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-- Program Elements Intro -- Program Elements Introductionduction

• We can now examine the core elements of programming

• Chapter 3 focuses on:– data types– variable declaration and use– operators and expressions– decisions and loops– input and output

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Primitive Data Types Primitive Data Types

• A data type is defined by a set of values an d the operators you can perform on them

• Each value stored in memory is associated with a particular data type

• The Java language has several predefined t ypes, called primitive data types

• The following reserved words represent eig ht different primitive types:

– byte , short , int , long , float , double , boolean , char

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IntegersIntegers• There are four separate integer primiti

ve data types

• They differ by the amount of memory u sed to store them

Type

byteshortintlong

Storage

8 bits16 bits32 bits64 bits

Min Value

-128-32,768-2,147,483,648< -9 x 1018

Max Value

12732,7672,147,483,647> 9 x 1018

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Floating Point Floating Point

• There are two floating point types:

• The float type stores 7 significant digits

• The double type stores 15 significant digits

Type

floatdouble

Storage

32 bits64 bits

ApproximateMin Value

-3.4 x 1038

-1.7 x 10308

ApproximateMax Value

3.4 x 1038

1.7 x 10308

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CharactersCharacters

• A char value stores a single character from th e Unicode character set

• A character set is an ordered list of characters

• The Unicode character set uses sixteen bits pe r character, allowing for 65,536 unique characters

• It is an international character set, containing symbols and characters frommany world lang

uages

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CharactersCharacters

• The ASCII character set is still the basis for many other programming language

s

• ASCII is a subset of Unicode, including:uppercase letterslowercase letterspunctuationdigitsspecial symbolscontrol characters

A, B, C, …a, b, c, …period, semi-colon, …0, 1, 2, …&, |, \, …carriage return, tab, ...

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BooleanBoolean

•A boolean value represents a true or f alse condition

• They can also be used to represent any two states, such as a light bulb being o

n or off

• The reserved words true and false a re the only valid values for a boolean ty

pe

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WrappersWrappers• For each primitive data type there is a cor

responding wrapper class . For example:

• Wrapper classes are useful in situations w here you need an object instead of a primi

tive type

• They also contain some useful methods

Primitive Type

intdoublechar

boolean

Wrapper Class

IntegerDouble

CharacterBoolean

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VariablesVariables

• A variable is an identifier that represents a l ocation in memory that holds a particular ty

pe of data

• Variables must be declared before they can be used

• The syntax of a variable declaration is: data-type variable-name;

• For example: int total;

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VariablesVariables

• Multiple variables can be declared on t he same line:

int total, count, sum;

• Variables can be initialized (given an in itial value) in the declaration:

int total = 0, count = 20; float unit_price = 57.25;

• See Piano_Keys.java

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Assignment Statements Assignment Statements• An assignment statement takes the followi

ng form: variable-name = expression;

• The expression is evaluated and the result is stored in the variable, overwriting the va

lue currently stored in the variable

• See United_States.java

• The expression can be a single value or a m ore complicated calculation

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ConstantsConstants

• A constant is similar to a variable exce pt that they keep the same value throu

ghout their existence

• They are specified using the reservedword final in the declaration

• For example: final double PI = 3.14159; final int STUDENTS = 25;

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ConstantsConstants

• When appropriate, constants are bette r than variables because:– they prevent inadvertent errors bec

ause their value cannot change

• They are better than literal values because:– they make code more readable by gi

ving meaning to a value– they facilitate change because the v

alue is only specified in one place

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Input and Output Input and Output

• Java I/O is based on input streams and o utput streams

• There are three predefined standard streams:

• The print and println methods write to standard output

Stream

System.inSystem.outSystem.err

Purpose

reading inputwriting outputwriting errors

Default Device

keyboardmonitormonitor

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Input and Output Input and Output

• The Java API allows you to create many k inds of streams to perform various kinds

of I/O• To read character strings, we will conve

rt the System.in stream to another kind of stream using:

• BufferedReader stdin = new BufferedReader

• (new InputStreamReader (System.in));

• This declaration creates a new stream called stdin

• Wewill discuss object creation in more d etail later

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Escape Sequences Escape Sequences

• See Echo.java• An escape sequence is a special seque

nce of characters preceded by a backsl ash (\)

• They indicate some special purpose, su ch as:Escape Sequence

\t\n\"\'\\

Meaning

tabnew line

double quotesingle quotebackslash

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BuffersBuffers• As you type, the characters are stored in an

input buffer

• When you press enter, the program begins processing the data

• Similarly, output information is temporarily stored in an output buffer

• The output buffer can be explicitly flushed ( sent to the screen) using the flush method

• See Python.java

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Numeric Input Numeric Input

• Converting a string that holds an integer into the integer value can be done with a

method in the Integer wrapper class:

value = Integer.parseInt (my_string);

• A value can be read and converted in oneline:

num = Integer.parseInt (stdin.readLine());

• See Addition.java and Addition2.java

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ExpressionsExpressions• An expression is a combination of operator

s and operands

• The arithmetic operators include addition (+ ), subtraction (- ), multiplication (* ), and di

vision (/)

• Operands can be literal values, variables, o r other sources of data

• The programmer determines what is done with the result of an expression (stored, pri nted, etc.)

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DivisionDivision• If the operands of the / operator are bo

th integers, the result is an integer (the f ractional part is truncated)

• If one or more operands to the / operat or are floating point values, the result is

a floating point value• The remainder operator (% ) returns the i

nteger remainder after dividing the first operand by the second

• The operands to the % operator must beintegers

• See Division.java • The remainder result takes the sign of th

e numerator

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DivisionDivision

Expression

17 / 517.0 / 517 / 5.0

9 / 129.0 / 12.0

6 % 214 % 5-14 % 5

Result

33.43.4

00.75

04-4

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Operator Precedence Operator Precedence

• The order in which operands are evaluated in - an expression is determined by a well defined

precedence hierarchy

• Operators at the same level of precedence ar e evaluated according to their associativity (r

ight to left or left to right)

• Parentheses can be used to force precedence

• Appendix D contains a complete operator pre cedence chart for all Java operators

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Operator Precedence Operator Precedence• Multiplication, division, and remainder

have a higher precedence than additio n and subtraction

• Both groups associate left to rightExpression:

Order of evaluation:

Result:

5 + 12 / 5 - 10 % 3

6

43 21

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Operator Precedence Operator Precedence

Expression

2 + 3 * 4 / 2

3 * 13 + 2(3 * 13) + 23 * (13 + 2)

4 * (11 - 6) * (-8 + 10)

(5 * (4 - 1)) / 2

Result

8

414145

40

7

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The if Statement The if Statement

• The Java if statement has the following syntax:

if (condition)

statement;

• If the boolean condition is true, the stateme nt is executed; if it is false, the statement i

s skipped• This provides basic decision making capabilities

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The if Statement The if Statement

statement

conditionfalse

true

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Boolean Expressions Boolean Expressions• The condition of an if statement must

evaluate to a true or false result• Java has several equality and relationaloperators:

• See Temperature.java

Operator

==!=<<=><=

Meaning

equal tonot equal to

less thanless than or equal to

greater thangreater than or equal to

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Block Statements Block Statements

• Several statements can be grouped tog ether into a block statement

• Blocks are delimited by braces

• A block statement can be used wherev er a statement is called for in the Java s

yntax

• See Temperature2.java

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- The if else Statement - The if else Statement

• An else clause can be added to an if state ment to make it an - if else statement:

if (condition) statement1; else statement2;

• If the condition is true, statement1 is execut 2ed; if the condition is false, statement is ex

eeeeee• eee Temperature3.java eee Right_Triangle.java

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- The if else Statement - The if else Statement

statement1

conditionfalse

true

statement2

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Nested if Statements Nested if Statements

• The body of an if statement or else clause can be another if statement

• These are called nested if statements

• See Football_Choice.java

• Note: an else clause is matched to th e last unmatched if (no matter what t

he indentation implies)

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The while Statement The while Statement

• A while statement has the following syntax:

while (condition)

statement;

• If the condition is true, the statement is exec uted; then the condition is evaluated again

• The statement is executed over and over unti l the condition becomes false

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The while Statement The while Statement

statement

conditionfalse

true

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The while Statement The while Statement

• If the condition of a while statement i s false initially, the statement is neverexecuted

• Therefore, we say that a while statem ent executes zero or more times

• See Counter.java, Factors.java , and Powers_of_Two.java

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Infinite Loops Infinite Loops

• The body of a while loop must eventually make the condition false

• If not, it is an infinite loop , which will execu te until the user interrupts the program

• -- This is a common type of logical error al ways double check that your loops will ter

minate normally

• See Forever.java

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Program Development Program Development

• The creation of software involves four basic activities:

– establishing the requirements– creating a design– implementing the code– testing the implementation

• The development process is muchmor e involved that this, but these basic ste

ps are a good starting point

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RequirementsRequirements

• Requirements specify the tasks a progr ammust accomplish (what to do, not h ow to do it)

• They often address the user interface• An initial set of requirements are often

provided, but usually must be critiqued , modified, and expanded

• It is often difficult to establish detailed, unambiguous, complete requirements

• Careful attention to the requirements c an save significant time andmoney in t he overall project

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DesignDesign• A program follows an algorithm , which is a

- - step by step process for solving a problem

• The design specifies the algorithms and da ta needed

• - In object oriented development, it establis hes the classes, objects, and methods that are required

• The details of a method may be expressed i n pseudocode - , which is code like, but does

not necessarily follow any specific syntax

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ImplementationImplementation• Implementation is the process of translating

a design into source code

• Most novice programmers think that writing code is the heart of software development, b

ut it actually should be the least creative

• Almost all important decisions are made duri ng requirements analysis and design

• Implementation should focus on coding detai ls, including style guidelines and documenta

tion

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TestingTesting

• A program should be executed multiple ti mes with various input in an attempt to fi

nd errors

•Debugging is the process of discovering t he cause of a problem and fixing it

• Programmers often erroneously think tha t there is "only one more bug" to fix

• Tests should focus on design details as we ll as overall requirements

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Program Development Program Development• See Average.java

• Follow the process of requirements analysi s, design, implementation, and testing

• There are always multiple ways to design a nd implement a program

• Any design has advantages and disadvanta -ges; there are always trade offs

• See Average2.java