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<section version="5.0" xml:id="sd1_exam_2024_summer" xml:lang="en"
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<title>SD1 examination summer 2024</title>
<section xml:id="sd1_exam_2024_summer_task1">
<title>Implementing tasks</title>
<section xml:id="sd1_exam_2024_summer_task1_preparation">
<title>Preparation</title>
<orderedlist>
<listitem>
<para>Download and unzip the above file
<filename>exam.zip</filename>. You should see a directory
»<filename>Exam</filename>« containing a
<filename>pom.xml</filename> file.</para>
</listitem>
<listitem>
<para>Open this project in your <productname>IDEA</productname> IDE
by selecting the <filename>Exam/pom.xml</filename> file.</para>
</listitem>
</orderedlist>
</section>
<section xml:id="sd1_exam_2024_summer_task1_task">
<title>Task</title>
<qandaset defaultlabel="qanda" xml:id="sd1_exam_2024_summer_task1Qanda">
<qandadiv>
<qandaentry>
<question>
<para>Open the <filename>Readme.md</filename> file in your
project's root. It contains all necessary instructions for
solving the implementation tasks.</para>
</question>
<answer>
<para>Solution see <link
xlink:href="https://gitlab.mi.hdm-stuttgart.de/goik/GoikLectures/-/tree/master/Klausuren/Sd1/2023winter/Solve">winter
2022 exam</link>.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1_exam_2024_summer_task1Caveats">
<title>Caveats</title>
<itemizedlist>
<listitem>
<para>When approaching end of examination check your input for
completeness prior to being automatically logged out by the system.
Remember: There is 120 minutes for the examination and another 5
minutes to check for completeness.</para>
</listitem>
<listitem>
<para>Projects residing just on your local workstation's file system
cannot be recovered after finishing the exam.</para>
</listitem>
</itemizedlist>
</section>
</section>
<section xml:id="sd1_exam_2024_summer_task2">
<title>Executing main(...)</title>
<qandaset defaultlabel="qanda" xml:id="sd1_exam_2024_summer_task3Qanda">
<qandadiv>
<qandaentry>
<question>
<para>Executing the following program</para>
<programlisting language="none"><emphasis role="red">01</emphasis> public class LackingInit {
<emphasis role="red">02</emphasis> ...
<emphasis role="red">03</emphasis> public static void main(String[] args) {
<emphasis role="red">04</emphasis> main(args);
<emphasis role="red">05</emphasis> }
<emphasis role="red">06</emphasis> ...</programlisting>
<para>yields:</para>
<screen>Exception in thread "main" java.lang.<link
xlink:href="https://freedocs.mi.hdm-stuttgart.de/doc/openjdk-17-doc/api/java.base/java/lang/StackOverflowError.html">StackOverflowError</link>
at de.hdm_stuttgart.mi.sd1.extra.LackingInit.main(LackingInit.java:4)
at de.hdm_stuttgart.mi.sd1.extra.LackingInit.main(LackingInit.java:4)
<emphasis role="red">... 1020 more identical lines omitted for brevity ...</emphasis>
at de.hdm_stuttgart.mi.sd1.extra.LackingInit.main(LackingInit.java:4)
at de.hdm_stuttgart.mi.sd1.extra.LackingInit.main(LackingInit.java:4)
Process finished with exit code 1</screen>
<para>What's happening here? Why is execution being terminated by
an exception?</para>
</question>
<answer>
<para>Our <methodname>main(...)</methodname> method calls itself
recursively until breaching the JVM's limit resulting in the
observed <link
xlink:href="https://freedocs.mi.hdm-stuttgart.de/doc/openjdk-17-doc/api/java.base/java/lang/StackOverflowError.html">StackOverflowError</link>
exception.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
<section xml:id="sd1_exam_2024_summer_task3">
<title>Strange arithmetics</title>
<qandaset defaultlabel="qanda" xml:id="sd1_exam_2024_summer_task2Qanda">
<qandadiv>
<qandaentry>
<question>
<para>From ordinary arithmetics we expect a + b - c = a - c + b.
But executing the subsequent code yields:</para>
<informaltable border="1">
<tr>
<th>Code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">System.out.println(1000000000 + 0.000000001 - 1000000000);
System.out.println(1000000000 - 1000000000 + 0.000000001);</programlisting></td>
<td valign="top"><screen>0.0
1.0E-9</screen></td>
</tr>
</informaltable>
<para>Explain these two different results.</para>
</question>
<answer>
<para>Short answer: The two values 1000000000 or <inlineequation>
<m:math display="inline">
<m:msup>
<m:mi>10</m:mi>
<m:mi>9</m:mi>
</m:msup>
</m:math>
</inlineequation>and 0.000000001 or <inlineequation>
<m:math display="inline">
<m:msup>
<m:mi>10</m:mi>
<m:mi>-9</m:mi>
</m:msup>
</m:math>
</inlineequation> differ by 18 orders of magnitude. An eight
byte <code language="java">double</code> cannot express enough
digits for expressing this.</para>
<para>Longer answer: For summing up the int literal <code
language="java">1000000000</code> gets being converted into a
<code language="java">double</code> value being represented as 8
byte IEEE 754. Due to its inherently limited precision the above
sums gets truncated. We consider:</para>
<informaltable border="1">
<tr>
<th>Code</th>
<th>Output</th>
</tr>
<tr>
<td valign="top"><programlisting language="java">System.out.println(1000000000 == 1000000000 + 0.000000001);</programlisting></td>
<td valign="top"><screen>true</screen></td>
</tr>
<tr>
<td valign="top"><programlisting language="java">System.out.println(1000000000.0 == 1000000000 + 0.000000001);</programlisting></td>
<td valign="top"><screen>true</screen></td>
</tr>
</informaltable>
<para>Thus in 8 byte IEEE 754 representation both <code
language="java">1000000000.0</code> and the expression <code
language="java">1000000000 + 0.000000001</code> amount to the
exact same value. Evaluating from left to right we thus
have:</para>
<itemizedlist>
<listitem>
<para><code language="java">1000000000 + 0.000000001 -
1000000000</code> = <code language="java">1000000000.0 -
1000000000</code> = <code language="java">0.0</code>.</para>
</listitem>
<listitem>
<para>Since <code language="java">1000000000</code> is an
<code language="java">int</code> literal we have <code
language="java">1000000000 - 1000000000 == 0</code> and
thus:</para>
<para><code language="java">1000000000 - 1000000000 +
0.000000001</code> = <code language="java">0 +
0.000000001</code> = <code
language="java">1.0E-9</code>.</para>
</listitem>
</itemizedlist>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
</section>
</section>