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Lecture 4

CMPUT101 Lecture Notes - Lecture 4: Dollar Sign

Computing Science
Course Code
Duane Szafron

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CMPUT 114 – Lecture 4 – References: Literals, Variables and
Referring to objects & values
In a written program, we can’t draw shapes so we need to
have some notation for referring to objects and values called a
There are different ways to reference objects & values in a
The simplest way is to use a literal in our code.
A literal refers to the same object or value at all times.
You can think of a literal as a nameplate that is automatically
pre-bound to an object or value
Literals and values cannot be re-bound
Literal Tokens
In general, a literal is recognized by the compiler and is
immediately translated into a language value or object
Common literals in programming languages include
characters, numbers, and strings
Literals are somewhat restrictive because:
Some objects cannot be referred to using literals
Sometimes we may want to rebind our nameplates.
A variable is like a soft nameplate that may be attached to an
object or value and may be re-attached to a different object or
value in the future.
More than one variable can be attached to the same object or
value at the same time
Java: 4 Kinds or Variables
local variables (Inside a method)
static variables (sometimes called “class” variables)
message parameters (specified in the method params)
instance variables (specified outside in the class.)
Variable Declarations
Every variable must be declared
The declarations syntax for each kind of variable is different
A declaration specifies:
The amount of memory to be allocated
How the data should be internally represented
How the data can be legally used once the variable has
been bound to it.
Characteristics of variables
Every variable has two characteristics:
The scope is the region of program that can use the
The lifetime is the length of time that the variable exists
Java Identifiers
When we declare a variable we give it an identifier (name)
In Java, an identifier:
Starts wit a letter, underscore or dollar sign.
The initial character is followed by zero or more letters,
digits, underscores or dollar signs.
Some other uses for Identifiers
Identifiers aren’t just used for variables. The following also
have identifiers:
Classes: String, Date, PrintStream
Messages: toUpperCase, trim, println
Boolean literals: true, false
Java Identifier Conventions
Class names start with an upper case letter
Message names start with a lower case letter
If an identifier consists of more than one word then the first
letter of subsequent words is capitalized
PrintStream (class)
Final Variables
A final variable is a variable that can only be bound once and
cannot be re-bound
Toe declare a final variable in Java, we precede the data type
witht eh keyword, final
For example:
Final double MAX_HOURS;
Once MAX_HOURS is bound, its value cannot be changed – its
value is final.
Local Variables
The type of every variable has to be declared before the
variable can be used.
Local variables are declared inside a block of code called a
Lifetime: is the time that the method is running
Scope: is the method it is declared in
If the keyword final is included, the variable can only be bound