The CS job market. Building Java Programs Chapter 1. Programming languages. What is programming? Introduction to Java Programming

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1 The CS job market 160,000 Buildig Java Programs Chapter 1 Itroductio to Java Programmig 140, , ,000 80,000 60,000 40,000 PhD Master's Bachelor's Projected Job Opeigs 20,000 Copyright (c) Pearso All rights reserved. - Computer sciece Biological sciece SOURCES: Tabulated by Natioal Sciece Foudatio/Divisio of Sciece Resources Statistics; data from Departmet of Educatio/Natioal Ceter for Educatio Statistics: Itegrated Postsecodary Educatio Data System Completios Survey; ad NSF/SRS: Sur 2 What is programmig? program: A set of istructios to be carried out by a computer. program executio: The act of carryig out the istructios cotaied i a program. programmig laguage: A systematic set of rules used to describe computatios i a format that is editable by humas. This textbook teaches programmig i a laguage amed Java. Programmig laguages Some ifluetial oes: FORTRAN sciece / egieerig COBOL busiess data LISP logic ad AI BASIC a simple laguage 3 4

2 Parts of a Computer large uits CPU GHz for desktops, cell phoes Primitive istructios DRAM GB for desktops Disk GB o phoe, 100s GB laptop, TB desktop Iput/Output devices kilo ,000 mega ,000,000 giga ,000,000,000 tera ,000,000,000,000 peta ,000,000,000,000,000 exa ,000,000,000,000,000,000 zetta ,000,000,000,000,000,000,000 yotta ,000,000,000,000,000,000,000, uits < 1 milli 10 3 micro 10 6 ao 10 9 pico femto atto zepto yocto ames for large umbers kilo 10 3 thousad mega 10 6 millio giga 10 9 billio tera trillio peta quadrillio exa quitillio zetta sextillio yotta septillio 7 8

3 kilo: 1,000 or 1,024? proposed prefixes powers of 2 powers of 10 powers of 2 kilo , ,024 mega ,000, ,048,576 giga ,000,000, ,073,741,824 tera ,000,000,000, ,099,511,627,776 peta ,000,000,000,000, ,125,899,906,842,624 exa ,000,000,000,000,000, ,152,921,504,606,846,976 zetta ,000,000,000,000,000,000, ,180,591,620,717,411,303,424 yotta ,000,000,000,000,000,000,000, ,208,925,819,614,629,174,706,176 usually use: powers of 2 for storage powers of 10 for just about everythig else powers of 10 powers of 2 kilo 10 3 kibi ,024 mega 10 6 mebi ,048,576 giga 10 9 gibi ,073,741,824 tera tebi ,099,511,627,776 peta pebi ,125,899,906,842,624 exa exbi ,152,921,504,606,846,976 zetta zebi ,180,591,620,717,411,303,424 yotta yobi ,208,925,819,614,629,174,706,176 have t exactly take the world by storm 9 10 Compile/ru a program Basic Java programs with pritl statemets 1. Write it. code or source code: The set of istructios i a program. 2. Compile it. compile: Traslate a program from oe laguage to aother. byte code: The Java compiler coverts your code ito a format amed byte code that rus o may computer types. 3. Ru (execute) it. output: The messages prited to the user by a program. source code compile byte code ru output 12

4 A Java program public class Hello { public static void mai(strig[] args) { System.out.pritl("Hello, world!"); System.out.pritl(); System.out.pritl("This program produces"); System.out.pritl("four lies of output"); Its output: Hello, world! Structure of a Java program public class ame { class: a program public static void mai(strig[] args) { statemet; statemet; statemet; method: a amed group of statemets statemet: a commad to be executed This program produces four lies of output Every executable Java program cosists of a class, cosole: Text box ito which the program's output is prited. 13 that cotais a method amed mai, that cotais the statemets (commads) to be executed. 14 System.out.pritl Names ad idetifiers A statemet that prits a lie of output o the cosole. proouced "prit-li" sometimes called a "pritl statemet" for short Two ways to use System.out.pritl : System.out.pritl("text"); Prits the give message as output. System.out.pritl(); Prits a blak lie of output. You must give your program a ame. public class GagstaRap { Namig covetio: capitalize each word (e.g. MyClassName) Your program's file must match exactly (GagstaRap.java) icludes capitalizatio (Java is "case-sesitive") idetifier: A ame give to a item i your program. must start with a letter or _ or $ subsequet characters ca be ay of those or a umber legal: _myname TheCure ANSWER_IS_42 $blig$ illegal: me+u 49ers side-swipe Ph.D's 15 16

5 Keywords keyword: A idetifier that you caot use because it already has a reserved meaig i Java. abstract default if private this boolea do implemets protected throw break double import public throws byte else istaceof retur trasiet case exteds it short try catch fial iterface static void char fially log strictfp volatile class float ative super while cost for ew switch cotiue goto package sychroized Sytax sytax: The set of legal structures ad commads that ca be used i a particular laguage. Every basic Java statemet eds with a semicolo ; The cotets of a class or method occur betwee { ad sytax error (compiler error): A problem i the structure of a program that causes the compiler to fail. Missig semicolo Too may or too few { braces Illegal idetifier for class ame Class ad file ames do ot match Sytax error example Strigs 1 public class Hello { 2 pooblic static void mai(strig[] args) { 3 System.owt.pritl("Hello, world!")_ 4 5 Compiler output: Hello.java:2: <idetifier> expected pooblic static void mai(strig[] args) { ^ Hello.java:3: ';' expected ^ 2 errors The compiler shows the lie umber where it foud the error. The error messages ca be tough to uderstad! 19 strig: A sequece of characters to be prited. Starts ad eds with a " quote " character. The quotes do ot appear i the output. Examples: "hello" "This is a strig. It's very log!" Restrictios: May ot spa multiple lies. "This is ot a legal Strig." May ot cotai a " character. "This is ot a "legal" Strig either." 20

6 Escape sequeces escape sequece: A special sequece of characters used to represet certai special characters i a strig. \t tab character \ ew lie character \" quotatio mark character \\ backslash character Questios What is the output of the followig pritl statemets? System.out.pritl("\ta\tb\tc"); System.out.pritl("\\\\"); System.out.pritl("'"); System.out.pritl("\"\"\""); System.out.pritl("C:\i\the dowward spiral"); Example: System.out.pritl("\\hello\how\tare \"you\"?\\\\"); Output: \hello how are "you"?\\ Write a pritl statemet to produce this output: //\\ Aswers Output of each pritl statemet: \\ ' """ C: i a b c he dowward spiral pritl statemet to produce the lie of output: System.out.pritl("\\/\\\\\"); 23 Questios What pritl statemets will geerate this output? This program prits a quote from the Gettysburg Address. "Four score ad seve years ago, our 'fore fathers' brought forth o this cotiet a ew atio." What pritl statemets will geerate this output? A "quoted" Strig is 'much' better if you lear the rules of "escape sequeces." Also, "" represets a empty Strig. Do't forget: use \" istead of "! '' is ot the same as " 24

7 Aswers pritl statemets to geerate the output: System.out.pritl("This program prits a"); System.out.pritl("quote from the Gettysburg Address."); System.out.pritl(); System.out.pritl("\"Four score ad seve years ago,"); System.out.pritl("our 'fore fathers' brought forth o"); System.out.pritl("this cotiet a ew atio.\""); pritl statemets to geerate the output: System.out.pritl("A \"quoted\" Strig is"); System.out.pritl("'much' better if you lear"); System.out.pritl("the rules of \"escape sequeces.\""); System.out.pritl(); System.out.pritl("Also, \"\" represets a empty Strig."); System.out.pritl("Do't forget: use \\\" istead of \"!"); System.out.pritl("'' is ot the same as \""); Commets commet: A ote writte i source code by the programmer to describe or clarify the code. Commets are ot executed whe your program rus. Sytax: // commet text, o oe lie or, /* commet text; may spa multiple lies */ Examples: // This is a oe-lie commet. /* This is a very log multi-lie commet. */ Where to place commets: Usig commets at the top of each file (a "commet header") at the start of every method (see later) to explai complex pieces of code Commets are useful for: Uderstadig larger, more complex programs. Multiple programmers workig together, who must uderstad each other's code. Commets example /* Suzy Studet, CS 101, Fall 2019 This program prits lyrics about somethig. */ public class BaWitDaBa { public static void mai(strig[] args) { // first verse System.out.pritl("Bawitdaba"); System.out.pritl("da bag a dag diggy diggy"); System.out.pritl(); // secod verse System.out.pritl("diggy said the boogy"); System.out.pritl("said up jump the boogy"); 27 28

8 Algorithms algorithm: A list of steps for solvig a problem. Static methods Example algorithm: "Bake sugar cookies" Mix the dry igrediets. Cream the butter ad sugar. Beat i the eggs. Stir i the dry igrediets. Set the ove temperature. Set the timer. Place the cookies ito the ove. Allow the cookies to bake. Spread frostig ad sprikles oto the cookies. 30 Problems with algorithms Structured algorithms lack of structure: May tiy steps; tough to remember. redudacy: Cosider makig a double batch Mix the dry igrediets. Cream the butter ad sugar. Beat i the eggs. Stir i the dry igrediets. Set the ove temperature. Set the timer. Place the first batch of cookies ito the ove. Allow the cookies to bake. Set the timer. Place the secod batch of cookies ito the ove. Allow the cookies to bake. Mix igrediets for frostig. structured algorithm: Split ito coheret tasks. 1 Make the cookie batter. Mix the dry igrediets. Cream the butter ad sugar. Beat i the eggs. Stir i the dry igrediets. 2 Bake the cookies. Set the ove temperature. Set the timer. Place the cookies ito the ove. Allow the cookies to bake. 3 Add frostig ad sprikles. Mix the igrediets for the frostig. Spread frostig ad sprikles oto the cookies

9 Removig redudacy A program with redudacy A well-structured algorithm ca describe repeated tasks with less redudacy. 1 Make the cookie batter. Mix the dry igrediets. 2a Bake the cookies (first batch). Set the ove temperature. Set the timer. 2b Bake the cookies (secod batch). 3 Decorate the cookies. 33 public class BakeCookies { public static void mai(strig[] args) { System.out.pritl("Mix the dry igrediets."); System.out.pritl("Cream the butter ad sugar."); System.out.pritl("Beat i the eggs."); System.out.pritl("Stir i the dry igrediets."); System.out.pritl("Set the ove temperature."); System.out.pritl("Set the timer."); System.out.pritl("Place a batch of cookies ito the ove."); System.out.pritl("Allow the cookies to bake."); System.out.pritl("Set the ove temperature."); System.out.pritl("Set the timer."); System.out.pritl("Place a batch of cookies ito the ove."); System.out.pritl("Allow the cookies to bake."); System.out.pritl("Mix igrediets for frostig."); System.out.pritl("Spread frostig ad sprikles."); 34 Static methods static method: A amed group of statemets. deotes the structure of a program elimiates redudacy by code reuse procedural decompositio: dividig a problem ito methods Writig a static method is like addig a ew commad to Java. class method A statemet statemet statemet method B statemet statemet method C statemet statemet statemet Desig the algorithm. Usig static methods Look at the structure, ad which commads are repeated. Decide what are the importat overall tasks. 2. Declare (write dow) the methods. Arrage statemets ito groups ad give each group a ame. 3. Call (ru) the methods. The program's mai method executes the other methods to perform the overall task. 36

10 Desig of a algorithm Declarig a method // This program displays a delicious recipe for bakig cookies. public class BakeCookies2 { public static void mai(strig[] args) { // Step 1: Make the cake batter. System.out.pritl("Mix the dry igrediets."); System.out.pritl("Cream the butter ad sugar."); System.out.pritl("Beat i the eggs."); System.out.pritl("Stir i the dry igrediets."); // Step 2a: Bake cookies (first batch). System.out.pritl("Set the ove temperature."); System.out.pritl("Set the timer."); System.out.pritl("Place a batch of cookies ito the ove."); System.out.pritl("Allow the cookies to bake."); // Step 2b: Bake cookies (secod batch). System.out.pritl("Set the ove temperature."); System.out.pritl("Set the timer."); System.out.pritl("Place a batch of cookies ito the ove."); System.out.pritl("Allow the cookies to bake."); // Step 3: Decorate the cookies. System.out.pritl("Mix igrediets for frostig."); System.out.pritl("Spread frostig ad sprikles."); 37 Sytax: Gives your method a ame so it ca be executed public static void ame() { statemet; statemet; statemet; Example: public static void pritwarig() { System.out.pritl("This product causes cacer"); System.out.pritl("i lab rats ad humas."); 38 Callig a method Program with static method Sytax: ame(); Executes the method's code public class FreshPrice { public static void mai(strig[] args) { rap(); // Callig (ruig) the rap method System.out.pritl(); rap(); // Callig the rap method agai You ca call the same method may times if you like. Example: pritwarig(); Output: This product causes cacer i lab rats ad humas. 39 Output: // This method prits the lyrics to my favorite sog. public static void rap() { System.out.pritl("Now this is the story all about how"); System.out.pritl("My life got flipped tured upside-dow"); Now this is the story all about how My life got flipped tured upside-dow Now this is the story all about how My life got flipped tured upside-dow 40

11 Fial cookie program // This program displays a delicious recipe for bakig cookies. public class BakeCookies3 { public static void mai(strig[] args) { makebatter(); bake(); // 1st batch bake(); // 2d batch decorate(); // Step 1: Make the cake batter. public static void makebatter() { System.out.pritl("Mix the dry igrediets."); System.out.pritl("Cream the butter ad sugar."); System.out.pritl("Beat i the eggs."); System.out.pritl("Stir i the dry igrediets."); // Step 2: Bake a batch of cookies. public static void bake() { System.out.pritl("Set the ove temperature."); System.out.pritl("Set the timer."); System.out.pritl("Place a batch of cookies ito the ove."); System.out.pritl("Allow the cookies to bake."); // Step 3: Decorate the cookies. public static void decorate() { System.out.pritl("Mix igrediets for frostig."); System.out.pritl("Spread frostig ad sprikles."); 41 Methods callig methods public class MethodsExample { public static void mai(strig[] args) { message1(); message2(); System.out.pritl("Doe with mai."); public static void message1() { System.out.pritl("This is message1."); public static void message2() { System.out.pritl("This is message2."); message1(); System.out.pritl("Doe with message2."); Output: This is message1. This is message2. This is message1. Doe with message2. Doe with mai. 42 Whe a method is called, the program's executio "jumps" ito that method, executig its statemets, the "jumps" back to the poit where the method was called. public class MethodsExample { public static void mai(strig[] public static args) void { message1() { message1(); System.out.pritl("This is message1."); message2(); Cotrol flow public static void message2() { System.out.pritl("This is message2."); message1(); System.out.pritl("Doe System.out.pritl("Doe with mai."); with message2."); public static void message1() { System.out.pritl("This is message1."); 43 Whe to use methods Place statemets ito a static method if: The statemets are related structurally, ad/or The statemets are repeated. You should ot create static methods for: A idividual pritl statemet. Oly blak lies. (Put blak pritls i mai.) Urelated or weakly related statemets. (Cosider splittig them ito two smaller methods.) 44

12 Static methods questio Drawig complex figures with static methods Write a program to prit these figures usig methods. \/ \/ STOP \/ 46 Developmet strategy Program versio 1 \/ \/ STOP \/ First versio (ustructured): Create a empty program ad mai method. Copy the expected output ito it, surroudig each lie with System.out.pritl sytax. Ru it to verify the output. 47 public class Figures1 { public static void mai(strig[] args) { System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(); System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(""); System.out.pritl(); System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl(" STOP "); System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(); System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl(""); 48

13 Developmet strategy 2 Output structure \/ \/ STOP \/ Secod versio (structured, with redudacy): Idetify the structure of the output. Divide the mai method ito static methods based o this structure. \/ \/ STOP \/ The structure of the output: iitial "egg" figure secod "teacup" figure third "stop sig" figure fourth "hat" figure This structure ca be represeted by methods: egg teacup stopsig hat Program versio 2 public class Figures2 { public static void mai(strig[] args) { egg(); teacup(); stopsig(); hat(); public static void egg() { System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(); public static void teacup() { System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(""); System.out.pritl(); Program versio 2, cot'd. public static void stopsig() { System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl(" STOP "); System.out.pritl("\"); System.out.pritl(" \\/"); System.out.pritl(); public static void hat() { System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); System.out.pritl(""); 51 52

14 Developmet strategy 3 Output redudacy \/ \/ STOP \/ Third versio (structured, without redudacy): Idetify redudacy i the output, ad create methods to elimiate as much as possible. Add commets to the program. \/ \/ STOP \/ The redudacy i the output: egg top: reused o stop sig, hat egg bottom: reused o teacup, stop sig divider lie: used o teacup, hat This redudacy ca be fixed by methods: eggtop eggbottom lie Program versio 3 Program versio 3, cot'd. // Suzy Studet, CSE 138, Sprig 2094 // Prits several figures, with methods for structure ad redudacy. public class Figures3 { public static void mai(strig[] args) { egg(); teacup(); stopsig(); hat(); // Draws the top half of a a egg figure. public static void eggtop() { System.out.pritl(" "); System.out.pritl(" \"); System.out.pritl("\"); // Draws the bottom half of a egg figure. public static void eggbottom() { System.out.pritl("\"); System.out.pritl(" \\/"); // Draws a complete egg figure. public static void egg() { eggtop(); eggbottom(); System.out.pritl(); 55 // Draws a teacup figure. public static void teacup() { eggbottom(); lie(); System.out.pritl(); // Draws a stop sig figure. public static void stopsig() { eggtop(); System.out.pritl(" STOP "); eggbottom(); System.out.pritl(); // Draws a figure that looks sort of like a hat. public static void hat() { eggtop(); lie(); // Draws a lie of dashes. public static void lie() { System.out.pritl(""); 56