

Answer 53

That part of a method enclosed by braces that follows the method header. The body of a method is an example of a statement block.

Question 54

method header

Answer 54

A method header consists of an access modifier (e.g. public), a return value (e.g. int or void) and a name (e.g. setPosition) followed by any
formal argument names enclosed in parentheses (e.g. (int aNumber)). For example, the method header for the method of the class Frog whose name is setPosition() is public void setPosition(int aNumber).

Question 55

method

Answer 55

The code that is invoked by the Java Virtual Machine at run-time when an object receives a message.

Question 56

method signature

Answer 56

The name of the method together with the parentheses and the types of any arguments. For example, the signature for the setPosition() method in the Frog class is setPosition(int).

getPosition();
getColour(OUColour);

Question 57

actual argument

Answer 57

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.

Question 58

formal argument

Answer 58

A typed identifier used in a method signature between parentheses to stand for a value (an actual argument) that is passed into the method body by a message.

public void myMethod (String formalArgument){}

Question 59

overloading

Answer 59

A method is said to be 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 60

overriding

Answer 60

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. A method that overrides a method from a superclass has the same signature and return type as the superclass method.

Question 61

@Override

Answer 61

A Java annotation used to indicate that a method is intended to override an inherited method.

Question 62

member

Answer 62

Members of a class: class variables, class methods, instance variables
and instance methods. Constructors are not members. Members are involved in inheritance.

Question 63

local variables

Answer 63

A variable that is declared inside a statement block, such as inside a method body, or for statement. The scope of such a variable is restricted to the statement block in which it is declared

Question 64

lazy evaluation

Answer 64

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 65

interpreter

Answer 65

A program that translates statements (rather than translating a whole program) from a high-level language, such as Java, to a machine code language, and executes the machine code.

Question 66

interface

Answer 66

An interface specifies a list of messages that a group of UNRELATED classes, which are said to implement the interface, must be able to receive.

An interface lists only method headers (no method code), cannot declare any instance variables and cannot be instantiated.

Classes implementing an interface must be able to receive all the messages corresponding to methods specified by that interface, either by defining the required methods or inheriting them.

Question 67

wrapper class

Answer 67

Every primitive type has a corresponding wrapper class whose purpose is to hold a primitive value in a place where a primitive value is intuitively what is needed, but where the type checking requires an object. For example, the wrapper class of int is Integer. Wrapper classes are used in auto-boxing.

Question 68

void

Answer 68

A keyword used in Java to indicate that a method does not return a value.

Question 69

virtual machine (VM)

Answer 69

A layer of software that simulates a computer capable of interpreting bytecode.

Question 70

unwrapping

Answer 70

The process of converting a wrapped value (one contained in a wrapper class) to its primitive value equivalent.

Question 71

unchecked exception

Answer 71

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 72

tree

Answer 72

A collection structure typified by branching connections between elements, often used to create collections of items sorted according to some criteria.

Question 73

token

Answer 73

An item of data that can be written to or retrieved from a file. Delimiters are used to separate tokens in a file.

Question 74

try-catch statement

Answer 74

An exception handling statement, a mechanism to catch exceptions. If a possible checked exception is not declared in a throws clause, it must be handled using a try-catch statement.

Question 75

throws clause

Answer 75

A part of a method header beginning with the keyword throws, followed by a comma-separated list of any exceptions the programmer wants to declare to be thrown. If a checked exception is not handled, it must be declared in a throws clause.

Question 76

supertype

Answer 76

A type that supports substitution of more specific types. Both interface types and superclass types (including abstract class types) are examples of supertypes. See also subtype.

Question 77

syntax

Answer 77

The syntax of a programming language is the set of rules governing the structure of the language, including its valid symbols, expressions and structures. Syntax rules define the form of the various constructs in the language, but say nothing about the meaning of these constructs.

Question 78

substitution

Answer 78

The technique of providing an instance of one class in a situation where an instance of a different class is expected. In Java, an object can be assigned to a variable whose type has been declared as some superclass of the object, or which has been declared as an interface type which that object’s class implements. Similarly, an object can be used as an actual argument to a method where the formal argument’s type has been declared as some superclass of the object, or which has been declared as an interface type which that object’s class implements.

Question 79

strong typing

Answer 79

A programming language feature that requires variables to have a declared type and enforces rules on the ways in which those types can be used, including what types of values can be assigned to variables of those types and what operators values of those types can be used with. Languages (such as Java) that feature strong typing are said to be strongly typed.

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

statement

Answer 81

A statement represents a single instruction for the compiler to translate into bytecode. In Java most statements end with a semicolon.

Question 82

stack trace

Answer 82

A stack of textual information ordered from the last thing that happened down to the first thing that happened that is displayed when an exception is thrown and not handled; a representation of a call stack.

Question 83

stable sort

Answer 83

A sort that does not reorder equal elements.

Question 84

sorted collection

Answer 84

A collection that always keeps its elements in their natural ordering, if any. This contrasts with an ordered collection, where the ordering may just be an accident of where elements happen to have been placed in the collection.

Question 85

setter method

Answer 85

A method whose purpose is to assign a new value to an instance variable. For example, the method setPosition() is a setter method for the instance variable position held by instances of the Frog class.

Question 86

set

Answer 86

A collection in which each item may appear only once – no duplicates are allowed.

Question 87

secondary sort

Answer 87

A sorting that can be applied after applying a primary sort in order to sort any elements that are considered equal by the primary sort.

Question 88

scope

Answer 88

The scope of a variable describes the areas of program code from which the variable may be used. The scope of a local variable is from the point where it is declared to the end of its enclosing statement bloc

Question 89

run-time error

Answer 89

A programming error that becomes apparent only when the program is run, and has not been detected beforehand. Run-time errors can be caused by logical errors, failure to account for different ways in which code might be used, or conditions outside of the control of the programmer.

Question 90

refactoring

Answer 90

Refactoring is when code is rewritten without changing its overall effect, but for the purpose of improving its design, removing code duplication, or improving maintainability.

Question 91

protected

Answer 91

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 92

public

Answer 92

An access modifier applied to a class member that allows all classes of objects to have access.

Question 93

private

Answer 93

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

Question 94

orchestrating instance

Answer 94

An orchestrating instance is a separate object used to tie together the different parts of a complicated interaction that do not seem to belong to any single one of the objects involved.

Question 95

ordered collection

Answer 95

A collection where each element in the collection has a well-defined place, as indicated by its index number. Thus, an ordered collection allows us to access and find the first, second or last element etc. However, the order does not have to reflect anything to do with the elements themselves. It is simply determined by where each element has been placed in the list. This contrasts with a sorted collection

Question 96

natural ordering

Answer 96

In Java, an element type is said to have a natural ordering if it implements the interface Comparable and consequently has a compareTo()method. For numbers and strings, the natural ordering corresponds simply to everyday numerical order and alphabetical order.

Question 97

map

Answer 97

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

Question 98

hiding

Answer 98

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 99

hash code

Answer 99

In Java, a hash code is an int that can be calculated for any object, by sending it the message hashCode(). This method is inherited from Object, but in general must be overridden if equals() has been overridden. Hash codes are used by sets and related collections to allow them to store elements efficiently and to check rapidly for duplicates. If a class redefines equals(), then the hashCode() method for that class must take the definition into account; otherwise the type will not behave properly in collections.

Question 100

flushing

Answer 100

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

Question 101

wrapper class

Answer 101

• every primitive type has one
• used where type checking requires an object (e.g. Collections)
• wrapper class of int is Integer

Question 102

visibiity

Answer 102

refers to whether or not parts of a class can access that item directly

Question 103

union

Answer 103

Results in a set containing all of the elements of all of the sets. The term is used to refer both to the result and to the operation.

Question 104

this()

Answer 104

used to invoke another constructor in the same class