copyright 1998-2010 curt hill variables what are they? why do we need them?

Copyright 1998-2010 Curt Hill

Variables

What are they?Why do we need them?


Why• Most programming languages have

variables• Variables store values• They are used in computations• Their values may determine how

the program executes


What is it?• A variable represents one or more

memory locations• This is a collection of bits that may

be interpreted in a variety of ways• A value may only be stored in a

computer if it can be digitized• The name comes from variables in

algebra


Variable characteristics• Each variable has three important

characteristics:– Name– Type– Value– Other less important characteristics

• Knowing how to set each of these is important to the first part of this course


Name

• A character string that identifies a variable or other item

• Java names start with a letter• Followed by letters, digits, underscores

and dollar signs• No blanks, punctuation or other

characters• Case sensitive• Any reference must exactly match the

declaration and not be a reserved word


Reserved words• A word that has special meaning to Java• May not be used as a name• These include:

– boolean, byte, char, double, float, int, void– false, true, main, new, null– class, private, protected, public, static, this– case, catch, do, else, for, if, switch, try, while– extends, implements, import, package, super– Among others

• There are also numerous predefined classes with names that you should avoid


Naming Conventions• Java has a convention for naming

things• Variables

– Start with lower case letter– Each new word is capitalized

• Classes– Start with upper case letter– Each new word is capitalized

• Constants– All in upper case

• Just a convention but nice


Types

• Two kinds of types: primitive and classes

• Most variables are objects– An instance of a class

• Primitives include the simple types of most programming languages

• Types determine two properties:– Legal values– Legal operators and operations

Aside• People are smart and computers are

dumb• This is obvious when we speak about

types• The type is usually understood when

people work in algebra– We do not care if the number is an

integer, fraction or real – it is just a number

• Not so in Java – we must explicitly state the type


Primitive Types• boolean - true or false• char - Unicode (16 bit)• byte - -128 to +127, 1 byte• short - -32768 to +32767, 2 bytes• int - 32 bit • long - 64 bit integer• float - 32 bit IEEE floating point• double - 64 bit IEEE floating point• void - function result or parameter



Constant Values for Numeric Types

• int: optional sign and digits– 25, -90, 1452912– No commas or decimal points

• double: integer or exponential• floats: must be tagged with f


Real constants• May be used for float or double• May be an integer• May have one decimal point• May have a trailing exponent • Examples:

– 3.4 -3.4982 2.3E5 3.95E+5 -3E-4


Non-numeric Constant Values• char: numeric or letter in

apostrophes– ‘a’, ‘-’, 1245

• byte: numeric or letter in apostrophes– ‘x’, ‘*’, -127, +90

• boolean: true or false


Integer Operators• Addition +• Subtraction – • Multiplication *• Division / (quotient)• Division % (remainder)• Examples:

– 7 / 2 is 3– 7 % 2 is 1


Real Operators• Addition +• Subtraction – • Multiplication *• Division / • Examples:

– 7.0 / 2.0 is 3.5– 7.0 % 2.0 is 1.0


Other Operators• char and byte may do increment

and decrement• boolean may do And (&&), Or (||)

or Not(!)– These will be discussed later


Variable Declaration• To connect a name to a type• Form:

[visibility] type name1, name2, … namen;

• A variable must be declared before it is used

• Visibility may be left off• The declaration is only known within the

enclosing set of braces


Visibility

• Control over accesses • Private - class only• Public - every one• Protected - class and derived classes• Friendly - public for this file, private

otherwise– No keyword, friendly if not any other

• This will be discussed later with objects


Example declaration• A declaration:

int a, count;double average;

• Three variables– Two integers– One real


Declarations Again• A variable may be initialized when

declared• Follow name with = and a value• Example:

int a = 5, b, c=-3;double d = 3.4;

• The type and constant value must be compatible


Value• The value of a variable may be set

in a variety of ways:– Initialization at declaration– A side effect operator– Input operation– Method call that changes the value

• Each of these are covered in a separate presentation


Objects• Declaration is almost the same as

primitive• Except what is declared is really a

handle to the item• It needs to be instantiated with the

new keyword• Sometimes done automatically,

usually not• Most will be considered later, strings

now


Strings• A string is a sequence of characters• Most often used to identify outputs

or give information • Every language needs to handle

strings in order to interface with the people who use them

• Java has two string types:– String– StringBuffer


Strings• This is the native string• Whenever a string is enclosed in

quotes it is mapped into a string object

• If the same string is used twice in a class it will become one string with multiple handles

• Thus String type does not need the new operator– Unlike every other object


String Declaration• String x; // null handle• String y = “Hello there”;• String z = new String(“Hi there”);• String n = z;• Almost all predefined classes start

with an uppercase letter

copyright 1998-2010 curt hill variables what are they? why do we need them?

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