COS 130 Computer Programming Methodology Introduction to Java

COS 130 Computer Programming Methodology Introduction to Java

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Algorithm “In mathematics and computing, an algorithm is a procedure (a finite set of well-defined instructions) for accomplishing some task which, given an initial state, will terminate in a defined end-state. ” Wikipedia Like a Recipe, but. . . n Strictly defines the order of steps

Program “A computer program is a list of instructions to be executed by the computer. ” - Wikipedia • • • Programs implement algorithms Programs are executable by “machines” Programs group instructions into a unit of work

Programs Exist in 2 Forms • Source – Designed to be understood by people – Letters, words • Executable – Designed to drive the computer – Binary – 1’s and 0’s

Computing a 2 Step Process Compilation Source Compiler Executable Execution Computer Data In Executable Data Out

Java • Recent language developed by Sun • Primarily OO, procedural programming also supported (somewhat awkwardly) • Most “popular” computer language – Tiobe Software’s TPC index (www. tiobe. com) • Write once, run everywhere

First Java Program A name you pick Capitalization Starts a java file important Must inside call fileclass the same name All code We will only use 1 public class Hello { public static void main(String args[]) Braces group code { They always come in pairs Special. World”); Method System. out. println(“Hello Your program starts running here } Make it just like this Weinstruction will alwaystohave one An printexactly something Statements always end in a semicolon } Your program will do this Or a brace

Literals • Do not change while program is running • Must edit and re-compile program to change

Literals (cont. ) • String Literal – Group of characters (letters) – Inside quotes – “Hello World” • Character Literals – One character (letter) – Inside single quotes – Example: ‘A’ Note: ‘A’ is not the same as “A”

Literals (cont) • Integer Literals – Numbers with no decimal point – Just by itself – Example: 7 • Floating Point Literals – Numbers with a decimal point – Just by itself – Example: 3. 14 Note: 7 is not the same as 7. 0 Note: 7 is not the same as “ 7” Note: 7 is not the same as ‘ 7’

Variable Declarations • Sets aside space in memory • Labels the space with a name • Declaration sets variable for a particular type – numbers, letters, etc. • Once type is set it can not be changed • Variable must be declared before it can be used

Name rules • • • First letter: A-Z a-z Remaining letters: A-Z a-z 0 -9 May be as long as you want Capitalization is important May not include spaces

int declaration int num. Students; • Declares a variable to hold an integer number • Start variable names with a small letter • This variable will be named num. Students • It will set aside 1 32 bit word of space • It can hold values between -2, 147, 483, 648 and +2, 147, 483, 647

A Shortcut • When we create a variable we can also set it to a particular value • This is called “initialization” • Example: int max. Students = 30;

Integer Operators • • • + Addition - Subtraction * Multiplication / Division % Remainder

Division and Remainder What is 5 / 9 ? 0 What is 5 % 9 ? 5 What is 28 / 9 ? 3 What is 28 % 9 ? 1

Integer Expressions • Use the integer operators on int variables and literals • Operators are applied left to right • Multiplication, Division, and Remainder are done before addition and subtraction What is 7 + 10 * 30 ? 307

Integer Expression (cont) • To control the order things happen use parens “(“ and “)” What is (7 + 10) * 30 ? n 510 Notice that parens do not imply multiplication What is (7 + 10) 30 ?

With Variables How many seats are left in a class? max. Students - cur. Enroll Where does the result go? How do we save the result? How do we use the result?

= Equal Sign?

Assignment Operator = max. Students = 30; • Left side must be a variable • The value from the right is put in the spot in memory named on the left. . . 30 max. Students . . .

Example seats. Left = max. Students – cur. Enroll ; . . . 33 cur. Enroll -3. . . 30 max. Students . . . -3 seats. Left . . .

Example • How to increase a number by 1 count = count + 1; n n This is a very common case, be sure you understand it. This is mathematical nonsense.

More Data Types • long • double • boolean

long • What if I need a number that is too big to fit into an int? long population; n Use pretty much like an int long pop. North. America, pop. South. America, pop. New. World; . . . pop. New. World = pop. North. America + pop. South. America; . . .

Caution • An int will fit into a long • But, a long will not fit into an int long my. Long; int my. Int; . . . my. Long = my. Int + 17; // this is ok my. Int = my. Long + 1; // this will not work

What if I need a fraction? • float and double give you numbers that have places to the right of the decimal point float pi = 3. 14; Using float is a bad idea double pi = 3. 14; Use double instead!

Arithmetic with doubles double area; double pi=3. 14; double radius=7. 5; …. . area = pi * (radius * radius);

Simple conversion int my. Int=7; long my. Long=77; double my. Double=7. 77; my. Int = my. Long; No! my. Long = my. Int; my. Int = my. Double; No! my. Long = my. Double; No! my. Double = my. Int; my. Double = my. Long;

boolean • A variable that can be true or false boolean is. Vegetarian; is. Vegetarian = true; is. Vegetarian = false; Note: true is not the same as “true”

Variable Declarations (Review) • Sets aside space in memory • Labels the space with a name • Declaration sets variable for a particular type – numbers, letters, etc. • Once type is set it can not be changed • Variable must be declared before it can be used

Comments • Comments are a way to stick notes to other people (and your self) in your program code • The compiler skips over comments • Therefore, no trace of your comments goes into the executable

Comments (cont) • Two slashes comments out the rest of the line • Example: num. Students = 30; // current enrollment

Comments (cont) • A slash asterisk comments out every line until an asterisk slash is found • Example: /* Current enrollment */ num. Students = 30;

Comments (cont) • What if we do this: /* Current enrollment num. Students = 30; */

Comments (cont) • Use comments to put your name on your programs • Use comments to identify what your program does