From fd8e9579b80e25f1887ab18494bd0908e42a3655 Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Thu, 20 Jan 2022 21:43:22 +0100
Subject: [PATCH] AutoCloseable, Exercise understanding initializations in try
... catch
---
Doc/Sd1/errorHandling.xml | 64 ++++++++++++++++++----
Doc/Sd1/workingWithNumbers.xml | 99 ++++++++++++++++++++++++++++------
2 files changed, 139 insertions(+), 24 deletions(-)
diff --git a/Doc/Sd1/errorHandling.xml b/Doc/Sd1/errorHandling.xml
index 405bd8b4f..bd8e2d614 100644
--- a/Doc/Sd1/errorHandling.xml
+++ b/Doc/Sd1/errorHandling.xml
@@ -336,7 +336,7 @@ try {
xml:id="sd1_errorHandling_fig_tryWithResources-2-co"/> = new Scanner(System.in)) {
... // Something may fail
}<co linkends="sd1_errorHandling_fig_tryWithResources-3"
- xml:id="sd1_errorHandling_fig_tryWithResources-3-co"/></programlisting>
+ xml:id="sd1_errorHandling_fig_tryWithResources-3-co"/> // implicitly calling scanner.close()</programlisting>
<calloutlist>
<callout arearefs="sd1_errorHandling_fig_tryWithResources-1-co"
@@ -360,6 +360,56 @@ try {
</callout>
</calloutlist>
</figure>
+
+ <figure xml:id="sd1_errorHandling_fig_ScannerAutoCloseable">
+ <title>Scanner implementing <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/AutoCloseable.html">AutoCloseable</classname></title>
+
+ <informaltable border="0">
+ <tr>
+ <td valign="top"><programlisting language="java">public class Scanner
+ implements AutoCloseable <co
+ linkends="sd1_errorHandlingAutoCloseablePromise-1"
+ xml:id="sd1_errorHandlingAutoCloseablePromise-1-co"/>, ... {
+
+ ...
+
+ public void close() {...} <co
+ linkends="sd1_errorHandlingAutoCloseablePromise-2"
+ xml:id="sd1_errorHandlingAutoCloseablePromise-2-co"/>
+
+}</programlisting></td>
+
+ <td valign="top"><programlisting language="java">Interface AutoCloseable {
+ public void close(); // Signature, no
+ // implementation
+}</programlisting></td>
+ </tr>
+ </informaltable>
+
+ <calloutlist>
+ <callout arearefs="sd1_errorHandlingAutoCloseablePromise-1-co"
+ xml:id="sd1_errorHandlingAutoCloseablePromise-1">
+ <para>Promise to implement all methods being declared in <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/AutoCloseable.html">AutoCloseable</classname>.</para>
+ </callout>
+
+ <callout arearefs="sd1_errorHandlingAutoCloseablePromise-2-co"
+ xml:id="sd1_errorHandlingAutoCloseablePromise-2">
+ <para>Actually implementing a <methodname>close()</methodname>
+ method.</para>
+ </callout>
+ </calloutlist>
+ </figure>
+
+ <figure xml:id="sd1_errorHandling_fig_tryWithResourcesString">
+ <title>No <methodname>close()</methodname> method in e.g.
+ <classname>class</classname> String</title>
+
+ <programlisting language="java">try (final String s = new String()) { // Error: Required type: AutoCloseable; Provided: String
+ ...
+}</programlisting>
+ </figure>
</section>
<section xml:id="sd1_errorhandling_sect_classException">
@@ -437,11 +487,9 @@ public class StackTrace {
new <link xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/File.html#%3Cinit%3E(java.lang.String)">File("test.txt"))</link>;
} catch(Exception e) {
System.err.println("General error");
-} catch (<emphasis role="red"
- xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/IOException.html">IOException e</emphasis>) {
+} catch (<emphasis role="red">IOException e</emphasis>) {
System.err.println( "IO error");
-} catch(<emphasis role="red"
- xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/FileNotFoundException.html">FileNotFoundException e</emphasis>) {
+} catch(<emphasis role="red">FileNotFoundException e</emphasis>) {
System.err.println("File not found");
}</programlisting></td>
@@ -457,11 +505,9 @@ public class StackTrace {
<figure xml:id="sd1_errorHandling_fig_implementCardinalSimple">
<title>Implementing <methodname>convert</methodname></title>
- <programlisting language="java">/**
- * Translate {"one", "two", "three"} to {"first", "second", "third"}
+ <programlisting language="java">/* Translate {"one", "two", "three"} to {"first", "second", "third"}
* @param input The input String to be translated.
- * @return See above explanation.
- */
+ * @return See above explanation. */
static public String convert(final String input) {
switch (input) {
case "one": return "first";
diff --git a/Doc/Sd1/workingWithNumbers.xml b/Doc/Sd1/workingWithNumbers.xml
index 2ba8a9b41..51157282d 100644
--- a/Doc/Sd1/workingWithNumbers.xml
+++ b/Doc/Sd1/workingWithNumbers.xml
@@ -324,16 +324,17 @@ Double d = intValue;</programlisting>
<colgroup width="35%"/>
<tr>
- <td valign="top"><programlisting language="java">String userInput = "dummyValue";
+ <td valign="top"><programlisting language="java">String userInput = null;
try (final Scanner scanner =
new Scanner(System.in)){
System.out.print("Enter an integer:");
userInput = scanner.nextLine();
+
final int value = Integer.parseInt(userInput);
+
System.out.println("You entered " + value);
} catch (final NumberFormatException e) {
- System.out.println("Sorry, but '" +
- userInput + "' is not an integer.");
+ System.out.println("Error: No integer value");
}</programlisting></td>
<td valign="top"><screen>Enter an integer:-34
@@ -346,14 +347,14 @@ not an integer.</screen></td>
<qandaset defaultlabel="qanda" xml:id="sd1_numbers_qanda_whyDummyString">
<title>Why using <code language="java">String userInput =
- "dummyValue"</code>?</title>
+ null</code>?</title>
<qandadiv>
<qandaentry>
<question>
<para>The former listing initializes the <code
language="java">userInput</code> variable using a default value
- <code language="java">"dummyValue"</code>. Why is that?</para>
+ <code language="java">null</code>. Why is that?</para>
<orderedlist>
<listitem>
@@ -379,26 +380,35 @@ not an integer.</screen></td>
</orderedlist>
<para>So the dummy assignment <code language="java">String
- userInput = "dummyValue"</code> seems to be redundant. Is that
+ userInput = null</code> seems to be redundant. Is that
true?</para>
</question>
<answer>
<para>Actually the <xref linkend="glo_Java"/> compiler is not
clever enough analyzing the <code language="java">try {...}</code>
- block. It cannot detect the impossibility of an uninitialized
- <code language="java">userInput</code> variable inside the <code
- language="java">catch {...}</code> clause.</para>
+ block. The catch clause can only be reached by inadequate user
+ input causing the <methodname>parseInt(...)</methodname> method.
+ In particular the <methodname>nextLine(...)</methodname> being
+ unable to throw a <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/NumberFormatException.html">NumberFormatException</classname>
+ cannot be the culprit for entering the <code
+ language="java">catch</code> clause. Thus if a <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/NumberFormatException.html">NumberFormatException</classname>
+ is being thrown our <code language="java">userInput</code>
+ variable will already carry a user entered input string. The
+ compiler however cannot analyze this type of dependency.</para>
- <para>Since the possibility of am uninitialized <code
+ <para>Thus the possibility of an uninitialized <code
language="java">userInput</code> variable using just <code
- language="java">String userInput;</code> causes a compile time
- error <code>Variable 'userInput' might not have been
+ language="java">String userInput;</code> would cause a compile
+ time error <code>Variable 'userInput' might not have been
initialized</code>.</para>
- <para>Thus <code language="java">String userInput = null</code>
- would be appropriate as well since the value will actually never
- be used.</para>
+ <para><code language="java">So userInput = null</code> is
+ sufficient for keeping the compiler happy and inside the catch
+ clause the variable will definitively carry a<classname>
+ String</classname> reference.</para>
</answer>
</qandaentry>
</qandadiv>
@@ -969,6 +979,25 @@ System.out.println(result);</programlisting>
<programlisting language="java">final BigDecimal zero_dot_99 = new BigDecimal("0.99");
final BigDecimal zero_dot_1 = new BigDecimal("0.1");
+BigDecimal
+ result = zero_dot_99.subtract(zero_dot_1); // Subtracting 0.1
+
+ result = result.subtract(zero_dot_1); // Subtracting 0.1
+ result = result.subtract(zero_dot_1); // Subtracting 0.1
+
+System.out.println(result);</programlisting>
+
+ <screen>0.69</screen>
+ </figure>
+
+ <figure xml:id="sd1_numbers_fig_usingBigDecimalChaining">
+ <title>Chaining <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>
+ operations</title>
+
+ <programlisting language="java">final BigDecimal zero_dot_99 = new BigDecimal("0.99");
+final BigDecimal zero_dot_1 = new BigDecimal("0.1");
+
BigDecimal result = zero_dot_99.
subtract(zero_dot_1).
subtract(zero_dot_1).
@@ -979,6 +1008,46 @@ System.out.println(result);</programlisting>
<screen>0.69</screen>
</figure>
+ <qandaset defaultlabel="qanda"
+ xml:id="sd1_workingWithNumbers_BigDecimalChaining">
+ <title>Chaining subtract method calls</title>
+
+ <qandadiv>
+ <qandaentry>
+ <question>
+ <para>Explain the chaining mechanism implementing three successive
+ subtractions in <xref
+ linkend="sd1_numbers_fig_usingBigDecimalChaining"/>.</para>
+ </question>
+
+ <answer>
+ <para>We may re-write the statement in question:</para>
+
+ <programlisting language="none">result = zero_dot_99.subtract(zero_dot_1).subtract(zero_dot_1).subtract(zero_dot_1);
+ \ / / /
+ \ / / /
+ \ / / /
+ 0.99 - 0.1 / /
+ \ / /
+ \ / /
+ \ / /
+ 0.98 - 0.1 /
+ \ /
+ \ /
+ 0.97 - 0.1
+ == 0.96</programlisting>
+
+ <para>Each <methodname>subtract(...)</methodname> call returns a
+ new result instance of class <classname
+ xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>.
+ We can thus chain a subsequent
+ <methodname>subtract(...)</methodname> call using this returned
+ instance.</para>
+ </answer>
+ </qandaentry>
+ </qandadiv>
+ </qandaset>
+
<figure xml:id="sd1_numbers_fig_usingBigDecimalFeatures">
<title><classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>
--
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