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access modifier
A keyword that controls the visibility of class members to objects of other classes. The modifiers private and public are the most frequently used access modifiers.
accessor message
Another name for a getter message.
accessor method
Another name for a getter method.
actual argument
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.
application domain
See problem domain.
See actual argument and formal argument.
See assignment.
Assignment is 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.
assignment operator
An operator (=) used to assign a value to a variable in an assignment statement.
assignment statement
A statement that assigns a particular value to a variable (see assignment).
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.
attribute value
The current value of an attribute, often the same as an instance variable value, but possibly computed using instance variables.
automatic type conversion
Where the Java compiler converts a value of some type to another type without the need for any explicit conversion of type on the part of the programmer. Automatic type conversion occurs in certain contexts, such as in an assignment statement when a compatible type on the right-hand side of the assignment is converted to the type of the variable on the left-hand side of the assignment.
A term used to describe the way in which an object behaves in response to the messages in its protocol.
binary digit
Either of the two digits 0 and 1 in the binary number system. Binary digits are used for the internal representation of numbers, characters and instructions. The binary digit is the smallest unit of storage.
binary operator
An operator that has two operands.
An abbreviation of binary digit.
See statement block.
Another word for a statement block; for example, the body of a while loop is a statement block.
Boolean condition
A Boolean expression used to control the conditional execution of a statement block.
Boolean expression
An expression that evaluates to either true or false.
Boolean operator
An operator used to combine simple Boolean expressions to form more complex Boolean expressions, which in turn can be combined with other Boolean expressions. They are also known as logical operators.
Bytecode is the intermediate code produced by the Java compiler. In BlueJ, compilation is done when the Compile button is pressed. This will create a bytecode file, for example, Frog.class, from the source code file The bytecode file is portable because each computer that can run Java programs has a Java Virtual Machine that understands bytecode and converts it into the machine code required for that particular computer. Bytecode is so-called because its instructions are a byte in size.
The prefixing of a value with the name of a type in parentheses in order to convert a copy of that value into a different type. For example, (int) 99.0 casts the value 99.0 into an int value.
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.
class header
The line of code in a class definition that provides information about a class such as its name and access modifier. A class header must include the keyword class. Example simple usage: public class Marionette.
class method
Classes, as well as defining instance methods, can also define class methods. These are methods that can be executed irrespective of whether any instances of the class have been created. A class method is executed as the result of an invocation (not by sending a message to an instance of a class). Class methods in Java are specified by including the static keyword in the method header.
A comment is a piece of text in program code to assist human readers in understanding the code, and which the compiler ignores. In Java multi-line comments are delimited by /* and */. End-of-line comments begin with two forward slashes (//) and continue to the end of the line on which they begin. Javadoc comments are placed between the symbols /** and */ and must appear immediately before a method, constructor or class header.
comparison operators
A set of operators used for comparing values of expressions, including ==, > and < .
A program that first checks that source code written in a high-level language is syntactically correct. If the check is successful the compiler translates the source code into bytecode or machine code. The Java compiler translates source code into bytecode.
See software component.
compound expression
An expression built up using other sub-expressions, for example, the following is a compound expression: (3 + 2) * (6 - 3).
The joining of two strings. In Java the string concatenation operator is + (the plus sign). For example, ""Milton "" + ""Keynes"" evaluates to ""Milton Keynes"".
See Boolean condition.
conditional selection
The use of if statements to select and execute alternative statement blocks based upon the value of a Boolean condition.
A programming construct, similar to a method, used to initialise a newly created object.
A commonly followed rule for implementing some feature of a software system that is not enforced by the language used, compiler, or platform on which the software is used.
data hiding
Where an object is treated as a black box, with access to the encapsulated data (the instance variables) being possible only through a limited set of methods.
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).
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.
dialogue box
A type of window with buttons through which users can be given information by a program or provide information to a program on request. In M250, dialogue boxes are provided by class methods of the OUDialog class.
direct access (of an instance variable)
Accessing an instance variable using its name, rather than using a getter method.
See problem domain.
dynamic compilation
A compilation technique (used by the Java environment) generating real machine code from bytecode (intermediate code). A chunk of bytecode is compiled into machine code just prior to being executed. (This is different from, and faster than, the piecemeal operation of an interpreter.) The real machine code is retained so that subsequent execution of that chunk of bytecode does not require the translation to be repeated.
Encapsulation is the parcelling up of information and behaviour into a reusable component. Objects allow you to encapsulate data by incorporating into a single entity (the object) both the data (instance variables) and the behaviour (methods) defined for that data.
end-of-line comment
See comment.
escape character
A character used to mark the start of an escape sequence in a string. In Java this is the \ (backslash) character.
escape sequence
A sequence of characters beginning with a special escape character, such as a backslash, that allows subsequent characters to take on a different meaning for the compiler.
To find the value of something. We say that expressions have a value or evaluate to some value.
Code that evaluates to a single value. Expressions can be formed from literals, variables, operators and messages.
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