From 7a2e1412295cfdac177e2ab4e0af0e76cfb3a2c9 Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Sat, 7 Dec 2019 19:28:46 +0100
Subject: [PATCH] Cosmetics + two new equals() / enum exercises
---
.../de/hdm_stuttgart/mi/sd1/use_enum/Day.java | 2 +-
Doc/Sd1/deployment.xml | 2 +-
Doc/Sd1/gettingStarted.xml | 14 +-
Doc/Sd1/inheritance.xml | 150 ++++++++++++++++++
Doc/Sd1/objectsClasses.xml | 29 +++-
5 files changed, 186 insertions(+), 11 deletions(-)
diff --git a/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Intro/src/main/java/de/hdm_stuttgart/mi/sd1/use_enum/Day.java b/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Intro/src/main/java/de/hdm_stuttgart/mi/sd1/use_enum/Day.java
index 6b649cd3f..03313efa1 100644
--- a/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Intro/src/main/java/de/hdm_stuttgart/mi/sd1/use_enum/Day.java
+++ b/Doc/Sd1/Ref/ObjectsAndClasses/Enum/Intro/src/main/java/de/hdm_stuttgart/mi/sd1/use_enum/Day.java
@@ -26,7 +26,7 @@ public enum Day {
return name;
}
- public static String getItalianDayname(final Day day) {
+ public static String getItalianDayName(final Day day) {
switch (day) {
case MONDAY: return "Lunedì";
diff --git a/Doc/Sd1/deployment.xml b/Doc/Sd1/deployment.xml
index ce63f6421..ce392181b 100644
--- a/Doc/Sd1/deployment.xml
+++ b/Doc/Sd1/deployment.xml
@@ -8,7 +8,7 @@
xmlns:m="http://www.w3.org/1998/Math/MathML"
xmlns:html="http://www.w3.org/1999/xhtml"
xmlns:db="http://docbook.org/ns/docbook">
- <title>Application deployment I (8.12.)</title>
+ <title>Application deployment I</title>
<section xml:id="sd1DeployPrepare">
<title>Preparations</title>
diff --git a/Doc/Sd1/gettingStarted.xml b/Doc/Sd1/gettingStarted.xml
index 25783b8de..1c9d31095 100644
--- a/Doc/Sd1/gettingStarted.xml
+++ b/Doc/Sd1/gettingStarted.xml
@@ -2361,7 +2361,7 @@ nano ~/.m2/settings.xml <co linkends="sd1_fig_mavenMiRepo-2"
refers to a <orgname
xlink:href="https://www.sonatype.com">Sonatype</orgname>
<productname>Nexus repository manager</productname> instance
- hosting both supplementary <link
+ hosting supplementary <link
xlink:href="https://maven.apache.org/guides/introduction/introduction-to-archetypes.html">Maven
archetypes</link> and <link
xlink:href="https://maven.apache.org/glossary.html">artifacts</link>.</para>
@@ -2711,7 +2711,7 @@ Generating /ma/goik/First/target/site/apidocs/help-doc.html...
<calloutlist role="slideExclude">
<callout arearefs="sd1_fig_mavenIdeaMiArchetypeRepo_co_10"
xml:id="sd1_fig_mavenIdeaMiArchetypeRepo-1">
- <para>Archetype URI:</para>
+ <para>You may want to copy the archetype URI below:</para>
<para><uri
xlink:href="https://maven.mi.hdm-stuttgart.de/nexus/repository/mi-maven/archetype-catalog.xml">https://maven.mi.hdm-stuttgart.de/nexus/repository/mi-maven/archetype-catalog.xml</uri></para>
@@ -2767,7 +2767,7 @@ Generating /ma/goik/First/target/site/apidocs/help-doc.html...
<callout arearefs="sd1_fig_mavenIdeaDefineProjectMetadata-2-co"
xml:id="sd1_fig_mavenIdeaDefineProjectMetadata-2.2">
- <para>Tick <guibutton>Create from archetype</guibutton>
+ <para>Tick the <option>Create from archetype</option>
option.</para>
</callout>
@@ -2777,9 +2777,11 @@ Generating /ma/goik/First/target/site/apidocs/help-doc.html...
<option>de.hdm_stuttgart.mi</option>.</para>
<caution>
- <para>Mind the underscore <quote>_</quote>. Do not use a
- dash as in the corresponding <uri>mi.hdm-stuttgart.de</uri>
- URL.</para>
+ <para>Mind the underscore <quote>_</quote>! Do not use a
+ dash (-) as in the corresponding <uri>mi.hdm<emphasis
+ role="red">-</emphasis>stuttgart.de</uri> URL. Doing so
+ would mess with <xref linkend="glo_Java"/>'s minus operator
+ when it comes to package names.</para>
</caution>
</callout>
diff --git a/Doc/Sd1/inheritance.xml b/Doc/Sd1/inheritance.xml
index 010679cf6..d2121e97c 100644
--- a/Doc/Sd1/inheritance.xml
+++ b/Doc/Sd1/inheritance.xml
@@ -429,6 +429,156 @@ public <link
</callout>
</calloutlist>
</figure>
+
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_stringEqualProblem">
+ <title>Let me in, please!</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>Consider the following snippet:</para>
+
+ <programlisting language="java">try (final Scanner scan = new Scanner(System.in)) {
+ do {
+ System.out.print("Please enter password: ");
+ final String password = scan.nextLine();
+ if (password == "secret") {
+ break; // Leave enclosing do ... while loop
+ } else {
+ System.out.println("Sorry, please try again");
+ }
+ } while (true);
+ System.out.println("You made it!");
+ // ...
+}</programlisting>
+
+ <para>Describe the above code's intended behaviour. Will it
+ succeed? Execute the above code and provide a clue for correcting
+ the underlying flaw.</para>
+ </question>
+
+ <answer>
+ <para>The user is being asked for a password. Only when entering
+ <code>"secret"</code> access shall be granted.</para>
+
+ <para>Unfortunately comparing the user's input and the
+ corresponding password is seriously flawed:</para>
+
+ <programlisting language="java">...
+ if (password == "secret")
+...</programlisting>
+
+ <para>On execution the string literal "secret" will be represented
+ as a <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/String.html">String</classname>
+ object in memory. Each new user input string will be represented
+ as a <emphasis>different</emphasis> <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/String.html">String</classname>
+ object in memory as well.</para>
+
+ <para>Unfortunately the usual <quote>==</quote> operator only
+ works as expected for the eight built in primitive <xref
+ linkend="glo_Java"/> types. With respect to class instances a
+ variable holds a reference to an object rather than the object
+ itself. Applying the <quote>==</quote> operator compares for
+ object identity rather than object equality.</para>
+
+ <para>Two different <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/String.html">String</classname>
+ instances albeit being different object instances may off course
+ be equal with respect to their <quote>payload</quote> namely the
+ strings they both represent. Comparing for object equality rather
+ than for object identity in <xref linkend="glo_Java"/> requires
+ overriding the <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">Object.equals(Object
+ o)</methodname> method. This override does exist in class
+ <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/String.html">String</classname>.
+ Within the given context we may thus simply use <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/String.html#equals(java.lang.Object)">String.equals(Object
+ o)</methodname>:</para>
+
+ <programlisting language="none">try (final Scanner scan = new Scanner(System.in)) {
+ do {
+ System.out.print("Please enter password: ");
+ final String password = scan.nextLine();
+ <emphasis role="red">if (password.equals("secret"))</emphasis> {
+ break; // Leave enclosing do ... while loop
+ } else {
+ System.out.println("Sorry, please try again");
+ }
+ } while (true);
+ System.out.println("You made it!");
+ // ...
+}</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+
+ <qandaset defaultlabel="qanda" xml:id="sd1_qanda_enomNoEqualsRequired">
+ <title>Why does == work for <code language="java">enum</code>
+ instances?</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>The preceding exercise <xref
+ linkend="sd1_qanda_stringEqualProblem"/> told us to override
+ <methodname
+ xlink:href="https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">Object.equals(Object
+ o)</methodname> when comparing for the equality of different
+ objects.</para>
+
+ <para>With respect to <code language="java">enum</code> instances
+ there seems to be an exception to the rule
+ <abbrev>e.g.</abbrev>:</para>
+
+ <programlisting language="java">Day day = Day.Saturday;
+
+// Sufficient using operator == ?
+if (day == Day.FRIDAY) ...
+
+// Or do we require equals?
+if (day.equals(Day.FRIDAY)) ...</programlisting>
+
+ <para>Do we thus require an <methodname>equals(Object
+ o)</methodname> method for comparisons? Give a precise
+ explanation.</para>
+ </question>
+
+ <answer>
+ <para>In case of <code language="java">enum</code> instances using
+ the <quote>==</quote> operator is sufficient. Due to the way <code
+ language="java">enum</code>'s are being constructed instance
+ creation is being limited to the underlying <code
+ language="java">enum</code>'s scope by its implicitly private
+ constructor. It is thus impossible to create new instances outside
+ the respective <code language="java">enum</code>'s scope.
+ Therefore object identity and equality of values are being
+ guaranteed to match exactly for two arbitrary instances belonging
+ to the same <code language="java">enum</code> class:</para>
+
+ <programlisting language="none">MyEnum e1 = ..., e2 = ...; // MyEnum representing some enum type
+
+final boolean
+ <emphasis role="red">objectIdentity</emphasis> = (e1 == e2), // Both boolean values will
+ <emphasis role="red">objectEquality</emphasis> = e1.equals(e2); // always be equal.</programlisting>
+
+ <para>In case of <code language="java">null</code> values using
+ the <quote>==</quote> operator avoids
+ <classname>java.lang.NullPointerException</classname>
+ problems:</para>
+
+ <programlisting language="java">Day day = null;
+
+if (day == Day.FRIDAY) ... // Just different, no problems
+
+if (day.equals(Day.FRIDAY)) ... // Oops: NPE approaching ...</programlisting>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
</section>
<section xml:id="sd1_inherit_sect_overrideToString">
diff --git a/Doc/Sd1/objectsClasses.xml b/Doc/Sd1/objectsClasses.xml
index 7bb579a38..cf1391366 100644
--- a/Doc/Sd1/objectsClasses.xml
+++ b/Doc/Sd1/objectsClasses.xml
@@ -12020,7 +12020,7 @@ class_wrapper.Day@63961c42</emphasis></screen></td>
<abstract>
<para>Clearing the mess.</para>
- <para><code>switch</code> still does not work.</para>
+ <para><code>Sad: switch</code> still not working.</para>
</abstract>
</info>
@@ -12126,7 +12126,7 @@ class_wrapper_private.Day'</emphasis></screen></td>
<programlisting language="java">public enum <link
xlink:href="https://gitlab.mi.hdm-stuttgart.de/goik/GoikLectures/tree/master/Doc/Sd1/Ref/ObjectsAndClasses/Intro/Enum/src/main/java/de/hdm_stuttgart/mi/sd1/use_enum/Day.java">Day</link> {
...
- public static String getItalianDayname(final Day day) {
+ public static String getItalianDayName(final Day day) {
switch (day) {
case MONDAY: return "Lunedì";
case TUESDAY: return "Martedì";
@@ -12134,7 +12134,7 @@ class_wrapper_private.Day'</emphasis></screen></td>
case SUNDAY: return "Domenica";
}
return null; // Actually unreachable, but static
- // code analysis is limited
+ // compiler code analysis is limited
}
}</programlisting>
</figure>
@@ -12368,6 +12368,29 @@ System.out.println(" Opposite: " + southWest.opposite());</programlisting></td>
<section xml:id="sd1_sect_usingGit">
<title>Using git</title>
+ <figure xml:id="sd1_fig_gitUrban">
+ <title>From <uri
+ xlink:href="https://www.urbandictionary.com">https://www.urbandictionary.com</uri></title>
+
+ <para>Git:</para>
+
+ <orderedlist>
+ <listitem>
+ <para>A completely ignorant, childish person with no manners.</para>
+ </listitem>
+
+ <listitem>
+ <para>A person who feels justified in their callow behaviour.</para>
+ </listitem>
+
+ <listitem>
+ <para>A pubescent kid who thinks it's totally cool to act like a
+ moron on the internet, only because no one can actually reach
+ through the screen and punch their lights out.</para>
+ </listitem>
+ </orderedlist>
+ </figure>
+
<figure xml:id="sd1_fig_gitBiblio">
<title>Useful links</title>
--
GitLab