<?xml version="1.0" encoding="UTF-8"?>
<chapter annotations="slide" version="5.1" xml:id="sd1GettingStarted"
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<title>Getting started</title>
<figure xml:id="sd1_fig_workload">
<title>How much time to invest?</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/weeklyWorkload.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<section xml:id="sw1Resources">
<title>Lecture related resources</title>
<figure xml:id="sd1_fig_recommendedReading">
<title>Recommended reading resources I</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/recommendedReadings.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_fig_recommendedReading2">
<title>Recommended reading resources II</title>
<glosslist>
<glossentry xml:id="glossSda1PrimaryLectures">
<glossterm>Primary</glossterm>
<glossdef>
<itemizedlist>
<listitem>
<para><link
xlink:href="https://brainysoftware.com/book/9781771970365">Java:
A Beginner's Tutorial (5th Edition)</link></para>
</listitem>
</itemizedlist>
</glossdef>
</glossentry>
<glossentry xml:id="glossSda1SecondaryLectures">
<glossterm>Secondary</glossterm>
<glossdef>
<itemizedlist>
<listitem>
<para><link
xlink:href="http://openbook.rheinwerk-verlag.de/javainsel"
xml:lang="de"><productname>Java</productname> ist auch eine
Insel</link> of 12-th edition book 2018 including <xref
linkend="glo_Java"/> 11.</para>
</listitem>
<listitem>
<para><link
xlink:href="https://www.hanser-fachbuch.de/buch/Programmieren+in+Java/9783446441347">Programmieren
in Java</link></para>
</listitem>
</itemizedlist>
</glossdef>
</glossentry>
</glosslist>
</figure>
<figure xml:id="sd1_fig_biggestEnemy">
<title>Your biggest enemies</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/biggestEnemy.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_fig_nurMalSchnell">
<title>German humour</title>
<para xml:lang="de">Aus <link
xlink:href="https://www.der-postillon.com/2021/01/smartphone.html"><quote>Der
Postillion</quote></link> :</para>
<blockquote xml:lang="de">
<para>Mann, der am Handy</para>
<para>nur mal eben die Uhrzeit nachschauen wollte,</para>
<para>chattet acht Minuten auf WhatsApp,</para>
<para>schaut drei YouTube-Videos</para>
<para>und liest einen Artikel über Peru,</para>
<para>weiß aber am Ende immer noch nicht, wie spät es ist</para>
</blockquote>
</figure>
<figure xml:id="sd1_fig_timeManagement">
<title>4 most imperative study objectives</title>
<orderedlist>
<listitem>
<para><link
xlink:href="https://www.intelligent.com/manage-your-time">MANAGE
YOUR TIME!</link></para>
</listitem>
<listitem>
<para><link
xlink:href="https://www.mindtools.com/pages/main/newMN_HTE.htm">MANAGE
YOUR TIME!</link></para>
</listitem>
<listitem>
<para><link
xlink:href="https://toggl.com/time-management-tips">MANAGE YOUR
TIME!</link></para>
</listitem>
<listitem>
<para><link
xlink:href="https://www.psychologytoday.com/intl/basics/time-management">MANAGE
YOUR TIME!</link></para>
</listitem>
</orderedlist>
</figure>
<figure xml:id="sd1_fig_onlineRurorial">
<title>Online tutorials</title>
<glosslist>
<glossentry>
<glossterm><link
xlink:href="http://programmedlessons.org/Java9/">Bradley Kjell:
Introduction to Computer Science using Java</link></glossterm>
<glossdef>
<itemizedlist>
<listitem>
<para><link
xlink:href="http://www.gailer-net.de/tutorials/java/java-toc.html">German
translation by Heinrich Gailer</link></para>
</listitem>
</itemizedlist>
</glossdef>
</glossentry>
<glossentry>
<glossterm><link
xlink:href="https://www.udemy.com/java-tutorial/learn/v4/content">Udemy:
Java Tutorial for Complete Beginners</link></glossterm>
<glossdef>
<para>Video tutorials and related source code examples.
Registration required.</para>
</glossdef>
</glossentry>
</glosslist>
</figure>
<figure xml:id="sd1_fig_unixIntro">
<title>Unix and the terminal</title>
<itemizedlist>
<listitem>
<para><link
xlink:href="https://swcarpentry.github.io/shell-novice">The Unix
Shell</link> / Software-carpentry, nice video collection. Each
section is also available in PDF and
<trademark>PowerPoint</trademark> format.</para>
</listitem>
<listitem>
<para><link
xlink:href="http://www.ee.surrey.ac.uk/Teaching/Unix">UNIX Tutorial
for Beginners</link>, text oriented.</para>
</listitem>
<listitem>
<para><link xlink:href="https://kb.iu.edu/d/afsk">Introduction to
Unix commands</link></para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_programmingExercisesAutomatedFeedback">
<title>Online programming, automated feedback</title>
<glosslist>
<glossentry>
<glossterm><link
xlink:href="http://codingbat.com">http://codingbat.com</link></glossterm>
<glossdef>
<para>No registration required.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><link
xlink:href="https://www.programmr.com/zone/java">https://www.programmr.com/zone/java</link></glossterm>
<glossdef>
<itemizedlist>
<listitem>
<para>Hunt for <quote>Challenges</quote> within page.</para>
</listitem>
<listitem>
<para>Registration required.</para>
</listitem>
</itemizedlist>
</glossdef>
</glossentry>
<glossentry>
<glossterm><link
xlink:href="https://www.codewars.com">https://www.codewars.com</link></glossterm>
<glossdef>
<itemizedlist>
<listitem>
<para>Registration or <link
xlink:href="https://github.com">github.com</link> login
required.</para>
</listitem>
</itemizedlist>
</glossdef>
</glossentry>
</glosslist>
</figure>
<figure xml:id="sd1_fig_programmingExercises">
<title>Online programming I</title>
<glosslist>
<glossentry>
<glossterm><uri
xlink:href="http://www.codeabbey.com">codeabbey.com</uri></glossterm>
<glossdef>
<para><link
xlink:href="http://www.codeabbey.com/index/task_list">Problem
list</link>.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><uri
xlink:href="https://rosettacode.org">rosettacode.org</uri></glossterm>
<glossdef>
<para><link
xlink:href="https://rosettacode.org/wiki/Category:Programming_Tasks">Programming
tasks</link> (including solutions for multiple languages).</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>reddit.com</glossterm>
<glossdef>
<para><link
xlink:href="https://www.reddit.com/r/dailyprogrammer">Daily
Programmer</link>.</para>
</glossdef>
</glossentry>
</glosslist>
</figure>
<figure xml:id="sd1_fig_programmingExercises_2">
<title>Online programming II</title>
<glosslist>
<glossentry>
<glossterm xlink:href="https://projecteuler.net">Project
Euler</glossterm>
<glossdef>
<para>Registration required for keeping track of your exercises'
status. The following exercises in particular are considered to be
useful with respect to this lecture:</para>
<para><link
xlink:href="https://projecteuler.net/problem=1">1</link>, <link
xlink:href="https://projecteuler.net/problem=2">2</link>, <link
xlink:href="https://projecteuler.net/problem=4">4</link>, <link
xlink:href="https://projecteuler.net/problem=5">5</link>, <link
xlink:href="https://projecteuler.net/problem=8">8</link>, <link
xlink:href="https://projecteuler.net/problem=9">9</link>, <link
xlink:href="https://projecteuler.net/problem=11">11</link>.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Java Programming Tutorial</glossterm>
<glossdef>
<para
xlink:href="https://www3.ntu.edu.sg/home/ehchua/programming/java/J2a_BasicsExercises.html">Basic
and more difficult exercises</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>Java Programming Exercises</glossterm>
<glossdef>
<para>Start from the <link
xlink:href="https://www.home.hs-karlsruhe.de/~pach0003/informatik_1/aufgaben/en/java.html">easier
exercises</link>.</para>
</glossdef>
</glossentry>
</glosslist>
</figure>
<figure xml:id="sd1_fig_javaVisualizer">
<title>Java Visualizer</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/javaVisualizer.svg"/>
</imageobject>
</mediaobject>
<para><link
xlink:href="https://prog.mi.hdm-stuttgart.de/java_visualize">http://prog.mi.hdm-stuttgart.de/java_visualize</link></para>
</figure>
<figure xml:id="sd1_fig_liveLectureAdditions">
<title>Live <link
xlink:href="https://cloud.mi.hdm-stuttgart.de/index.php/s/df9f296af3298f96361a15a679390e59">lecture
additions</link></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/liveLectureAdditions.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_fig_IntelliJidea">
<title><xref linkend="glo_Soft_IntellijIDEA"/> <xref linkend="glo_IDE"/>
(See <link linkend="sd1_sect_usingMavenIdea">installation
details)</link></title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/idea.svg"/>
</imageobject>
</mediaobject>
</figure>
<figure xml:id="sd1_fig_MiLinuxImages">
<title><productname
xlink:href="https://www.virtualbox.org">Virtualbox</productname> /
<productname
xlink:href="https://www.vmware.com/products/workstation-player.html">VMware</productname>
player based virtualized Linux image</title>
<itemizedlist>
<listitem>
<para>Contain all MI pool workstation Linux software.</para>
</listitem>
<listitem>
<para>Available for free <productname
xlink:href="https://www.vmware.com/products/workstation-pro/workstation-pro-evaluation.html">VMware
Workstation Player</productname> as <link
xlink:href="https://mirror.mi.hdm-stuttgart.de/ubuntu/Vmware/Mibuntu">compressed
image</link>. Apple users: Consider buying <productname
xlink:href="https://www.vmware.com/products/fusion.html">VMWare
Fusion</productname>.</para>
</listitem>
<listitem>
<para>Available for free <productname
xlink:href="https://www.virtualbox.org/wiki/Downloads">VirtualBox</productname>
desktop virtualization as <link
xlink:href="https://mirror.mi.hdm-stuttgart.de/ubuntu/VirtualBox/Mibuntu">compressed
image</link>.</para>
</listitem>
<listitem>
<para>The beasts are quite big (~20 GB on disk, ~ 5GB compressed
download)! <emphasis role="red">You may prefer a wired connection in
favour of WiFi !</emphasis></para>
</listitem>
<listitem>
<para>Alternative: Native or dual boot <orgname
xlink:href="https://www.ubuntu.com">Ubuntu</orgname> <quote
xlink:href="https://www.ubuntu.com/download/desktop">Desktop</quote>
installation.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_virtualboxConfigScreenshots">
<title><trademark
xlink:href="https://www.virtualbox.org">Virtualbox</trademark>
settings</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/virtualboxConfig.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
</section>
<section xml:id="sd1_sect_usingTheExercises">
<title>Using the exercises</title>
<figure xml:id="sd1_fig_exerciseList">
<title>Embedded exercises</title>
<itemizedlist>
<listitem>
<para>Tight relationship to the E-examination.</para>
</listitem>
<listitem>
<para>Complete <link xlink:href="apb.html">list of exercises on
offer</link>.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="fose2017_assetNavigation">
<title>Using the exercises</title>
<mediaobject>
<imageobjectco>
<areaspec otherunits="imagemap" units="other">
<area coords="120,39,502,184"
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<area coords="120,196,349,237"
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linkends="sd1_callout_usingExercises-3" units="calspair"
xml:id="sd1_callout_usingExercises-3-co"/>
</areaspec>
<imageobject>
<imagedata fileref="Fig/exercise.svg.marked.svg"/>
</imageobject>
</imageobjectco>
</mediaobject>
<calloutlist role="slideExclude">
<callout arearefs="sd1_callout_usingExercises-1-co"
xml:id="sd1_callout_usingExercises-1">
<para>The question to be answered. See <link
linkend="sd1_qanda_hotelKeyCard">current example in
real</link>.</para>
</callout>
<callout arearefs="sd1_callout_usingExercises-2-co"
xml:id="sd1_callout_usingExercises-2">
<para>The corresponding solution: <emphasis role="red">Try hard
yourself before even considering the easy way!</emphasis>.</para>
<para>If you have no clue how to begin ask fellow course mates for
hints.</para>
</callout>
<callout arearefs="sd1_callout_usingExercises-3-co"
xml:id="sd1_callout_usingExercises-3">
<para>Create a comment in order to:</para>
<itemizedlist>
<listitem>
<para>Find help among your colleagues, tutors, lecturer or
technical assistant.</para>
</listitem>
<listitem>
<para>Report issues concerning ill-defined questions, missing
explanations or support, wrong or deficient answers and so
on.</para>
</listitem>
</itemizedlist>
</callout>
</calloutlist>
</figure>
</section>
<section xml:id="sd1CommonRessources">
<title>Common software development related resources</title>
<figure xml:id="sd1_fig_text_HdM_mailserver">
<title><xref linkend="glo_HdM"/> mail server</title>
<para>Either of:</para>
<itemizedlist>
<listitem>
<para>Read your mails at <link
xlink:href="https://ox.hdm-stuttgart.de">https://ox.hdm-stuttgart.de</link>
regularly.</para>
<blockquote>
<para>or</para>
</blockquote>
</listitem>
<listitem>
<para>Activate mail forwarding from <link
xlink:href="https://ox.hdm-stuttgart.de">https://ox.hdm-stuttgart.de</link>
to your <quote>real</quote> email account.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_text_MiVpn">
<title>Configure <acronym>MI <xref linkend="glo_VPN"/></acronym> client
access</title>
<itemizedlist>
<listitem>
<para>External <link
xlink:href="https://klausur.mi.hdm-stuttgart.de/ilias">MI
E-examination system</link> access requires <xref
linkend="glo_VPN"/>:</para>
<itemizedlist>
<listitem>
<para>Past years' E-examinations.</para>
</listitem>
<listitem>
<para>Your personal exam results.</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>OpenVPN <acronym>wiki</acronym> <link
xlink:href="https://wiki.mi.hdm-stuttgart.de/doku.php?id=studium:infrastruktur:vpn">installation
page</link> (Login required).</para>
</listitem>
<listitem>
<para><filename>HdM_MI_stud.ovpn</filename> allows for using a
maximum of MI services.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_text_MiCloud">
<title>MI Cloud server</title>
<informaltable border="0">
<tr>
<td valign="top"><itemizedlist>
<listitem>
<para><link
xlink:href="https://cloud.mi.hdm-stuttgart.de">https://cloud.mi.hdm-stuttgart.de</link>.</para>
</listitem>
<listitem>
<para>25 GB free disk space.</para>
</listitem>
<listitem>
<para><link
xlink:href="https://nextcloud.com/install/#install-clients">Desktop
and mobile clients</link>.</para>
</listitem>
</itemizedlist></td>
<td valign="top"><mediaobject>
<imageobject>
<imagedata fileref="Fig/miCloud.svg"/>
</imageobject>
</mediaobject></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sd1_fig_text_Mi_E_examination">
<title>E-examination resources</title>
<informaltable border="0">
<colgroup width="75%"/>
<colgroup width="25%"/>
<tr>
<td valign="top"><itemizedlist>
<listitem>
<para>Examination server <link
xlink:href="https://klausur.mi.hdm-stuttgart.de">https://klausur.mi.hdm-stuttgart.de</link>
providing recent examinations. (External access requires <link
linkend="sd1_fig_text_MiVpn">Mi VPN</link>.)</para>
</listitem>
<listitem>
<para><link
xlink:href="https://learn.mi.hdm-stuttgart.de/files/CheatSheet"><quote>Cheat
sheet uploads</quote></link> of <filename>zip</filename>
Archives to be used during the examination.</para>
<caution>
<para>Restrict yourself to standard formats e.g.
<acronym>png</acronym>, <acronym>gif</acronym>,
<acronym>pdf</acronym> rather than using proprietary stuff
like <acronym>.doc</acronym>, <acronym>.docx</acronym>,
<acronym>.rtf</acronym> possibly without tool support during
the examination.</para>
</caution>
</listitem>
<listitem>
<para>Tests at <link
xlink:href="https://learn.mi.hdm-stuttgart.de">https://learn.mi.hdm-stuttgart.de</link>.</para>
</listitem>
</itemizedlist></td>
<td valign="top"><mediaobject>
<imageobject>
<imagedata fileref="Fig/exam.svg"/>
</imageobject>
</mediaobject></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sd1_fig_text_MiFileserver">
<title>MI File server</title>
<itemizedlist>
<listitem>
<para>Accessing your computer pool home directory.</para>
</listitem>
<listitem>
<para>Windows share \\mi-ad1.srv.mi.hdm-stuttgart.de\xy123 or
\\192.168.111.15\xy123.</para>
</listitem>
<listitem>
<para>Requires <link linkend="sd1_fig_text_MiVpn">Mi
VPN</link>.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_text_MiGitSvnServer">
<title>MI <xref linkend="glo_Soft_Git"/> versioning server</title>
<itemizedlist>
<listitem>
<para><link
xlink:href="https://gitlab.mi.hdm-stuttgart.de">https://gitlab.mi.hdm-stuttgart.de</link>.</para>
</listitem>
<listitem>
<para>Collaborative software development.</para>
</listitem>
</itemizedlist>
</figure>
</section>
<section xml:id="sd1SectTutorials">
<title>Coached exercises</title>
<figure xml:id="sd1_fig_tuitions">
<title>Coached exercises</title>
<itemizedlist>
<listitem>
<para>Tuesday and Wednesday 17:45-19:15.</para>
</listitem>
<listitem>
<para>Alternating seminar and software exercises:
<abbrev>E.g.</abbrev> seminar on Tuesdays, exercises on
Wednesdays.</para>
</listitem>
<listitem>
<para>Seminar groups of ~12 participants assigned to a tutor.</para>
</listitem>
</itemizedlist>
</figure>
<para>Each lecture participant may start a project to gain bonus points
with respect to the final examination. This bonus may be added to your
final examination's mark:</para>
<figure xml:id="sd1_fig_points">
<title>Bonus points</title>
<itemizedlist>
<listitem>
<para>Precondition: You must pass the examination based on its own
score excluding bonus points.</para>
</listitem>
<listitem>
<para>Examination: 90 points resulting in <quote>1,0</quote>, 45
points resulting in <quote>4.0</quote>.</para>
</listitem>
<listitem>
<para>0-10 bonus points on top of examination score in case of
reaching at least 45 examination points.</para>
</listitem>
<listitem>
<para>Examples:</para>
<itemizedlist>
<listitem>
<para>40 examination points: <quote>Failed</quote> regardless of
any number of bonus points</para>
</listitem>
<listitem>
<para>45 Examination points, 10 bonus points. Result: 55 points
resulting in a 3.0 mark rather than 4.0.</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_seminarRules">
<title>Seminar rules and bonus points</title>
<itemizedlist>
<listitem>
<para>Join exactly one group at the <link
xlink:href="https://learn.mi.hdm-stuttgart.de/ilias/goto.php?target=crs_2137&client_id=iliasclient1">MI
E-learning system</link>.</para>
</listitem>
<listitem>
<para>Bonus point requirements:</para>
<orderedlist>
<listitem>
<para>80% participation rate of all weekly appointments</para>
</listitem>
<listitem>
<para>Presenting <emphasis role="bold">at least three
exercise</emphasis> solutions of <uri
xlink:href="https://freedocs.mi.hdm-stuttgart.de/apb.html">https://freedocs.mi.hdm-stuttgart.de/apb.html</uri>.</para>
</listitem>
</orderedlist>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_seminarRulesExercises">
<title>Presenting exercise solutions</title>
<itemizedlist>
<listitem>
<para>Give a brief account of the exercise in question.</para>
</listitem>
<listitem>
<para>Explain your solution's concept and present your code.</para>
</listitem>
<listitem>
<para>Explain possible problems / pitfalls.</para>
</listitem>
<listitem>
<para>Ask your tutor for exercises to avoid thematic clashes</para>
</listitem>
</itemizedlist>
</figure>
</section>
<section xml:id="sd1SectToolsOfTheTrade1">
<title>Using plain <xref linkend="glo_Java"/></title>
<para>We outline the process of:</para>
<orderedlist>
<listitem>
<para>Editing source code.</para>
</listitem>
<listitem>
<para>Compile <xref linkend="glo_Java"/> source code thereby
generating so called class files.</para>
</listitem>
<listitem>
<para>Executing class files by virtue of a piece of software being
called the <quote
xlink:href="https://www.javaworld.com/article/3272244/core-java/what-is-the-jvm-introducing-the-java-virtual-machine.html">Java
Virtual Machine</quote>.</para>
<para>Main idea: <emphasis>Write once, run
everywhere.</emphasis></para>
</listitem>
</orderedlist>
<figure xml:id="sd1_fig_editCompileExecute">
<title>Edit - compile - execute</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/javaWorkingTerminal.multi.svg"/>
</imageobject>
</mediaobject>
</figure>
<para>We now examine these three steps in greater detail starting with
editing source code:</para>
<figure xml:id="sd1_fig_imageco_editJava">
<title>Editing <xref linkend="glo_Java"/> files</title>
<mediaobject>
<imageobjectco>
<areaspec otherunits="imagemap" units="other">
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xml:id="sd1_fig_imageco_editJava-2.2-co"/>
</areaspec>
<imageobject>
<imagedata fileref="Fig/javaEdit.svg.marked.svg"/>
</imageobject>
</imageobjectco>
</mediaobject>
<calloutlist role="slideExclude">
<callout arearefs="sd1_fig_imageco_editJava-1-co"
xml:id="sd1_fig_imageco_editJava-1.2">
<para>The process of creating a text file from scratch or editing an
existing one is called editing. It requires a text editor like
<command xlink:href="https://www.nano-editor.org">nano</command>,
<command xlink:href="https://www.vim.org">vi(m)</command>, <command
xlink:href="https://atom.io">atom</command> or similar.</para>
<tip>
<para>You should learn quickly using a text editor of your choice.
A basic command is sufficient. Both <command
xlink:href="https://www.nano-editor.org">nano</command> and
<command xlink:href="https://www.vim.org">vi(m)</command> are good
choices on Linux/UNIX systems due to their widespread
distribution. Windows and Apple users have different choices as
well.</para>
</tip>
</callout>
<callout arearefs="sd1_fig_imageco_editJava-2.2-co"
xml:id="sd1_fig_imageco_editJava-2.2">
<para>The resulting files are just plain text. Unlike office
documents they won't contain any layout or formatting like
<abbrev>e.g.</abbrev> different font sizes, background colours or
similar.</para>
</callout>
</calloutlist>
</figure>
<para>We shed some light on <xref linkend="glo_Java"/> class
definitions:</para>
<figure xml:id="sd1_fig_listing_structureClassDefinition">
<title>Defining class <classname>HelloWorld</classname></title>
<programlisting language="java">// Filename HelloWorld.java <co
linkends="sd1_fig_listing_structureClassDefinition-1"
xml:id="sd1_fig_listing_structureClassDefinition-1-co"/>
public class HelloWorld <co
linkends="sd1_fig_listing_structureClassDefinition-2"
xml:id="sd1_fig_listing_structureClassDefinition-2-co"/> {
public static void main(String[] args) <co
linkends="sd1_fig_listing_structureClassDefinition-3"
xml:id="sd1_fig_listing_structureClassDefinition-3-co"/> {
System.out.println("Hello, world"); <co
linkends="sd1_fig_listing_structureClassDefinition-4"
xml:id="sd1_fig_listing_structureClassDefinition-4-co"/>
}
}</programlisting>
<calloutlist role="slideExclude">
<callout arearefs="sd1_fig_listing_structureClassDefinition-1-co"
xml:id="sd1_fig_listing_structureClassDefinition-1">
<para>The text file <filename>HelloWorld.java</filename> contains a
class definition. Classes are <xref linkend="glo_Java"/>'s main
building blocks. Real world <xref linkend="glo_Java"/> applications
consist of up to thousands of interacting classes.</para>
</callout>
<callout arearefs="sd1_fig_listing_structureClassDefinition-2-co"
xml:id="sd1_fig_listing_structureClassDefinition-2">
<para><code language="java">... public class HelloWorld ...</code>
actually defines a so called <xref linkend="glo_Java"/> class of
name HelloWorld. We will refer to it as <quote>class
HelloWorld</quote> for short. You'll learn about classes starting
from <xref linkend="sw1ChapterObjectsClasses"/>. The following
restrictions apply:</para>
<orderedlist>
<listitem>
<para>Class names must obey <xref linkend="glo_Java"/> rules for
so called identifiers. Spaces, operators like <quote>+</quote>
and a lot of other characters are disallowed. More details are
to be found in <xref
linkend="sd1_fig_legalVariableName"/>.</para>
</listitem>
<listitem>
<para>A <xref linkend="glo_Java"/> file's base name must match
the primary (the one being defined using the <code
language="java">public</code> modifier) class name it contains.
In the given example the file's base name is
<classname>HelloWorld</classname>. Defining
<abbrev>e.g.</abbrev> <code language="java">... public class
Hello {...</code> inside a file
<filename>HelloWorld.java</filename> would be flagged as a
compile time error.</para>
</listitem>
</orderedlist>
</callout>
<callout arearefs="sd1_fig_listing_structureClassDefinition-3-co"
xml:id="sd1_fig_listing_structureClassDefinition-3">
<para><code language="java">public static void main(String[]
args)</code> is a so called <link
linkend="sd1SectClassMembers">class method</link> definition.</para>
<para>For the time being we present a rule of thumb: Every <xref
linkend="glo_Java"/> program to become executable requires at least
one class containing (nearly) exactly the following<code
language="java"> main(..)</code> method definition:</para>
<programlisting language="java">public static void main(String[] args)</programlisting>
</callout>
<callout arearefs="sd1_fig_listing_structureClassDefinition-4-co"
xml:id="sd1_fig_listing_structureClassDefinition-4">
<para>A <link linkend="sd1_chap_Statements">statement</link>
example. This is our toy example's actual <quote>payload</quote>:
Only this line of code has an observable effect on execution. All
surrounding text is just boilerplate code being required to keep the
<xref linkend="glo_Java"/> environment happy.</para>
</callout>
</calloutlist>
</figure>
<para>Prior to execution we require a compilation step:</para>
<figure xml:id="sd1_fig_imageco_compileJava">
<title>Compiling <xref linkend="glo_Java"/> file</title>
<mediaobject>
<imageobjectco>
<areaspec otherunits="imagemap" units="other">
<area coords="113,61,273,111"
linkends="sd1_fig_imageco_compileJava-sourcecode"
xml:id="sd1_fig_compileJava-sourcecode">
<alt>Sourcecode</alt>
</area>
<area coords="347,134,439,182"
linkends="sd1_fig_imageco_compileJava-compiler"
xml:id="sd1_fig_compileJava-compiler">
<alt>Java compiler</alt>
</area>
<area coords="481,129,641,178"
linkends="sd1_fig_imageco_compileJava-bytecodeFile"
xml:id="sd1_fig_compileJava-bytecodeFile">
<alt>Executable bytecode file</alt>
</area>
</areaspec>
<imageobject>
<imagedata fileref="Fig/javaCompile.svg.marked.svg"/>
</imageobject>
</imageobjectco>
</mediaobject>
<calloutlist role="slideExclude">
<callout arearefs="sd1_fig_compileJava-sourcecode"
xml:id="sd1_fig_imageco_compileJava-sourcecode">
<para>The text file <filename>HelloWorld.java</filename> containing
our <classname>HelloWorld</classname> class.</para>
</callout>
<callout arearefs="sd1_fig_compileJava-compiler"
xml:id="sd1_fig_imageco_compileJava-compiler">
<para>The <xref linkend="glo_Java"/> compiler is a piece of software
belonging to a <xref linkend="glo_JDK"/>. It will read the input
filename <filename>HelloWorld.java</filename> and create a resulting
output filename <filename>HelloWorld.class</filename>.</para>
</callout>
<callout arearefs="sd1_fig_compileJava-bytecodeFile"
xml:id="sd1_fig_imageco_compileJava-bytecodeFile">
<para>A <xref linkend="glo_Java"/> bytecode file contains
instructions to be interpreted by a <xref linkend="glo_Java"/> run
time system (<xref linkend="glo_JRE"/>), see next step. Bytecode
files are not meant to be viewed or edited.</para>
<para>The main advantage of bytecode files is becoming independent
of operating systems. Thus a given
<classname>HelloWorld.class</classname> file may be executed on
Linux, Windows, Mac-OS or any other <xref linkend="glo_JRE"/>
providing system in exactly the same way.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sd1_fig_imageco_compileJavaShell">
<title>Command line <xref linkend="glo_Java"/> file compilation</title>
<screen>~/tmp$ ls -al <emphasis role="red">HelloWorld.class</emphasis>
ls: cannot access '<emphasis role="red">HelloWorld.class</emphasis>': No such file or directory
~/tmp$ <emphasis role="red">javac</emphasis> HelloWorld.java
~/tmp$ ls -al <emphasis role="red">HelloWorld.class</emphasis>
-rw-r--r-- 1 goik fb1prof 419 Sep 23 15:44 <emphasis role="red">HelloWorld.class</emphasis></screen>
</figure>
<para>Besides human readable portions bytecode files mainly contain binary
data providing instructions to be executed:</para>
<figure xml:id="sd1_fig_listing_javaBytecodeFile">
<title>Java byte code file <filename>HelloWorld.class</filename></title>
<screen>Êþº¾^@^@^@6^@^]
^@^F^@^O ^@^P^@^Q^H^@^R
^@^S^@^T^G^@^U^G^@^V^A^@^F<init>^A^@^C()V^A^@^DCode^A^@^OLineNumberTable^A^@^Dmain^A^@^V(\
[Ljava/lang/String;)V^A^@
<emphasis role="red">SourceFile</emphasis>^A^@^O<emphasis role="red">HelloWorld.java</emphasis>^L^@^G^@^H^G^@^W^L^@^X^@^Y^A^@^L<emphasis
role="red">Hello, world</emphasis>^G^@^Z^L^@^[^@\
^\^A^@
<emphasis role="red">HelloWorld</emphasis>^A^@^Pjava/lang/Object^A^@^Pjava/lang/System^A^@^Cout^A^@^ULjava/io/PrintStream;\
^A^@^Sjava/io/PrintStream^A^@^G<emphasis role="red">println</emphasis>^A^@^U(Ljava/lang/String;)V^@!^@^E^@^F^@^@^@^@^@^\
B^@^A^@^G^@^H^@^A^@ ^@^@^@^]^@^A^@^A^@^@^@^E*·^@^A±^@^@^@^A^@
^@^@^@^F^@^A^@^@^@^B^@ ^@^K^@^L^@^A^@ ^@^@^@%^@^B^@^A^@^@^@ ²^@^B^R^C¶^@^D±^@^@^@^A^@
^@^@^@
^@^B^@^@^@^D^@^H^@^E^@^A^@^M^@^@^@^B^@^N</screen>
</figure>
<figure xml:id="sd1_fig_listing_javaSourceVsBytecode">
<title>Source code <abbrev>vs.</abbrev> bytecode</title>
<informaltable border="1">
<tr>
<th><filename>HelloWorld.java</filename></th>
<th><filename>HelloWorld.class</filename></th>
</tr>
<tr>
<td valign="top"><itemizedlist>
<listitem>
<para>Human readable (kind of 😆).</para>
</listitem>
<listitem>
<para>High abstraction level.</para>
</listitem>
<listitem>
<para>Text file</para>
</listitem>
</itemizedlist></td>
<td valign="top"><itemizedlist>
<listitem>
<para>Machine readable instructions.</para>
</listitem>
<listitem>
<para>Non-editable (usually).</para>
</listitem>
<listitem>
<para>Binary file.</para>
</listitem>
</itemizedlist></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sd1_fig_imageco_executeJava">
<title>Executing byte code file
<filename>HelloWorld.class</filename></title>
<mediaobject>
<imageobjectco>
<areaspec otherunits="imagemap" units="other">
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xml:id="__area-ni0p8r"/>
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<imageobject>
<imagedata fileref="Fig/javaExecute.svg.marked.svg"/>
</imageobject>
</imageobjectco>
</mediaobject>
<calloutlist role="slideExclude">
<callout arearefs="__area-5fxifu"
xml:id="sd1_fig_imageco_executeJava-1">
<para>Passing a bytecode file to the Java Runtime System (<xref
linkend="glo_JRE"/>):</para>
<screen>java HelloWorld</screen>
<caution>
<para>The <filename>.class</filename> extension must be omitted!
Trying <command
xlink:href="https://docs.oracle.com/javase/10/tools/java.htm">java</command>
<option>HelloWorld.class</option> will yield an error.</para>
</caution>
</callout>
<callout arearefs="__area-6imq1s"
xml:id="sd1_fig_imageco_executeJava-2">
<para>The bytecode input will pass various steps inside the <xref
linkend="glo_JRE"/>. On successful processing the underlying
operating system will eventually execute the desired actions on
behalf of the <xref linkend="glo_JRE"/>.</para>
</callout>
<callout arearefs="__area-ni0p8r"
xml:id="sd1_fig_imageco_executeJava-3">
<para>Terminal and keyboard are being linked to the operating system
by two pre-connected communication channels called <quote>standard
input</quote> and <quote>standard output</quote> respectively. In
our toy example <classname>System.out</classname> represents the
<quote>standard output</quote> channel and thus allows for execution
of <code language="java">println("Hello, world")</code>.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sd1_fig_imageco_executeJavaScreen">
<title>Command line byte code file <filename>HelloWorld.class</filename>
execution</title>
<screen>> java HelloWorld
Hello, world</screen>
<para>Remark: This executes <filename>HelloWorld.class</filename> rather
than <filename>HelloWorld.java</filename>.</para>
</figure>
<figure xml:id="sd1_fig_imageco_installJdk">
<title><xref linkend="glo_JDK"/> installation options</title>
<itemizedlist>
<listitem>
<para><link
xlink:href="https://www.oracle.com/technetwork/java/javase/downloads/jdk10-downloads-4416644.html">Manual
installation</link>.</para>
</listitem>
<listitem>
<para>Linux <productname>Debian</productname> / <productname>Ubuntu
22.04:</productname></para>
<screen>apt install openjdk-17-jdk</screen>
</listitem>
</itemizedlist>
</figure>
</section>
<section xml:id="sd1CodeSnippets">
<title>Play!</title>
<para>The following code snippets may be executed and modified without
prior knowledge. They are intended to give you an idea about programming
without exactly understanding whats going on. Just enjoy! We'll present
the gory details later ...</para>
<section xml:id="sd1SectSnippetHelloWorld">
<title><quote>Hello, World</quote> and friends.</title>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_SnippetHelloWorld">
<title>Extending class <classname>HelloWorld</classname></title>
<qandadiv>
<qandaentry>
<question>
<orderedlist>
<listitem>
<para>Create a class <classname>HelloWorld</classname> by
using your favourite text editor ( <command
xlink:href="https://www.nano-editor.org">nano</command>,
<command xlink:href="https://www.vim.org">vim</command>,
<command xlink:href="https://atom.io">atom</command> ...)
inserting the following code into a file
<filename>HelloWorld.java</filename>:</para>
<programlisting language="java">public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World ...");
}
}</programlisting>
</listitem>
<listitem>
<para>Compile <filename>HelloWorld.java</filename>:</para>
<programlisting language="none">javac HelloWorld.java</programlisting>
<para>This should result in a newly generated bytecode file
<filename>HelloWorld.class</filename>:</para>
<screen>ls
<emphasis role="red">HelloWorld.class</emphasis>
HelloWorld.java</screen>
</listitem>
<listitem>
<para>Execute <filename>HelloWorld.class</filename>
by:</para>
<programlisting language="none">java HelloWorld</programlisting>
<warning>
<para>Executing <code>java HelloWorld.class</code> will
not work.</para>
</warning>
<para>You should see the following output in your
terminal:</para>
<screen>Hello, World ...</screen>
<para>Extend the above code to produce a whole paragraph of
sentences rather than just one single line. Your output
might read:</para>
<screen>Those who can do.
Those, who cannot, teach.
Those, who cannot teach, lecture trainee teachers teaching methodology.</screen>
</listitem>
</orderedlist>
</question>
<answer>
<para>After creating a file <filename>HelloWorld.java</filename>
we execute the compilation step:</para>
<screen>>javac HelloWorld.java <co
linkends="sd1QandaSnippetHelloWorldCompileStep-1"
xml:id="sd1QandaSnippetHelloWorldCompileStep-1-co"/>
>
>ls -l <co linkends="sd1QandaSnippetHelloWorldCompileStep-2"
xml:id="sd1QandaSnippetHelloWorldCompileStep-2-co"/>
-rw-r--r-- 1 goik fb1prof 426 <emphasis role="red">Sep 25 09:14</emphasis> HelloWorld.class <co
linkends="sd1QandaSnippetHelloWorldCompileStep-3"
xml:id="sd1QandaSnippetHelloWorldCompileStep-3-co"/>
-rw-r--r-- 1 goik fb1prof 124 <emphasis role="red">Sep 23 17:44</emphasis> HelloWorld.java <co
linkends="sd1QandaSnippetHelloWorldCompileStep-4"
xml:id="sd1QandaSnippetHelloWorldCompileStep-4-co"/></screen>
<calloutlist>
<callout arearefs="sd1QandaSnippetHelloWorldCompileStep-1-co"
xml:id="sd1QandaSnippetHelloWorldCompileStep-1">
<para>The <command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
command will read <filename>HelloWorld.java</filename>. If
no errors are found a <filename>HelloWorld.class</filename>
bytecode file is being generated.</para>
</callout>
<callout arearefs="sd1QandaSnippetHelloWorldCompileStep-2-co"
xml:id="sd1QandaSnippetHelloWorldCompileStep-2">
<para>The <command
xlink:href="https://linux.die.net/man/1/ls">ls</command>
command lists all files and directories being found in your
current working directory. The <option>-l</option> option
will create a longer listing providing privilege, user and
group ownership, file size and modification time meta
data.</para>
</callout>
<callout arearefs="sd1QandaSnippetHelloWorldCompileStep-3-co"
xml:id="sd1QandaSnippetHelloWorldCompileStep-3">
<para>The bytecode file
<filename>HelloWorld.class</filename> results from
compilation in <coref
linkend="sd1QandaSnippetHelloWorldCompileStep-1-co"/>.</para>
</callout>
<callout arearefs="sd1QandaSnippetHelloWorldCompileStep-4-co"
xml:id="sd1QandaSnippetHelloWorldCompileStep-4">
<para>The source file containing our class definition code.
Note <emphasis role="red">Sep 25 09:14</emphasis> being more
recent than <emphasis role="red">Sep 23 17:44</emphasis>.
This reflects the fact that compilation happened after
editing <filename>HelloWorld.java</filename>.</para>
</callout>
</calloutlist>
<para>Execution requires a <xref linkend="glo_Java"/> runtime by
virtue of the <command
xlink:href="https://docs.oracle.com/javase/10/tools/java.htm">java</command>
command:</para>
<informaltable border="0">
<tr>
<td valign="top"><calloutlist>
<callout arearefs="sd1QandaSnippetHelloWorldExecuteStep-1-co"
xml:id="sd1QandaSnippetHelloWorldExecuteStep-1">
<para>Starting the <xref linkend="glo_Java"/> runtime.
<option>HelloWorld</option> is a command line
parameter to <command
xlink:href="https://docs.oracle.com/javase/10/tools/java.htm">java</command>.</para>
</callout>
<callout arearefs="sd1QandaSnippetHelloWorldExecuteStep-2-co"
xml:id="sd1QandaSnippetHelloWorldExecuteStep-2">
<para>The source line <code
language="java">System.out.println("Hello,
world")</code> causes the <xref linkend="glo_Java"/>
runtime writing the string <code
language="java">"Hello, world"</code> to the operating
system's standard output channel appending a newline
character.</para>
</callout>
</calloutlist></td>
<td valign="top"><screen>>java HelloWorld <co
linkends="sd1QandaSnippetHelloWorldExecuteStep-1"
xml:id="sd1QandaSnippetHelloWorldExecuteStep-1-co"/>
Hello, world <co linkends="sd1QandaSnippetHelloWorldExecuteStep-2"
xml:id="sd1QandaSnippetHelloWorldExecuteStep-2-co"/></screen></td>
</tr>
</informaltable>
<para>There are several options for creating a whole paragraph
spanning multiple text lines:</para>
<orderedlist>
<listitem>
<para>Using <link
xlink:href="https://blogs.oracle.com/javamagazine/text-blocks-come-to-java">Java
13 multiline string literals</link> (easy):</para>
<programlisting language="none">public static void main( String[] args ) {
System.out.println(<emphasis role="red">"""</emphasis>
Those who can do.
Those, who cannot, teach.
Those, who cannot teach, lecture trainee teachers teaching methodology.<emphasis
role="red">"""</emphasis> );
}</programlisting>
</listitem>
<listitem>
<para>Straightforward: Using multiple print
statements:</para>
<programlisting language="java">public class HelloWorld {
public static void main(String[] args) {
System.out.println("Those who can do.");
System.out.println("Those, who cannot, teach.");
System.out.println(
"Those who cannot teach lecture trainee teachers teaching methodology.");
}
}</programlisting>
</listitem>
<listitem>
<para>Long strings exceeding your editor's desired maximum
line length may be decomposed into smaller chunks using the
<quote>+</quote> <coref
linkend="sd1QandaSnippetHelloWorldPlusOperator-1-co"/>
string concatenation operator:</para>
<programlisting language="java">public static void main(String[] args) {
System.out.println("Those who can do.");
System.out.println("Those, who cannot, teach.");
System.out.println("Those, who cannot teach, " <emphasis role="red">+</emphasis> <co
xml:id="sd1QandaSnippetHelloWorldPlusOperator-1-co"/>
"lecture trainee teachers teaching methodology."); // continuation line
}</programlisting>
</listitem>
<listitem>
<para>The <methodname>System.out.println(...)</methodname>
method adds a final line break to your output. The <xref
linkend="glo_ASCII"/> newline character <quote><code
language="java">\n</code></quote> <coref
linkend="sd1QandaSnippetHelloWorldNewline-1-co"/> allows for
using just a single <methodname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/PrintStream.html#println(java.lang.String)">System.out.println("...")</methodname>
statement:</para>
<programlisting language="java">public static void main(String[] args) {
System.out.println(
"Those who can do.<emphasis role="red">\n</emphasis>" <co
xml:id="sd1QandaSnippetHelloWorldNewline-1-co"/> +
"Those, who cannot, teach.<emphasis role="red">\n</emphasis>" <coref
linkend="sd1QandaSnippetHelloWorldNewline-1-co"/> +
"Those, who cannot teach, " +
"lecture trainee teachers teaching methodology."); // No <emphasis
role="red">\n</emphasis> at end, print<emphasis
role="red">ln</emphasis> does it.
}</programlisting>
</listitem>
</orderedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_renameClassFile">
<title>Renaming a class file</title>
<qandadiv>
<qandaentry>
<question>
<para>Consider HelloWorld.java again:</para>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java">public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}</programlisting></td>
<td valign="top"><orderedlist>
<listitem>
<para>Rename your existing HelloWorld.java file to
Advance.java and try to compile it. What happens and
why does it happen?</para>
</listitem>
<listitem>
<para>How do you solve the problem by editing the
file's content?</para>
</listitem>
</orderedlist></td>
</tr>
</informaltable>
</question>
<answer>
<orderedlist>
<listitem>
<para>Renaming a <xref linkend="glo_Java"/> class file
without renaming a contained <code
language="java">public</code> class name results in a
compile time error:</para>
<screen>>mv HelloWorld.java Advance.java <co
linkends="sd1_qanda_renameClassFile-1"
xml:id="sd1_qanda_renameClassFile-1-co"/>
>
>javac Advance.java <co linkends="sd1_qanda_renameClassFile-2"
xml:id="sd1_qanda_renameClassFile-2-co"/>
Advance.java:2: error: class HelloWorld is public, should be declared in a file named HelloWorld.java
public class HelloWorld {
^ <co linkends="sd1_qanda_renameClassFile-3"
xml:id="sd1_qanda_renameClassFile-3-co"/>
1 error </screen>
<calloutlist>
<callout arearefs="sd1_qanda_renameClassFile-1-co"
xml:id="sd1_qanda_renameClassFile-1">
<para>We rename <filename>HelloWorld.java</filename> to
<filename>Advance.java</filename> on file system level
without touching its content.</para>
</callout>
<callout arearefs="sd1_qanda_renameClassFile-2-co"
xml:id="sd1_qanda_renameClassFile-2">
<para>Trying to compile
<filename>Advance.java</filename>.</para>
</callout>
<callout arearefs="sd1_qanda_renameClassFile-3-co"
xml:id="sd1_qanda_renameClassFile-3">
<para>Failure: The file
<filename>Advance.java</filename> still contains a <code
language="java">public class HelloWorld</code>
definition. But <xref linkend="glo_Java"/> requires the
class name being equal to the containing file's base
name appending a <filename>.java</filename>
extension.</para>
<para>This error belongs to the wide class of compile
time errors. More specific it is being called a syntax
error violating <xref linkend="glo_Java"/> language
constraints.</para>
</callout>
</calloutlist>
</listitem>
<listitem>
<para>Compilation requires replacing the class name
<classname>HelloWorld</classname> by
<classname>Advance</classname>:</para>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java">public class <emphasis
role="red">Advance</emphasis> {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}</programlisting></td>
<td valign="top"><screen>>javac Advance.java
>
>java Advance
Hello, world</screen></td>
</tr>
</informaltable>
</listitem>
</orderedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_compileTimeStamps">
<title>Editing and compilation</title>
<qandadiv>
<qandaentry>
<question>
<para>Consider the following class definition in a HelloWord
file:</para>
<programlisting language="java">public class HelloWorld {
public static void main(String[] args) {
System.out.println("<emphasis role="red">I have changed!</emphasis>");
}
}</programlisting>
<para>On execution a newbie programmer is stunned still seeing
the former <code language="java">"Hello, world"</code>
string:</para>
<screen>>java HelloWorld
<emphasis role="red">Hello, world</emphasis></screen>
<para>The file system view reads:</para>
<screen>>ls -l
-rw-r--r-- 1 goik fb1prof 426 Sep 25 10:45 HelloWorld.class
-rw-r--r-- 1 goik fb1prof 126 Sep 25 10:48 HelloWorld.java</screen>
<para>Whats wrong here? Why does the "<emphasis role="red">I
have changed!</emphasis>" string does not show up?</para>
</question>
<answer>
<para>The file system reveals
<filename>HelloWorld.class</filename> being about three minutes
older than <filename>HelloWorld.java</filename>:</para>
<screen>>ls -l
-rw-r--r-- 1 goik fb1prof 426 Sep 25 <emphasis role="red">10:45</emphasis> HelloWorld.class
-rw-r--r-- 1 goik fb1prof 126 Sep 25 <emphasis role="red">10:48</emphasis> HelloWorld.java</screen>
<para>The programmer likely edited
<filename>HelloWorld.java</filename> and forgot to invoke the
compiler by executing <command>javac</command>
<filename>HelloWorld.java</filename>. As a consequence the now
outdated <filename>HelloWorld.class</filename> bytecode file was
not replaced by a current version thus not reflecting the change
of <filename>HelloWorld.java</filename> and thus became
outdated.</para>
<para>This is a common beginner's error: Integrated development
environments like <xref linkend="glo_Soft_IntellijIDEA"/> and
build tools like <xref linkend="glo_Maven"/> relieve you from
the <quote>manually keeping track of dependencies</quote>
burden: Editing a source code file will trigger compilation
automatically. We'll <link
linkend="sd1_sect_usingMavenIdea">turn to those higher level
tools</link> after finishing our basic tour on
<quote>pure</quote> <xref linkend="glo_Java"/> <command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
/ <command
xlink:href="https://docs.oracle.com/javase/10/tools/java.htm">java</command>
commands and plain text source code editing.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_editingBytecode">
<title>Editing bytecode</title>
<qandadiv>
<qandaentry>
<question>
<para>We reconsider the bytecode example from <xref
linkend="sd1_fig_listing_javaBytecodeFile"/> being generated
from a <filename>HelloWorld.java</filename> class file:</para>
<screen>Êþº¾^@^@^@6^@^]
^@^F^@^O ^@^P^@^Q^H^@^R
^@^S^@^T^G^@^U^G^@^V^A^@^F<init>^A^@^C()V^A^@^DCode^A^@^OLineNumberTable^A^@^Dmain^A^@^V(\
[Ljava/lang/String;)V^A^@
SourceFile^A^@^OHelloWorld.java^L^@^G^@^H^G^@^W^L^@^X^@^Y^A^@^L<emphasis
role="red">Hello, world</emphasis>^G^@^Z^L^@^[^@\
...</screen>
<orderedlist>
<listitem>
<para>Use your favourite text editor for overwriting the
string <code language="java">"Hello, world"</code> by <code
language="java">"Software joy"</code> in
<filename>HelloWorld.class</filename> (Not in
<filename>HelloWorld.java</filename>!).</para>
<screen>Êþº¾^@^@^@6^@^]
^@^F^@^O ^@^P^@^Q^H^@^R
^@^S^@^T^G^@^U^G^@^V^A^@^F<init>^A^@^C()V^A^@^DCode^A^@^OLineNumberTable^A^@^Dmain^A^@^V(\
[Ljava/lang/String;)V^A^@
SourceFile^A^@^OHelloWorld.java^L^@^G^@^H^G^@^W^L^@^X^@^Y^A^@^L<emphasis
role="red">Software joy</emphasis>^G^@^Z^L^@^[^@\
...</screen>
<para>Try to execute this bytecode file (without
recompiling). What do you observe?</para>
</listitem>
<listitem>
<para>Recompile and repeat the process starting from a sound
<filename>HelloWorld.class</filename> file. This time use a
binary editor like <command
xlink:href="https://linux.die.net/man/1/hexedit">hexedit</command>
or <command
xlink:href="https://manpages.debian.org/unstable/ghex/ghex.1.en.html">ghex</command>
instead.</para>
<tip>
<para>Read your editor's documentation. Using <command
xlink:href="https://linux.die.net/man/1/hexedit">hexedit</command>
<option>HelloWorld.class</option> in a terminal just
requires:</para>
<itemizedlist>
<listitem>
<para>The <quote>tab</quote> key switching to the
<xref linkend="glo_ASCII"/> section in the right
pane.</para>
</listitem>
<listitem>
<para><quote>Ctrl-X</quote> for leaving the editor and
hitting the <quote>Y</quote> key to persist your
changes.</para>
</listitem>
</itemizedlist>
</tip>
</listitem>
<listitem>
<para>Start again from a freshly compiled
<filename>HelloWorld.class</filename>. Use a binary editor
again to append an exclamation mark as in <code
language="java">"Hello, world<emphasis
role="red">!</emphasis>"</code>. The string's length will
thus be increased by 1. What happens on execution?</para>
</listitem>
</orderedlist>
</question>
<answer>
<orderedlist>
<listitem>
<para>Using a simple text editor doesn't work:</para>
<screen>>java HelloWorld
Error: LinkageError occurred while loading main class HelloWorld
java.lang.ClassFormatError: Extra bytes at the end of class file HelloWorld
</screen>
<para>This error actually depends on your text editor
choice: When using <command
xlink:href="https://www.vim.org">vim</command> or <command
xlink:href="https://www.nano-editor.org">nano</command>
analyzing the modified bytecode file's content reveals the
culprit at the very end:</para>
<screen>...
00000090 17 0C 00 18 00 19 01 00 0C 43 65 6C 6C 6F 2C 20 .........<emphasis
role="red">Hello,</emphasis>
000000A0 77 6F 72 6C 64 07 00 1A 0C 00 1B 00 1C 01 00 0A <emphasis
role="red">world</emphasis>...........
...
00000190 00 0A 00 00 00 0A 00 02 00 00 00 03 00 08 00 04 ................
000001A0 00 01 00 0D 00 00 00 02 00 0E <emphasis role="red">0A</emphasis> <co
linkends="sd1_qanda_editingBytecodeBytecodeNewline-1"
xml:id="sd1_qanda_editingBytecodeBytecodeNewline-1-co"/> ...........</screen>
<calloutlist>
<callout arearefs="sd1_qanda_editingBytecodeBytecodeNewline-1-co"
xml:id="sd1_qanda_editingBytecodeBytecodeNewline-1">
<para>The <emphasis role="red">0A</emphasis> (10 in
hexadecimal notation) is being appended to the bytecode
file both by <command
xlink:href="https://www.vim.org">vim</command> and
<command
xlink:href="https://www.nano-editor.org">nano</command>.
This is the <xref linkend="glo_ASCII"/> <link
xlink:href="https://en.wikipedia.org/wiki/ASCII#ASCII_control_code_chart">value
corresponding to a linefeed character</link>. Both
editors thus add an empty new line on saving thereby
corrupting the bytecode file.</para>
</callout>
</calloutlist>
<para>The <command
xlink:href="https://linux.die.net/man/1/emacs">emacs</command>
editor for example refrains from auto-adding newlines and
doesn't cause this problem.</para>
</listitem>
<listitem>
<para>In any case using a binary editor like <command
xlink:href="https://linux.die.net/man/1/hexedit">hexedit</command>
or <command
xlink:href="https://www.systutorials.com/docs/linux/man/1-ghex">ghex</command>
leads to success :</para>
<screen> ... ...
00000070 ... 61 0C 00 07 rceFile...HelloWorld.java...
0000008C ... 79 07 00 1A .............<emphasis role="red">Software joy</emphasis>...
000000A8 ... 61 2F 6C 61 ........HelloWorld...java/la
... ...</screen>
<informaltable border="0">
<tr>
<td valign="top"><para>Executing <command
xlink:href="https://docs.oracle.com/javase/10/tools/java.htm">java</command>
<option>HelloWorld</option>:</para></td>
<td valign="top"><screen>>java HelloWorld
<emphasis role="red">Software joy</emphasis></screen></td>
</tr>
</informaltable>
</listitem>
<listitem>
<para>Extending the string even by just one character
corrupts the bytecode file:</para>
<screen>>java HelloWorld
Error: LinkageError occurred while loading main class HelloWorld
java.lang.ClassFormatError: Unknown constant tag 33 in class file HelloWorld</screen>
<para>Using <command
xlink:href="https://linux.die.net/man/1/hexedit">hexedit</command>
we find the following content within the unspoiled bytecode
file:</para>
<screen>00 08 07 00 17 0C 00 18 00 19 01 00 0C <emphasis
role="red">48 65 6C 6C 6F 2C 20 77 6F 72 6C 64</emphasis> 07 00 1A
<emphasis role="red">▲</emphasis> <emphasis
role="red">▲</emphasis>
<emphasis role="red">┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛</emphasis> <emphasis
role="red">┃</emphasis>
<emphasis role="red">▼</emphasis> will be overridden by
.............<emphasis role="red">Hello, world</emphasis>... »!« or 21 respectively</screen>
<para>Thus appending an exclamation mark will override the
hexadecimal <code language="java">07</code> value
immediately following the <code language="java">"Hello,
world"</code> string with a new value of <code
language="java">21 (<xref linkend="glo_ASCII"/> for
<quote>!</quote>)</code>. Apparently this value serves some
purpose with respect to the virtual machine and must
therefore not be altered. This is the byte code verifier's
responsibility as being outlined in <xref
linkend="sd1_fig_imageco_executeJava"/>.</para>
</listitem>
</orderedlist>
<para>Overall conclusion: Tinkering with bytecode files is a bad
idea unless you have a good reason and know what you are
doing.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1SectSimpleMath">
<title>Simple calculations</title>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_simpleMath">
<title>Working with variables</title>
<qandadiv>
<qandaentry>
<question>
<para>This exercise is about two <link
linkend="sd1_sect_variables">variables</link> <code
language="java">a</code> and <code language="java">b</code>
representing values being subject to change. Embed the following
snippet in a class and execute the beast:</para>
<programlisting language="java" linenumbering="numbered">public static void main(String[] args) {
int a = 4,
b = 7;
System.out.println("a=" + a);
System.out.println("b=" + b);
int sum = a + b;
System.out.println("Sum: " + sum);
}</programlisting>
<orderedlist>
<listitem>
<para>Modify these values and code more sophisticated
expressions.</para>
</listitem>
<listitem>
<para>Replace all three
<methodname>System.out.println(...)</methodname> statements
into a single one:</para>
<informaltable border="0">
<tr>
<th>Source code</th>
<th>Expected output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">int a = 4,
b = 7;
System.out.println(...); // TODO</programlisting></td>
<td valign="top"><screen>4+7=11</screen></td>
</tr>
</informaltable>
<para>Your code is expected to produce correct output when
changing the values of the two <link
linkend="sd1_sect_variables">variables</link> <code
language="java">a</code> and <code
language="java">b</code>:</para>
<informaltable border="0">
<tr>
<th>Source code</th>
<th>Expected output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">int a = -4, // Changing just values
b = 100;
System.out.println(...); // Remains unchanged</programlisting></td>
<td valign="top"><screen>-4+100=96</screen></td>
</tr>
</informaltable>
<tip>
<para><xref linkend="glo_Java"/> does not only provide an
arithmetic <code language="java">+</code> operator: You
have already seen <link
linkend="sd1_qanda_SnippetHelloWorld">the <code
language="java">+</code> operator connecting two
strings</link>. <xref linkend="glo_Java"/> extends this
concept of concatenation to non-string values as
well.</para>
</tip>
</listitem>
</orderedlist>
</question>
<answer>
<para>A naive solution reads:</para>
<programlisting language="java" linenumbering="unnumbered">int a = -4,
b = 100;
System.out.println(a + "+" + b + "=" + a + b);</programlisting>
<para>This unfortunately does not work. In ordinary math
<abbrev>e.g.</abbrev> <code language="java">2 + 3 * 4</code> is
being evaluated as <code language="java">2 + (3 * 4)</code>
rather than <code language="java">(2 + 3) * 4</code>. Likewise
the above expression is <emphasis>fully</emphasis> equivalent
to:</para>
<programlisting language="java" linenumbering="unnumbered">System.out.println(((((a + "+") + b) + "=") + a) + b);</programlisting>
<para>As we shall see in <xref linkend="sd1SectOperator"/> each
of these constituents evaluates to a string thus
producing:</para>
<screen>-4+<emphasis role="red">100</emphasis>=-4<emphasis
role="red">100</emphasis></screen>
<para>A better solution thus forcing an arithmetic
<quote>+</quote> operation requires a pair of braces grouping
<code language="java">(a+b)</code>:</para>
<informaltable border="0">
<tr>
<th>Source code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">int a = -4,
b = 100;
System.out.println(a + "+" + b+ "=" + (a + b));</programlisting></td>
<td valign="top"><screen>-4+100=96</screen></td>
</tr>
</informaltable>
<para>Reverting to the original variable values 4 and 7 still
yields a correct output:</para>
<informaltable border="0">
<tr>
<th>Source code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">int a = 4,
b = 7;
System.out.println(a + "+" + b+ "=" + (a + b));</programlisting></td>
<td valign="top"><screen>4+7=11</screen></td>
</tr>
</informaltable>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_AssocCodeEquivalence">
<title>Code equivalence</title>
<qandadiv>
<qandaentry>
<question>
<para>The solution of <xref linkend="sd1_qanda_simpleMath"/>
claims full source code equivalence between <code
language="java">System.out.println(a + "+" + b + "=" + a +
b)</code> and <code language="java">System.out.println(((((a +
"+") + b) + "=") + a) + b)</code>. Is there a way to prove this
statement?</para>
<tip>
<para>Think about the <xref linkend="glo_Java"/> compile and
execution process. What exactly does equivalence refer to? You
may also want to read the <command
xlink:href="https://linux.die.net/man/1/diff">diff</command>
command's documentation.</para>
</tip>
</question>
<answer>
<para>We start from the following two source code
versions:</para>
<informaltable border="0">
<tr>
<th>Version 1</th>
<th>Version 2</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">public class HelloWorld {
public static void main(String[] args) {
int a = 4,
b = 7;
System.out.println(a + "+" + b+ "=" + a + b);
}
}</programlisting></td>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">public class HelloWorld {
public static void main(String[] args) {
int a = 4,
b = 7;
System.out.println(((((a + "+") + b) + "=") + a) + b);
}
}</programlisting></td>
</tr>
</informaltable>
<para>Source code equivalence means equivalence of executing the
generated bytecode. The easiest case happens if compiling both
versions yields identical bytecode. This is indeed true and can
be proven by:</para>
<orderedlist>
<listitem>
<para>Compile source code version 1 by executing <command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
<option>HelloWorld.java</option>.</para>
</listitem>
<listitem>
<para>Rename the resulting bytecode file
<filename>HelloWorld.class</filename> to
<filename>HelloWorld.1.class</filename>.</para>
</listitem>
<listitem>
<para>Repeat the previous steps for source code version 2
resulting in <filename>HelloWorld.2.class</filename>.</para>
</listitem>
<listitem>
<para>Compare the two files
<filename>HelloWorld.1.class</filename> and
<filename>HelloWorld.2.class</filename>. Result:</para>
<screen><command
xlink:href="https://linux.die.net/man/1/diff">diff</command> HelloWorld.1.class HelloWorld.2.class</screen>
</listitem>
</orderedlist>
<para>The <command
xlink:href="https://linux.die.net/man/1/diff">diff</command>
command returning no result states the the two files to be
identical. Otherwise we would see e.g.:</para>
<screen><command xlink:href="https://linux.die.net/man/1/diff">diff</command> HelloWorld.class HelloWorld.2.class
Binary files HelloWorld.class and HelloWorld.2.class differ</screen>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<qandaset defaultlabel="qanda"
xml:id="sd1_qanda_sourceDiffButEquivalence">
<title>Different byte code, same execution results</title>
<qandadiv>
<qandaentry>
<question>
<para>Can you imagine two different byte code files being fully
equivalent with respect to execution?</para>
</question>
<answer>
<para>There are various scenario for this to happen:</para>
<orderedlist>
<listitem>
<para>Two source code versions may already be equivalent
albeit syntactically different. Consider the following
example differing only by the definition order of two
variables <code language="java">a</code> and <code
language="java">b</code>:</para>
<informaltable border="0">
<tr>
<th>Version 1</th>
<th>Version 2</th>
</tr>
<tr>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">public class HelloWorld {
public static void main(String[] args) {
int a = 4;
int b = 7;
System.out.println(a + b);
}
}</programlisting></td>
<td valign="top"><programlisting language="java"
linenumbering="unnumbered">public class HelloWorld {
public static void main(String[] args) {
int b = 7;
int a = 4;
System.out.println(a + b);
}
}</programlisting></td>
</tr>
</informaltable>
<para>Obviously these two versions are equivalent with
respect to execution. However the derived byte code
differs:</para>
<screen>>diff HelloWorld.class.1 HelloWorld.class.2
Binary files HelloWorld.class.1 and HelloWorld.class.2 differ</screen>
</listitem>
<listitem>
<para>The <command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
compiler allows for additional options. You may add e.g.
<command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
<option>-g</option> <option>HelloWorld.java</option>. This
adds so called debugging information to the generated byte
code causing the latter to differ.</para>
</listitem>
<listitem>
<para><command
xlink:href="https://docs.oracle.com/javase/10/tools/javac.htm">javac</command>
compilers from different vendors (Oracle, IBM, OpenJDK) or
versions (1.7, 1.8, 1.9, 10, 11 and so on) will most likely
produce different byte code hopefully executing
identically.</para>
</listitem>
</orderedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1SectSimpleMathConditional">
<title xml:id="sd1SnippetConditional">A conditional</title>
<qandaset defaultlabel="qanda" xml:id="sd1QandaSnippetConditional">
<qandadiv>
<qandaentry>
<question>
<para>The following snippet only contains a
<methodname>main()</methodname> method. Add a surrounding class
and execute:</para>
<programlisting language="java">public static void main(String[] args) {
int a = 3,
b = 87;
if (a < b) {
System.out.println("a is smaller than b");
} else if (b < a) {
System.out.println("b is smaller than a");
} else {
System.out.println("b equals a");
}
}</programlisting>
<para>Test different values for <code language="java">a</code>
and <code language="java">b</code> reaching all three
branches.</para>
</question>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1SectSimpleMathLoop">
<title xml:id="sd1SectSnippeLoop">A loop</title>
<qandaset defaultlabel="qanda" xml:id="sd1QandaSimpleMathLoop">
<qandadiv>
<qandaentry>
<question>
<para>Execute</para>
<programlisting language="java">public static void main(String[] args) {
int i = 0;
while ( i < 5) {
System.out.println("loop # " + i);
i = i + 1;
}
}</programlisting>
<para>Examine the result and implement the following
modifications:</para>
<orderedlist>
<listitem>
<para>Create five additional lines of output</para>
</listitem>
<listitem>
<para>Enlarge the gap between two adjacent lines of output
to a value of 3 retaining the same number of output
lines:</para>
<screen>loop # 0
loop # 3
loop # 6
loop # 9
loop # 12</screen>
</listitem>
</orderedlist>
</question>
<answer>
<orderedlist>
<listitem>
<para>The number of output lines is being determined by the
loop's termination condition <code language="java">i <
5</code>. Replacing this limit by <code
language="java">while ( i < 10)</code> achieves the
desired result.</para>
</listitem>
<listitem>
<para>We may adjust two related parameters:</para>
<informaltable border="0">
<tr>
<td valign="top"><calloutlist>
<callout arearefs="qandaPlayLoop-1-co"
xml:id="qandaPlayLoop-1">
<para>Adjusting the limit to accommodate larger
values.</para>
</callout>
<callout arearefs="qandaPlayLoop-2-co"
xml:id="qandaPlayLoop-2">
<para>Raising the gap to a value of 3.</para>
</callout>
</calloutlist></td>
<td valign="top"><programlisting language="java">while ( i < 15 <co
linkends="qandaPlayLoop-1"
xml:id="qandaPlayLoop-1-co"/>) {
System.out.println("loop # " + i);
i = i + 3 <co linkends="qandaPlayLoop-2" xml:id="qandaPlayLoop-2-co"/>;
}</programlisting></td>
</tr>
</informaltable>
<para>Alternatively we may as well modify the print
statement:</para>
<programlisting language="java">System.out.println("loop # " + <emphasis
role="bold">i * 3</emphasis>);</programlisting>
</listitem>
</orderedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
</section>
<section xml:id="sdi_sect_ideaBaseUsage">
<title>Using <xref linkend="glo_Soft_IntellijIDEA"/></title>
<figure xml:id="sd1_fig_ideaDownload">
<title>Downloading IntelliJ idea</title>
<itemizedlist>
<listitem>
<para><link
xlink:href="https://www.jetbrains.com/help/idea/installing-and-launching.html">Installation
notes</link>.</para>
</listitem>
<listitem>
<para><link
xlink:href="https://www.jetbrains.com/idea/download">https://www.jetbrains.com/idea/download</link></para>
<para>Choose <quote>Ultimate</quote>.</para>
</listitem>
<listitem>
<para>Ubuntu Linux users:</para>
<para><command>snap</command> <option>install</option>
<option>--classic</option>
<option>intellij-idea-ultimate</option></para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_ideaUltimateLicenseTypes">
<title>Idea »Ultimate« license types</title>
<itemizedlist>
<listitem>
<para>By HdM license server:</para>
<para><uri><emphasis
role="red">http</emphasis>://jetbrains.mi.hdm-stuttgart.de:11111</uri></para>
</listitem>
<listitem>
<para>Activation code for offline usage, apply at:</para>
<para><uri
xlink:href="https://www.jetbrains.com/shop/eform/students">www.jetbrains.com/shop/eform/students</uri></para>
</listitem>
<listitem>
<para>See <link
xlink:href="https://wiki.mi.hdm-stuttgart.de/doku.php?id=studium:infrastruktur:software:intellijidea&s[]=jetbrains">
MI wiki</link> for further details.</para>
</listitem>
</itemizedlist>
</figure>
<figure xml:id="sd1_fig_ideaUltimateHdmLicenseServer">
<title>Alternative: Using the HdM license server</title>
<informaltable border="0">
<colgroup width="50%"/>
<colgroup width="50%"/>
<tr>
<td valign="top"><mediaobject>
<imageobject>
<imagedata fileref="Fig/intelliJIDEALicenseActivation.png"
scale="80"/>
</imageobject>
</mediaobject></td>
<td valign="top"><para>Troubleshooting: Insert address
manually:</para><para><uri>http://jetbrains.mi.hdm-stuttgart.de:11111</uri></para><para>External
usage requires <link linkend="sd1_fig_text_MiVpn">VPN</link>
!</para></td>
</tr>
</informaltable>
</figure>
<figure xml:id="sd1_fig_ideaNewJavaProject">
<title>Creating a new Java project</title>
<mediaobject>
<imageobject>
<imagedata fileref="Fig/ideaNewJavaProject.multi.svg" scale="80"/>
</imageobject>
</mediaobject>
</figure>
<qandaset defaultlabel="qanda" xml:id="sd1_qanda_SnippetHelloWorldIdea">
<title>Getting used to <xref linkend="glo_Soft_IntellijIDEA"/></title>
<qandadiv>
<qandaentry>
<question>
<para>This exercise is about getting used to <xref
linkend="glo_Soft_IntellijIDEA"/> in favour of using the command
line. Repeat at least two of your exercises starting from <xref
linkend="sd1_qanda_SnippetHelloWorld"/>.</para>
</question>
</qandaentry>
</qandadiv>
</qandaset>
</section>
</chapter>