Java Coding

Java Coding

David Davenport

Computer Eng. Dept.,Bilkent UniversityAnkara - Turkey.

email: david@bilkent.edu.tr

Syntax for Variables & ConstantsInput, Output and Assignment

a complete Java programdata representations

IMPORTANT… Students…

This presentation is designed to be used in class as part of a guided discovery sequence. It is not self-explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn!

Instructors…You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an email saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it.

thank you, David.

From problem to program… The story so far...

JavaSourceCode

Javabytecode

Machinecode

Problem AlgorithmData/Memory requirements

Need Java Syntax for… Algorithm (in pseudo-code)

Sequence, Decision & Repetition, of Data flow operations

• Input, Output & Assignment

Data/Memory requirements Meaningfully named memory locations Restriction on data (data types) Variables or Constants & initial value

Plus comments & methods!

Comments & White space Comments Syntax:

// any text on remainder of current line /* any text across multiple lines */

Examples: // Author: David.

// Date: Oct. 2002

/* This program blah, blah, blah */

Java ignores line endings, blanks lines & white space!

Layout program code for ease of reading!

Identifiers User-defined names

Used for variables, constants, methods, etc. Any sequence of letters, digits and the

underscore character only. First character may not be a digit! Upper and lower case are considered different

(i.e. case sensitive!) Cannot use Java reserved words

• i.e. words such as while, for, class, if, etc.

CS101 rule: Names must be meaningful!

Data Types For now, use only the following…

Primitive int (for numeric integer, e.g. 5, -27, 0, 510…) double (for numeric real, e.g. 5.75, 3.0, -2.6…) char (for any character, e.g. A, a, B, b, 3, ?, &, … ) boolean (for true / false only)

Non-primitive String (for any sequence of zero or more characters

e.g. “CS101”, “A”, “Well done!”, … )

Declaring Variables Syntax:

Type Any Java type

Name (identifier) Convention:

first letter of embedded words capital, except first!

Examples: int age; double area; long initialSpeed; char letterGrade; char lettergrade; boolean exists;

CAUTIONJava is case

sensitive!

type name;

Declaring Constants Syntax:

Type Any Java type

Name (identifier) Convention: all capital letters (& underscore!)

Value (literal, variable, constant, expression)

Examples: final int SPEEDOFLIGHT = 300; final float PI = 3.142; final String COMPANY = “Bilkent”; final char LETTER_GRADE = ‘A’;

Literal valuesString use “…”

char use ‘.’

final type name = value;

Output (1) Syntax:

where output is Literal value

eg. “The area is “, ‘?’, 12.5, …

Named variable or constanteg. area, userName, TAXRATE, …

Expressioneg. 2 * PI * radius,

“The area is “ + area

System.out.println( output );

Value is output exactly as is!

Value in named memory location

is output

Resulting value of expression is

output

Note use of + for string concatenation

Output (2) Use

To output the value & leave text cursor on current line.

System.out.print( output );

System.out.println( “Welcome to CS101”);System.out.println( “The tax rate is “ + TAXRATE + ‘%’);

System.out.println( “Welcome to CS101”);System.out.print( “The tax rate is “);System.out.print( TAXRATE);System.out.println( ‘%’);

System.out.println();Output blank line!

Outline Java Program The CS101 console template…

import java.util.Scanner;

/** …description… @author …yourname… @version 1.00, date*/public class ClassName {

public static void main( String[] args) {Scanner scan = new Scanner( System.in);

// constants

// variables

// program code }

}

ClassName.java

In Java program = class

ClassnameConvention:first letters capitalised

Filename & classname MUST be the same.

Input Syntax:

Examples

StringVariable = scan.next();

intVariable = scan.nextInt();

doubleVariable = scan.nextDouble();

userName = scan.next();age = scan.nextInt();salary = scan.nextDouble();str = scan.nextLine();

• Standard from Java5.0 on• Invalid input may give run-time error!• Program must include:

•import java.util.Scanner;•Scanner scan = new Scanner( System.in);

Variables must be declared

before use

Assignment Syntax:

where expression is operand or operand operator operand

& Operand is

• Literal value• Named Variable or constant• Result of method call• Expression (can use brackets to disambiguate)!

Operator is• +, -, *, /, % (modulus, remainder after integer division)

resultVariable = expression;

Result of expression must

be of suitable type to put into resultVariable!

… is assigned the result of …

Assignment Examples

What is the result of this?

Evaluation rules Bracketed sub-expressions first Operator precedence ( * / % before + - ) Left to right

total = 0; x = y;sum = firstNumber + secondNumber;netPay = grossPay * ( 1 – TAX_RATE/100);count = count + 1;c = Math.sqrt( a * a + b * b );

4 + 2 / 3 – 1

… is assigned the result of …

Outline Java Program The CS101 console template…

import java.util.Scanner;

/** …description… @author …yourname… @version 1.00, date*/public class ClassName {

public static void main( String[] args) {Scanner scan = new Scanner( System.in);

// constants

// variables

// program code }

}

ClassName.java

In Java program = class

ClassnameConvention:first letters capitalised

Filename & classname MUST be the same.

A Complete Example (1)

Problem – find area & circumference… Algorithm

Data requirements

1. Print welcome message2. Ask for & get radius from user3. Compute area as pi.radius.radius4. Compute circumference as 2.pi.radius5. Report area, circumference & radius

L radius - intL area, circumference - double

PI – double, constant = 3.142

A Complete Example (2)

import java.util.Scanner;

/** …description… @author …yourname… @version 1.00, 2005/10/07*/

public class AreaCircum {

public static void main( String[] args) {

// constants

// variables

// 1. Print welcome message

// 2. Ask for & get radius from user

// 3. Compute area as pi.radius.radius

// 4. Compute circumference as 2.pi.radius

// 5. Report area, circumference & radius}

}

AreaCircum.java

A Complete Example (3)

import java.util.Scanner;

/**

* AreaCircum - computes area & circum of circle given radius

*

* @author David

* @version 1.00, 2005/10/07

*/

public class AreaCircum

{

public static void main( String[] args)

{

// constants

final double PI = 3.142;

// variables

int radius;

double area;

double circumference;

Header has been edited to include program description & author name

AreaCircum.java

A Complete Example (3)Scanner scan = new Scanner( System.in);

// 1. Print welcome message

System.out.println( "Welcome to area circumference finder.");

// 2. Ask for & get radius from user

System.out.print( "Please enter the radius: ");

radius = scan.nextInt();

// 3. Compute area as pi.radius.radius

area = PI * radius * radius;

// 4. Compute circumference as 2.pi.radius

circumference = 2 * PI * radius;

// 5. Report area, circumference & radius

System.out.print( "The area of a circle of radius ");

System.out.print( radius);

System.out.print( " is ");

System.out.println( area);

System.out.print( "and its circumference is ");

System.out.print( circumference);

System.out.println();

}

} // end of class AreaCircum

Template line required for Keyboard input.

Steps 2 & 5 expanded as per original algorithm.

Compile & Run…

Um… why the weird circumference value?

Testing… It compiled & ran, but…is it correct? How can you tell?

Enter input & check results it outputs(e.g. radius 5 area 78.55 & circumference 31.42)

are these actually the right answers? Really need more input/output sets

what input values should we use? & how many do we need?

Thinking about testing during designcan help produce better programs!

DATA & DATA TYPESIn more depth…

Communication…

Communicating…

a) One man multiple messages

b) Many men

• multiple messages

• combine state for single message

Communicating… Can use

anything…Abstractions

Up / down On / Off X / O True / False 1 / 0

Data Types For now, use only the following…

Primitive int (for numeric integer, e.g. 5, -27, 0, 510…) double (for numeric real, e.g. 5.75, 3.0, -2.6…) char (for any character, e.g. A, a, B, b, 3, ?, &, … ) boolean (for true / false only)

Non-primitive String (for any sequence of zero or more characters

e.g. “CS101”, “A”, “Well done!”, … )

Numeric representations

Number bases

58310

5.102 + 8.101 + 3.100

4178

4.82 + 1.81 + 7.80

1102

1.22 + 1.21 + 0.20

Base 2 - binary

digits 0 & 1 2n values 0 (2n – 1)

000001010011100101110111

00011011

0000000100100011010001010110011110001001101010111100110111101111

01

Characters… Coding

Size…?

ASCII•128 bit, •English only!

UNICODE•16 bit•All

languages!

0000 — ‘a’ 0001 — ‘b’0010 — ‘c’0011 — ‘d’0100 — ‘+’0101 — ‘-’0110 — ‘*’0111 — ‘/’

1000 — ‘0’1001 — ‘1’1010 — ‘2’1011 — ‘3’1100 — ‘ ’1101 — ‘x’1110 — ‘y’1111 — ‘z’

‘A’.. ‘Z’ 26‘a’.. ‘z’ 26‘0’.. ‘9’ 10punc. ??

62

26 = 6427 = 12828 = 256

Standard Codes

Data Types Primitive

byte, short, int, long (numeric integer) float, double (numeric real)

char - any character, e.g. A, a, B, b, 3, ?, &, … (Java uses ISO Unicode standard, 16 bit/char)

boolean - true / false

Non-primitive String - any sequence of zero or more characters enum – an ordered set of user-defined values

anything & everything else! (we will come to these shortly)

Primitive Numeric Types

Type

byteshortintlong

floatdouble

Storage

8 bits16 bits32 bits64 bits

32 bits64 bits

Min Value

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

3.4 x 10 38 7 significant digits 1.7 x 10 308 15 significant digits

Max Value

12732,7672,147,483,6479 x 1018

integer

real

Data Types Primitive

byte, short, int, long (numeric integer) float, double (numeric real)

char - any character, e.g. A, a, B, b, 3, ?, &, … (Java uses ISO Unicode standard, 16 bit/char)

boolean - true / false

Non-primitive String - any sequence of zero or more characters enum – an ordered set of user-defined values

anything & everything else! (we will come to these shortly)

Misc… Why so many numeric types? Error in reals? Typecasting

int into double, but not double into int! Overflow/underflow Division by zero Why not use String for everything?